Fix poll_expired incorrectly returning Pending when there are no outstanding requests.

Fixes https://github.com/google/tarpc/issues/352.

Cargo.toml

@@ -1,7 +1,11 @@
 [workspace]
+resolver = "2"
 
 members = [
     "example-service",
     "tarpc",
     "plugins",
 ]
+
+[profile.dev]
+split-debuginfo = "unpacked"

README.md

@@ -51,6 +51,14 @@ Some other features of tarpc:
   requests sent by the server that use the request context will propagate the request deadline.
   For example, if a server is handling a request with a 10s deadline, does 2s of work, then
   sends a request to another server, that server will see an 8s deadline.
+- Distributed tracing: tarpc is instrumented with
+  [tracing](https://github.com/tokio-rs/tracing) primitives extended with
+  [OpenTelemetry](https://opentelemetry.io/) traces. Using a compatible tracing subscriber like
+  [Jaeger](https://github.com/open-telemetry/opentelemetry-rust/tree/main/opentelemetry-jaeger),
+  each RPC can be traced through the client, server, amd other dependencies downstream of the
+  server. Even for applications not connected to a distributed tracing collector, the
+  instrumentation can also be ingested by regular loggers like
+  [env_logger](https://github.com/env-logger-rs/env_logger/).
 - Serde serialization: enabling the `serde1` Cargo feature will make service requests and
   responses `Serialize + Deserialize`. It's entirely optional, though: in-memory transports can
   be used, as well, so the price of serialization doesn't have to be paid when it's not needed.
@@ -59,7 +67,7 @@ Some other features of tarpc:
 Add to your `Cargo.toml` dependencies:
 
 ```toml
-tarpc = "0.25"
+tarpc = "0.27"
 ```
 
 The `tarpc::service` attribute expands to a collection of items that form an rpc service.
@@ -72,8 +80,9 @@ This example uses [tokio](https://tokio.rs), so add the following dependencies t
 your `Cargo.toml`:
 
 ```toml
-futures = "1.0"
+anyhow = "1.0"
-tarpc = { version = "0.25", features = ["tokio1"] }
+futures = "0.3"
+tarpc = { version = "0.27", features = ["tokio1"] }
 tokio = { version = "1.0", features = ["macros"] }
 ```
 
@@ -91,9 +100,8 @@ use futures::{
 };
 use tarpc::{
     client, context,
-    server::{self, Incoming},
+    server::{self, incoming::Incoming},
 };
-use std::io;
 
 // This is the service definition. It looks a lot like a trait definition.
 // It defines one RPC, hello, which takes one arg, name, and returns a String.
@@ -132,7 +140,7 @@ available behind the `tcp` feature.
 
 ```rust
 #[tokio::main]
-async fn main() -> io::Result<()> {
+async fn main() -> anyhow::Result<()> {
     let (client_transport, server_transport) = tarpc::transport::channel::unbounded();
 
     let server = server::BaseChannel::with_defaults(server_transport);
@@ -140,7 +148,7 @@ async fn main() -> io::Result<()> {
 
     // WorldClient is generated by the #[tarpc::service] attribute. It has a constructor `new`
     // that takes a config and any Transport as input.
-    let mut client = WorldClient::new(client::Config::default(), client_transport).spawn()?;
+    let mut client = WorldClient::new(client::Config::default(), client_transport).spawn();
 
     // The client has an RPC method for each RPC defined in the annotated trait. It takes the same
     // args as defined, with the addition of a Context, which is always the first arg. The Context

RELEASES.md

@@ -1,3 +1,111 @@
+## 0.27.1 (2021-09-22)
+
+### Breaking Changes
+
+### RPC error type is changing
+
+RPC return types are changing from `Result<Response, io::Error>` to `Result<Response,
+tarpc::client::RpcError>`.
+
+Becaue tarpc is a library, not an application, it should strive to
+use structured errors in its API so that users have maximal flexibility
+in how they handle errors. io::Error makes that hard, because it is a
+kitchen-sink error type.
+
+RPCs in particular only have 3 classes of errors:
+
+- The connection breaks.
+- The request expires.
+- The server decides not to process the request.
+
+RPC responses can also contain application-specific errors, but from the
+perspective of the RPC library, those are opaque to the framework, classified
+as successful responsees.
+
+### Open Telemetry
+
+The Opentelemetry dependency is updated to version 0.16.x.
+
+## 0.27.0 (2021-09-22)
+
+This version was yanked due to tarpc-plugins version mismatches.
+
+
+## 0.26.0 (2021-04-14)
+
+### New Features
+
+#### Tracing
+
+tarpc is now instrumented with tracing primitives extended with
+OpenTelemetry traces. Using a compatible tracing-opentelemetry
+subscriber like Jaeger, each RPC can be traced through the client,
+server, amd other dependencies downstream of the server. Even for
+applications not connected to a distributed tracing collector, the
+instrumentation can also be ingested by regular loggers like env_logger.
+
+### Breaking Changes
+
+#### Logging
+
+Logged events are now structured using tracing. For applications using a
+logger and not a tracing subscriber, these logs may look different or
+contain information in a less consumable manner. The easiest solution is
+to add a tracing subscriber that logs to stdout, such as
+tracing_subscriber::fmt.
+
+####  Context
+
+- Context no longer has parent_span, which was actually never needed,
+  because the context sent in an RPC is inherently the parent context.
+  For purposes of distributed tracing, the client side of the RPC has all
+  necessary information to link the span to its parent; the server side
+  need do nothing more than export the (trace ID, span ID) tuple.
+- Context has a new field, SamplingDecision, which has two variants,
+  Sampled and Unsampled. This field can be used by downstream systems to
+  determine whether a trace needs to be exported. If the parent span is
+  sampled, the expectation is that all child spans be exported, as well;
+  to do otherwise could result in lossy traces being exported. Note that
+  if an Openetelemetry tracing subscriber is not installed, the fallback
+  context will still be used, but the Context's sampling decision will
+  always be inherited by the parent Context's sampling decision.
+- Context::scope has been removed. Context propagation is now done via
+  tracing's task-local spans. Spans can be propagated across tasks via
+  Span::in_scope. When a service receives a request, it attaches an
+  Opentelemetry context to the local Span created before request handling,
+  and this context contains the request deadline. This span-local deadline
+  is retrieved by Context::current, but it cannot be modified so that
+  future Context::current calls contain a different deadline. However, the
+  deadline in the context passed into an RPC call will override it, so
+  users can retrieve the current context and then modify the deadline
+  field, as has been historically possible.
+- Context propgation precedence changes: when an RPC is initiated, the
+  current Span's Opentelemetry context takes precedence over the trace
+  context passed into the RPC method. If there is no current Span, then
+  the trace context argument is used as it has been historically. Note
+  that Opentelemetry context propagation requires an Opentelemetry
+  tracing subscriber to be installed.
+
+#### Server
+
+- The server::Channel trait now has an additional required associated
+  type and method which returns the underlying transport. This makes it
+  more ergonomic for users to retrieve transport-specific information,
+  like IP Address. BaseChannel implements Channel::transport by returning
+  the underlying transport, and channel decorators like Throttler just
+  delegate to the Channel::transport method of the wrapped channel.
+
+#### Client
+
+- NewClient::spawn no longer returns a result, as spawn can't fail.
+
+### References
+
+1. https://github.com/tokio-rs/tracing
+2. https://opentelemetry.io
+3. https://github.com/open-telemetry/opentelemetry-rust/tree/main/opentelemetry-jaeger
+4. https://github.com/env-logger-rs/env_logger
+
 ## 0.25.0 (2021-03-10)
 
 ### Breaking Changes
@@ -170,7 +278,7 @@ nameable futures and will just be boxing the return type anyway. This macro does
 
 ### Breaking Changes
 
-- Enums had _non_exhaustive fields replaced with the #[non_exhaustive] attribute.
+- Enums had `_non_exhaustive` fields replaced with the #[non_exhaustive] attribute.
 
 ### Bug Fixes
 

example-service/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "tarpc-example-service"
-version = "0.9.0"
+version = "0.10.0"
 authors = ["Tim Kuehn <tikue@google.com>"]
 edition = "2018"
 license = "MIT"
@@ -13,12 +13,18 @@ readme = "../README.md"
 description = "An example server built on tarpc."
 
 [dependencies]
-clap = "2.33"
+anyhow = "1.0"
-env_logger = "0.8"
+clap = "3.0.0-beta.2"
+log = "0.4"
 futures = "0.3"
-serde = { version = "1.0" }
+opentelemetry = { version = "0.16", features = ["rt-tokio"] }
-tarpc = { version = "0.25", path = "../tarpc", features = ["full"] }
+opentelemetry-jaeger = { version = "0.15", features = ["rt-tokio"] }
-tokio = { version = "1", features = ["full"] }
+rand = "0.8"
+tarpc = { version = "0.27", path = "../tarpc", features = ["full"] }
+tokio = { version = "1", features = ["macros", "net", "rt-multi-thread"] }
+tracing = { version = "0.1" }
+tracing-opentelemetry = "0.15"
+tracing-subscriber = "0.2"
 
 [lib]
 name = "service"

example-service/src/client.rs

@@ -4,57 +4,49 @@
 // license that can be found in the LICENSE file or at
 // https://opensource.org/licenses/MIT.
 
-use clap::{App, Arg};
+use clap::Clap;
-use std::{io, net::SocketAddr};
+use service::{init_tracing, WorldClient};
+use std::{net::SocketAddr, time::Duration};
 use tarpc::{client, context, tokio_serde::formats::Json};
+use tokio::time::sleep;
+use tracing::Instrument;
+
+#[derive(Clap)]
+struct Flags {
+    /// Sets the server address to connect to.
+    #[clap(long)]
+    server_addr: SocketAddr,
+    /// Sets the name to say hello to.
+    #[clap(long)]
+    name: String,
+}
 
 #[tokio::main]
-async fn main() -> io::Result<()> {
+async fn main() -> anyhow::Result<()> {
-    env_logger::init();
+    let flags = Flags::parse();
+    init_tracing("Tarpc Example Client")?;
 
-    let flags = App::new("Hello Client")
+    let transport = tarpc::serde_transport::tcp::connect(flags.server_addr, Json::default);
-        .version("0.1")
-        .author("Tim <tikue@google.com>")
-        .about("Say hello!")
-        .arg(
-            Arg::with_name("server_addr")
-                .long("server_addr")
-                .value_name("ADDRESS")
-                .help("Sets the server address to connect to.")
-                .required(true)
-                .takes_value(true),
-        )
-        .arg(
-            Arg::with_name("name")
-                .short("n")
-                .long("name")
-                .value_name("STRING")
-                .help("Sets the name to say hello to.")
-                .required(true)
-                .takes_value(true),
-        )
-        .get_matches();
-
-    let server_addr = flags.value_of("server_addr").unwrap();
-    let server_addr = server_addr
-        .parse::<SocketAddr>()
-        .unwrap_or_else(|e| panic!(r#"--server_addr value "{}" invalid: {}"#, server_addr, e));
-
-    let name = flags.value_of("name").unwrap().into();
-
-    let mut transport = tarpc::serde_transport::tcp::connect(server_addr, Json::default);
-    transport.config_mut().max_frame_length(usize::MAX);
 
     // WorldClient is generated by the service attribute. It has a constructor `new` that takes a
     // config and any Transport as input.
-    let client = service::WorldClient::new(client::Config::default(), transport.await?).spawn()?;
+    let client = WorldClient::new(client::Config::default(), transport.await?).spawn();
 
-    // The client has an RPC method for each RPC defined in the annotated trait. It takes the same
+    let hello = async move {
-    // args as defined, with the addition of a Context, which is always the first arg. The Context
+        // Send the request twice, just to be safe! ;)
-    // specifies a deadline and trace information which can be helpful in debugging requests.
+        tokio::select! {
-    let hello = client.hello(context::current(), name).await?;
+            hello1 = client.hello(context::current(), format!("{}1", flags.name)) => { hello1 }
+            hello2 = client.hello(context::current(), format!("{}2", flags.name)) => { hello2 }
+        }
+    }
+    .instrument(tracing::info_span!("Two Hellos"))
+    .await;
 
-    println!("{}", hello);
+    tracing::info!("{:?}", hello);
+
+    // Let the background span processor finish.
+    sleep(Duration::from_micros(1)).await;
+    opentelemetry::global::shutdown_tracer_provider();
 
     Ok(())
 }

example-service/src/lib.rs

@@ -4,6 +4,9 @@
 // license that can be found in the LICENSE file or at
 // https://opensource.org/licenses/MIT.
 
+use std::env;
+use tracing_subscriber::{fmt::format::FmtSpan, prelude::*};
+
 /// This is the service definition. It looks a lot like a trait definition.
 /// It defines one RPC, hello, which takes one arg, name, and returns a String.
 #[tarpc::service]
@@ -11,3 +14,21 @@ pub trait World {
     /// Returns a greeting for name.
     async fn hello(name: String) -> String;
 }
+
+/// Initializes an OpenTelemetry tracing subscriber with a Jaeger backend.
+pub fn init_tracing(service_name: &str) -> anyhow::Result<()> {
+    env::set_var("OTEL_BSP_MAX_EXPORT_BATCH_SIZE", "12");
+
+    let tracer = opentelemetry_jaeger::new_pipeline()
+        .with_service_name(service_name)
+        .with_max_packet_size(2usize.pow(13))
+        .install_batch(opentelemetry::runtime::Tokio)?;
+
+    tracing_subscriber::registry()
+        .with(tracing_subscriber::EnvFilter::from_default_env())
+        .with(tracing_subscriber::fmt::layer().with_span_events(FmtSpan::NEW | FmtSpan::CLOSE))
+        .with(tracing_opentelemetry::layer().with_tracer(tracer))
+        .try_init()?;
+
+    Ok(())
+}

example-service/src/server.rs

@@ -4,18 +4,30 @@
 // license that can be found in the LICENSE file or at
 // https://opensource.org/licenses/MIT.
 
-use clap::{App, Arg};
+use clap::Clap;
 use futures::{future, prelude::*};
-use service::World;
+use rand::{
+    distributions::{Distribution, Uniform},
+    thread_rng,
+};
+use service::{init_tracing, World};
 use std::{
-    io,
+    net::{IpAddr, Ipv6Addr, SocketAddr},
-    net::{IpAddr, SocketAddr},
+    time::Duration,
 };
 use tarpc::{
     context,
-    server::{self, Channel, Incoming},
+    server::{self, incoming::Incoming, Channel},
     tokio_serde::formats::Json,
 };
+use tokio::time;
+
+#[derive(Clap)]
+struct Flags {
+    /// Sets the port number to listen on.
+    #[clap(long)]
+    port: u16,
+}
 
 // This is the type that implements the generated World trait. It is the business logic
 // and is used to start the server.
@@ -25,35 +37,19 @@ struct HelloServer(SocketAddr);
 #[tarpc::server]
 impl World for HelloServer {
     async fn hello(self, _: context::Context, name: String) -> String {
-        format!("Hello, {}! You are connected from {:?}.", name, self.0)
+        let sleep_time =
+            Duration::from_millis(Uniform::new_inclusive(1, 10).sample(&mut thread_rng()));
+        time::sleep(sleep_time).await;
+        format!("Hello, {}! You are connected from {}", name, self.0)
     }
 }
 
 #[tokio::main]
-async fn main() -> io::Result<()> {
+async fn main() -> anyhow::Result<()> {
-    env_logger::init();
+    let flags = Flags::parse();
+    init_tracing("Tarpc Example Server")?;
 
-    let flags = App::new("Hello Server")
+    let server_addr = (IpAddr::V6(Ipv6Addr::LOCALHOST), flags.port);
-        .version("0.1")
-        .author("Tim <tikue@google.com>")
-        .about("Say hello!")
-        .arg(
-            Arg::with_name("port")
-                .short("p")
-                .long("port")
-                .value_name("NUMBER")
-                .help("Sets the port number to listen on")
-                .required(true)
-                .takes_value(true),
-        )
-        .get_matches();
-
-    let port = flags.value_of("port").unwrap();
-    let port = port
-        .parse()
-        .unwrap_or_else(|e| panic!(r#"--port value "{}" invalid: {}"#, port, e));
-
-    let server_addr = (IpAddr::from([0, 0, 0, 0]), port);
 
     // JSON transport is provided by the json_transport tarpc module. It makes it easy
     // to start up a serde-powered json serialization strategy over TCP.
@@ -64,12 +60,12 @@ async fn main() -> io::Result<()> {
         .filter_map(|r| future::ready(r.ok()))
         .map(server::BaseChannel::with_defaults)
         // Limit channels to 1 per IP.
-        .max_channels_per_key(1, |t| t.as_ref().peer_addr().unwrap().ip())
+        .max_channels_per_key(1, |t| t.transport().peer_addr().unwrap().ip())
         // serve is generated by the service attribute. It takes as input any type implementing
         // the generated World trait.
         .map(|channel| {
-            let server = HelloServer(channel.as_ref().as_ref().peer_addr().unwrap());
+            let server = HelloServer(channel.transport().peer_addr().unwrap());
-            channel.requests().execute(server.serve())
+            channel.execute(server.serve())
         })
         // Max 10 channels.
         .buffer_unordered(10)

hooks/pre-commit

@@ -67,7 +67,7 @@ else
 fi
 
 printf "${PREFIX} Checking for rustfmt ... "
-command -v cargo fmt &>/dev/null
+command -v rustfmt &>/dev/null
 if [ $? == 0 ]; then
 	printf "${SUCCESS}\n"
 else
@@ -93,19 +93,19 @@ diff=""
 for file in $(git diff --name-only --cached);
 do
 	if [ ${file: -3} == ".rs" ]; then
-        diff="$diff$(cargo fmt -- --check $file)"
+        diff="$diff$(rustfmt --edition 2018 --check $file)"
+        if [ $? != 0 ]; then
+            FMTRESULT=1
+        fi
 	fi
 done
-if grep --quiet "^[-+]" <<< "$diff"; then
-    FMTRESULT=1
-fi
 
 if [ "${TARPC_SKIP_RUSTFMT}" == 1 ]; then
 	printf "${SKIPPED}\n"$?
 elif [ ${FMTRESULT} != 0 ]; then
 	FAILED=1
 	printf "${FAILURE}\n"
-    echo "$diff" | sed 's/Using rustfmt config file.*$/d/'
+    echo "$diff"
 else
 	printf "${SUCCESS}\n"
 fi

plugins/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "tarpc-plugins"
-version = "0.10.0"
+version = "0.12.0"
 authors = ["Adam Wright <adam.austin.wright@gmail.com>", "Tim Kuehn <timothy.j.kuehn@gmail.com>"]
 edition = "2018"
 license = "MIT"
@@ -30,4 +30,4 @@ proc-macro = true
 assert-type-eq = "0.1.0"
 futures = "0.3"
 serde = { version = "1.0", features = ["derive"] }
-tarpc = { path = "../tarpc" }
+tarpc = { path = "../tarpc", features = ["serde1"] }

plugins/src/lib.rs

@@ -267,18 +267,25 @@ pub fn service(attr: TokenStream, input: TokenStream) -> TokenStream {
         None
     };
 
+    let methods = rpcs.iter().map(|rpc| &rpc.ident).collect::<Vec<_>>();
+    let request_names = methods
+        .iter()
+        .map(|m| format!("{}.{}", ident, m))
+        .collect::<Vec<_>>();
+
     ServiceGenerator {
         response_fut_name,
         service_ident: ident,
         server_ident: &format_ident!("Serve{}", ident),
-        response_fut_ident: &Ident::new(&response_fut_name, ident.span()),
+        response_fut_ident: &Ident::new(response_fut_name, ident.span()),
         client_ident: &format_ident!("{}Client", ident),
         request_ident: &format_ident!("{}Request", ident),
         response_ident: &format_ident!("{}Response", ident),
         vis,
         args,
         method_attrs: &rpcs.iter().map(|rpc| &*rpc.attrs).collect::<Vec<_>>(),
-        method_idents: &rpcs.iter().map(|rpc| &rpc.ident).collect::<Vec<_>>(),
+        method_idents: &methods,
+        request_names: &*request_names,
         attrs,
         rpcs,
         return_types: &rpcs
@@ -399,11 +406,7 @@ fn verify_types_were_provided(
 ) -> syn::Result<()> {
     let mut result = Ok(());
     for (method, expected) in expected {
-        if provided
+        if !provided.iter().any(|typedecl| typedecl.ident == expected) {
-            .iter()
-            .find(|typedecl| typedecl.ident == expected)
-            .is_none()
-        {
             let mut e = syn::Error::new(
                 span,
                 format!("not all trait items implemented, missing: `{}`", expected),
@@ -441,6 +444,7 @@ struct ServiceGenerator<'a> {
     camel_case_idents: &'a [Ident],
     future_types: &'a [Type],
     method_idents: &'a [&'a Ident],
+    request_names: &'a [String],
     method_attrs: &'a [&'a [Attribute]],
     args: &'a [&'a [PatType]],
     return_types: &'a [&'a Type],
@@ -475,7 +479,7 @@ impl<'a> ServiceGenerator<'a> {
                     ),
                     output,
                 )| {
-                    let ty_doc = format!("The response future returned by {}.", ident);
+                    let ty_doc = format!("The response future returned by [`{}::{}`].", service_ident, ident);
                     quote! {
                         #[doc = #ty_doc]
                         type #future_type: std::future::Future<Output = #output>;
@@ -524,6 +528,7 @@ impl<'a> ServiceGenerator<'a> {
             camel_case_idents,
             arg_pats,
             method_idents,
+            request_names,
             ..
         } = self;
 
@@ -534,6 +539,16 @@ impl<'a> ServiceGenerator<'a> {
                 type Resp = #response_ident;
                 type Fut = #response_fut_ident<S>;
 
+                fn method(&self, req: &#request_ident) -> Option<&'static str> {
+                    Some(match req {
+                        #(
+                            #request_ident::#camel_case_idents{..} => {
+                                #request_names
+                            }
+                        )*
+                    })
+                }
+
                 fn serve(self, ctx: tarpc::context::Context, req: #request_ident) -> Self::Fut {
                     match req {
                         #(
@@ -563,6 +578,7 @@ impl<'a> ServiceGenerator<'a> {
 
         quote! {
             /// The request sent over the wire from the client to the server.
+            #[allow(missing_docs)]
             #[derive(Debug)]
             #derive_serialize
             #vis enum #request_ident {
@@ -583,6 +599,7 @@ impl<'a> ServiceGenerator<'a> {
 
         quote! {
             /// The response sent over the wire from the server to the client.
+            #[allow(missing_docs)]
             #[derive(Debug)]
             #derive_serialize
             #vis enum #response_ident {
@@ -603,6 +620,7 @@ impl<'a> ServiceGenerator<'a> {
 
         quote! {
             /// A future resolving to a server response.
+            #[allow(missing_docs)]
             #vis enum #response_fut_ident<S: #service_ident> {
                 #( #camel_case_idents(<S as #service_ident>::#future_types) ),*
             }
@@ -714,6 +732,7 @@ impl<'a> ServiceGenerator<'a> {
             method_attrs,
             vis,
             method_idents,
+            request_names,
             args,
             return_types,
             arg_pats,
@@ -727,9 +746,9 @@ impl<'a> ServiceGenerator<'a> {
                     #[allow(unused)]
                     #( #method_attrs )*
                     #vis fn #method_idents(&self, ctx: tarpc::context::Context, #( #args ),*)
-                        -> impl std::future::Future<Output = std::io::Result<#return_types>> + '_ {
+                        -> impl std::future::Future<Output = Result<#return_types, tarpc::client::RpcError>> + '_ {
                         let request = #request_ident::#camel_case_idents { #( #arg_pats ),* };
-                        let resp = self.0.call(ctx, request);
+                        let resp = self.0.call(ctx, #request_names, request);
                         async move {
                             match resp.await? {
                                 #response_ident::#camel_case_idents(msg) => std::result::Result::Ok(msg),

tarpc/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "tarpc"
-version = "0.25.1"
+version = "0.27.1"
 authors = ["Adam Wright <adam.austin.wright@gmail.com>", "Tim Kuehn <timothy.j.kuehn@gmail.com>"]
 edition = "2018"
 license = "MIT"
@@ -17,10 +17,12 @@ default = []
 
 serde1 = ["tarpc-plugins/serde1", "serde", "serde/derive"]
 tokio1 = ["tokio/rt-multi-thread"]
-serde-transport = ["serde1", "tokio1", "tokio-serde/json", "tokio-util/codec"]
+serde-transport = ["serde1", "tokio1", "tokio-serde", "tokio-util/codec"]
+serde-transport-json = ["tokio-serde/json"]
+serde-transport-bincode = ["tokio-serde/bincode"]
 tcp = ["tokio/net"]
 
-full = ["serde1", "tokio1", "serde-transport", "tcp"]
+full = ["serde1", "tokio1", "serde-transport", "serde-transport-json", "serde-transport-bincode", "tcp"]
 
 [badges]
 travis-ci = { repository = "google/tarpc" }
@@ -30,26 +32,31 @@ anyhow = "1.0"
 fnv = "1.0"
 futures = "0.3"
 humantime = "2.0"
-log = "0.4"
 pin-project = "1.0"
-rand = "0.7"
+rand = "0.8"
 serde = { optional = true, version = "1.0", features = ["derive"] }
 static_assertions = "1.1.0"
-tarpc-plugins = { path = "../plugins", version = "0.10" }
+tarpc-plugins = { path = "../plugins", version = "0.12" }
+thiserror = "1.0"
 tokio = { version = "1", features = ["time"] }
 tokio-util = { version = "0.6.3", features = ["time"] }
 tokio-serde = { optional = true, version = "0.8" }
+tracing = { version = "0.1", default-features = false, features = ["attributes", "log"] }
+tracing-opentelemetry = { version = "0.15", default-features = false }
+opentelemetry = { version = "0.16", default-features = false }
+
 
 [dev-dependencies]
 assert_matches = "1.4"
 bincode = "1.3"
 bytes = { version = "1", features = ["serde"] }
-env_logger = "0.8"
 flate2 = "1.0"
 futures-test = "0.3"
-log = "0.4"
+opentelemetry = { version = "0.16", default-features = false, features = ["rt-tokio"] }
+opentelemetry-jaeger = { version = "0.15", features = ["rt-tokio"] }
 pin-utils = "0.1.0-alpha"
 serde_bytes = "0.11"
+tracing-subscriber = "0.2"
 tokio = { version = "1", features = ["full", "test-util"] }
 tokio-serde = { version = "0.8", features = ["json", "bincode"] }
 trybuild = "1.0"
@@ -63,7 +70,7 @@ name = "compression"
 required-features = ["serde-transport", "tcp"]
 
 [[example]]
-name = "server_calling_server"
+name = "tracing"
 required-features = ["full"]
 
 [[example]]

tarpc/examples/compression.rs

@@ -7,8 +7,8 @@ use tarpc::{
     client, context,
     serde_transport::tcp,
     server::{BaseChannel, Channel},
+    tokio_serde::formats::Bincode,
 };
-use tokio_serde::formats::Bincode;
 
 /// Type of compression that should be enabled on the request. The transport is free to ignore this.
 #[derive(Debug, PartialEq, Eq, Clone, Copy, Deserialize, Serialize)]
@@ -118,7 +118,7 @@ async fn main() -> anyhow::Result<()> {
     });
 
     let transport = tcp::connect(addr, Bincode::default).await?;
-    let client = WorldClient::new(client::Config::default(), add_compression(transport)).spawn()?;
+    let client = WorldClient::new(client::Config::default(), add_compression(transport)).spawn();
 
     println!(
         "{}",

tarpc/examples/custom_transport.rs

@@ -1,9 +1,7 @@
-use futures::future;
-use tarpc::context::Context;
 use tarpc::serde_transport as transport;
 use tarpc::server::{BaseChannel, Channel};
+use tarpc::{context::Context, tokio_serde::formats::Bincode};
 use tokio::net::{UnixListener, UnixStream};
-use tokio_serde::formats::Bincode;
 use tokio_util::codec::length_delimited::LengthDelimitedCodec;
 
 #[tarpc::service]
@@ -14,16 +12,13 @@ pub trait PingService {
 #[derive(Clone)]
 struct Service;
 
+#[tarpc::server]
 impl PingService for Service {
-    type PingFut = future::Ready<()>;
+    async fn ping(self, _: Context) {}
-
-    fn ping(self, _: Context) -> Self::PingFut {
-        future::ready(())
-    }
 }
 
 #[tokio::main]
-async fn main() -> std::io::Result<()> {
+async fn main() -> anyhow::Result<()> {
     let bind_addr = "/tmp/tarpc_on_unix_example.sock";
 
     let _ = std::fs::remove_file(bind_addr);
@@ -44,7 +39,9 @@ async fn main() -> std::io::Result<()> {
     let conn = UnixStream::connect(bind_addr).await?;
     let transport = transport::new(codec_builder.new_framed(conn), Bincode::default());
     PingServiceClient::new(Default::default(), transport)
-        .spawn()?
+        .spawn()
         .ping(tarpc::context::current())
-        .await
+        .await?;
+
+    Ok(())
 }

tarpc/examples/pubsub.rs

@@ -38,10 +38,11 @@ use futures::{
     future::{self, AbortHandle},
     prelude::*,
 };
-use log::info;
 use publisher::Publisher as _;
 use std::{
     collections::HashMap,
+    env,
+    error::Error,
     io,
     net::SocketAddr,
     sync::{Arc, Mutex, RwLock},
@@ -54,6 +55,8 @@ use tarpc::{
 };
 use tokio::net::ToSocketAddrs;
 use tokio_serde::formats::Json;
+use tracing::info;
+use tracing_subscriber::prelude::*;
 
 pub mod subscriber {
     #[tarpc::service]
@@ -83,10 +86,7 @@ impl subscriber::Subscriber for Subscriber {
     }
 
     async fn receive(self, _: context::Context, topic: String, message: String) {
-        info!(
+        info!(local_addr = %self.local_addr, %topic, %message, "ReceivedMessage")
-            "[{}] received message on topic '{}': {}",
-            self.local_addr, topic, message
-        );
     }
 }
 
@@ -120,7 +120,7 @@ impl Subscriber {
         let (handler, abort_handle) = future::abortable(handler.execute(subscriber.serve()));
         tokio::spawn(async move {
             match handler.await {
-                Ok(()) | Err(future::Aborted) => info!("[{}] subscriber shutdown.", local_addr),
+                Ok(()) | Err(future::Aborted) => info!(?local_addr, "subscriber shutdown."),
             }
         });
         Ok(SubscriberHandle(abort_handle))
@@ -153,13 +153,13 @@ impl Publisher {
             subscriptions: self.clone().start_subscription_manager().await?,
         };
 
-        info!("[{}] listening for publishers.", publisher_addrs.publisher);
+        info!(publisher_addr = %publisher_addrs.publisher, "listening for publishers.",);
         tokio::spawn(async move {
             // Because this is just an example, we know there will only be one publisher. In more
             // realistic code, this would be a loop to continually accept new publisher
             // connections.
             let publisher = connecting_publishers.next().await.unwrap().unwrap();
-            info!("[{}] publisher connected.", publisher.peer_addr().unwrap());
+            info!(publisher.peer_addr = ?publisher.peer_addr(), "publisher connected.");
 
             server::BaseChannel::with_defaults(publisher)
                 .execute(self.serve())
@@ -174,7 +174,7 @@ impl Publisher {
             .await?
             .filter_map(|r| future::ready(r.ok()));
         let new_subscriber_addr = connecting_subscribers.get_ref().local_addr();
-        info!("[{}] listening for subscribers.", new_subscriber_addr);
+        info!(?new_subscriber_addr, "listening for subscribers.");
 
         tokio::spawn(async move {
             while let Some(conn) = connecting_subscribers.next().await {
@@ -215,7 +215,7 @@ impl Publisher {
                 },
             );
 
-            info!("[{}] subscribed to topics: {:?}", subscriber_addr, topics);
+            info!(%subscriber_addr, ?topics, "subscribed to new topics");
             let mut subscriptions = self.subscriptions.write().unwrap();
             for topic in topics {
                 subscriptions
@@ -226,18 +226,18 @@ impl Publisher {
         }
     }
 
-    fn start_subscriber_gc(
+    fn start_subscriber_gc<E: Error>(
         self,
         subscriber_addr: SocketAddr,
-        client_dispatch: impl Future<Output = anyhow::Result<()>> + Send + 'static,
+        client_dispatch: impl Future<Output = Result<(), E>> + Send + 'static,
         subscriber_ready: oneshot::Receiver<()>,
     ) {
         tokio::spawn(async move {
             if let Err(e) = client_dispatch.await {
                 info!(
-                    "[{}] subscriber connection broken: {:?}",
+                    %subscriber_addr,
-                    subscriber_addr, e
+                    error = %e,
-                )
+                    "subscriber connection broken");
             }
             // Don't clean up the subscriber until initialization is done.
             let _ = subscriber_ready.await;
@@ -281,13 +281,29 @@ impl publisher::Publisher for Publisher {
     }
 }
 
+/// Initializes an OpenTelemetry tracing subscriber with a Jaeger backend.
+fn init_tracing(service_name: &str) -> anyhow::Result<()> {
+    env::set_var("OTEL_BSP_MAX_EXPORT_BATCH_SIZE", "12");
+    let tracer = opentelemetry_jaeger::new_pipeline()
+        .with_service_name(service_name)
+        .with_max_packet_size(2usize.pow(13))
+        .install_batch(opentelemetry::runtime::Tokio)?;
+
+    tracing_subscriber::registry()
+        .with(tracing_subscriber::EnvFilter::from_default_env())
+        .with(tracing_subscriber::fmt::layer())
+        .with(tracing_opentelemetry::layer().with_tracer(tracer))
+        .try_init()?;
+
+    Ok(())
+}
+
 #[tokio::main]
 async fn main() -> anyhow::Result<()> {
-    env_logger::init();
+    init_tracing("Pub/Sub")?;
 
-    let clients = Arc::new(Mutex::new(HashMap::new()));
     let addrs = Publisher {
-        clients,
+        clients: Arc::new(Mutex::new(HashMap::new())),
         subscriptions: Arc::new(RwLock::new(HashMap::new())),
     }
     .start()
@@ -309,7 +325,7 @@ async fn main() -> anyhow::Result<()> {
         client::Config::default(),
         tcp::connect(addrs.publisher, Json::default).await?,
     )
-    .spawn()?;
+    .spawn();
 
     publisher
         .publish(context::current(), "calculus".into(), "sqrt(2)".into())
@@ -337,6 +353,7 @@ async fn main() -> anyhow::Result<()> {
         )
         .await?;
 
+    opentelemetry::global::shutdown_tracer_provider();
     info!("done.");
 
     Ok(())

tarpc/examples/readme.rs

@@ -5,7 +5,6 @@
 // https://opensource.org/licenses/MIT.
 
 use futures::future::{self, Ready};
-use std::io;
 use tarpc::{
     client, context,
     server::{self, Channel},
@@ -35,7 +34,7 @@ impl World for HelloServer {
 }
 
 #[tokio::main]
-async fn main() -> io::Result<()> {
+async fn main() -> anyhow::Result<()> {
     let (client_transport, server_transport) = tarpc::transport::channel::unbounded();
 
     let server = server::BaseChannel::with_defaults(server_transport);
@@ -43,7 +42,7 @@ async fn main() -> io::Result<()> {
 
     // WorldClient is generated by the #[tarpc::service] attribute. It has a constructor `new`
     // that takes a config and any Transport as input.
-    let client = WorldClient::new(client::Config::default(), client_transport).spawn()?;
+    let client = WorldClient::new(client::Config::default(), client_transport).spawn();
 
     // The client has an RPC method for each RPC defined in the annotated trait. It takes the same
     // args as defined, with the addition of a Context, which is always the first arg. The Context

tarpc/examples/tracing.rs

@@ -6,12 +6,13 @@
 
 use crate::{add::Add as AddService, double::Double as DoubleService};
 use futures::{future, prelude::*};
-use std::io;
+use std::env;
 use tarpc::{
     client, context,
-    server::{BaseChannel, Incoming},
+    server::{incoming::Incoming, BaseChannel},
 };
 use tokio_serde::formats::Json;
+use tracing_subscriber::prelude::*;
 
 pub mod add {
     #[tarpc::service]
@@ -54,9 +55,25 @@ impl DoubleService for DoubleServer {
     }
 }
 
+fn init_tracing(service_name: &str) -> anyhow::Result<()> {
+    env::set_var("OTEL_BSP_MAX_EXPORT_BATCH_SIZE", "12");
+    let tracer = opentelemetry_jaeger::new_pipeline()
+        .with_service_name(service_name)
+        .with_max_packet_size(2usize.pow(13))
+        .install_batch(opentelemetry::runtime::Tokio)?;
+
+    tracing_subscriber::registry()
+        .with(tracing_subscriber::EnvFilter::from_default_env())
+        .with(tracing_subscriber::fmt::layer())
+        .with(tracing_opentelemetry::layer().with_tracer(tracer))
+        .try_init()?;
+
+    Ok(())
+}
+
 #[tokio::main]
-async fn main() -> io::Result<()> {
+async fn main() -> anyhow::Result<()> {
-    env_logger::init();
+    init_tracing("tarpc_tracing_example")?;
 
     let add_listener = tarpc::serde_transport::tcp::listen("localhost:0", Json::default)
         .await?
@@ -69,7 +86,7 @@ async fn main() -> io::Result<()> {
     tokio::spawn(add_server);
 
     let to_add_server = tarpc::serde_transport::tcp::connect(addr, Json::default).await?;
-    let add_client = add::AddClient::new(client::Config::default(), to_add_server).spawn()?;
+    let add_client = add::AddClient::new(client::Config::default(), to_add_server).spawn();
 
     let double_listener = tarpc::serde_transport::tcp::listen("localhost:0", Json::default)
         .await?
@@ -83,10 +100,14 @@ async fn main() -> io::Result<()> {
 
     let to_double_server = tarpc::serde_transport::tcp::connect(addr, Json::default).await?;
     let double_client =
-        double::DoubleClient::new(client::Config::default(), to_double_server).spawn()?;
+        double::DoubleClient::new(client::Config::default(), to_double_server).spawn();
 
-    for i in 1..=5 {
+    let ctx = context::current();
-        eprintln!("{:?}", double_client.double(context::current(), i).await?);
+    for _ in 1..=5 {
+        tracing::info!("{:?}", double_client.double(ctx, 1).await?);
     }
+
+    opentelemetry::global::shutdown_tracer_provider();
+
     Ok(())
 }

tarpc/src/client.rs

@@ -8,16 +8,14 @@
 
 mod in_flight_requests;
 
-use crate::{
+use crate::{context, trace, ClientMessage, Request, Response, ServerError, Transport};
-    context, trace::SpanId, ClientMessage, PollContext, PollIo, Request, Response, Transport,
-};
 use futures::{prelude::*, ready, stream::Fuse, task::*};
-use in_flight_requests::InFlightRequests;
+use in_flight_requests::{DeadlineExceededError, InFlightRequests};
-use log::{info, trace};
 use pin_project::pin_project;
 use std::{
     convert::TryFrom,
-    fmt, io,
+    error::Error,
+    fmt, mem,
     pin::Pin,
     sync::{
         atomic::{AtomicUsize, Ordering},
@@ -25,6 +23,7 @@ use std::{
     },
 };
 use tokio::sync::{mpsc, oneshot};
+use tracing::Span;
 
 /// Settings that control the behavior of the client.
 #[derive(Clone, Debug)]
@@ -61,19 +60,18 @@ pub struct NewClient<C, D> {
 impl<C, D, E> NewClient<C, D>
 where
     D: Future<Output = Result<(), E>> + Send + 'static,
-    E: std::fmt::Display,
+    E: std::error::Error + Send + Sync + 'static,
 {
     /// Helper method to spawn the dispatch on the default executor.
     #[cfg(feature = "tokio1")]
     #[cfg_attr(docsrs, doc(cfg(feature = "tokio1")))]
-    pub fn spawn(self) -> io::Result<C> {
+    pub fn spawn(self) -> C {
-        use log::warn;
+        let dispatch = self.dispatch.unwrap_or_else(move |e| {
-
+            let e = anyhow::Error::new(e);
-        let dispatch = self
+            tracing::warn!("Connection broken: {:?}", e);
-            .dispatch
+        });
-            .unwrap_or_else(move |e| warn!("Connection broken: {}", e));
         tokio::spawn(dispatch);
-        Ok(self.client)
+        self.client
     }
 }
 
@@ -114,100 +112,117 @@ impl<Req, Resp> Clone for Channel<Req, Resp> {
 
 impl<Req, Resp> Channel<Req, Resp> {
     /// Sends a request to the dispatch task to forward to the server, returning a [`Future`] that
-    /// resolves when the request is sent (not when the response is received).
+    /// resolves to the response.
-    fn send(
+    #[tracing::instrument(
+        name = "RPC",
+        skip(self, ctx, request_name, request),
+        fields(
+            rpc.trace_id = tracing::field::Empty,
+            otel.kind = "client",
+            otel.name = request_name)
+        )]
+    pub async fn call(
         &self,
         mut ctx: context::Context,
+        request_name: &str,
         request: Req,
-    ) -> impl Future<Output = io::Result<DispatchResponse<Resp>>> + '_ {
+    ) -> Result<Resp, RpcError> {
-        // Convert the context to the call context.
+        let span = Span::current();
-        ctx.trace_context.parent_id = Some(ctx.trace_context.span_id);
+        ctx.trace_context = trace::Context::try_from(&span).unwrap_or_else(|_| {
-        ctx.trace_context.span_id = SpanId::random(&mut rand::thread_rng());
+            tracing::warn!(
-
+                "OpenTelemetry subscriber not installed; making unsampled child context."
-        let (response_completion, response) = oneshot::channel();
+            );
-        let cancellation = self.cancellation.clone();
+            ctx.trace_context.new_child()
+        });
+        span.record("rpc.trace_id", &tracing::field::display(ctx.trace_id()));
+        let (response_completion, mut response) = oneshot::channel();
         let request_id =
             u64::try_from(self.next_request_id.fetch_add(1, Ordering::Relaxed)).unwrap();
 
-        // DispatchResponse impls Drop to cancel in-flight requests. It should be created before
+        // ResponseGuard impls Drop to cancel in-flight requests. It should be created before
         // sending out the request; otherwise, the response future could be dropped after the
-        // request is sent out but before DispatchResponse is created, rendering the cancellation
+        // request is sent out but before ResponseGuard is created, rendering the cancellation
         // logic inactive.
-        let response = DispatchResponse {
+        let response_guard = ResponseGuard {
-            response,
+            response: &mut response,
             request_id,
-            cancellation: Some(cancellation),
+            cancellation: &self.cancellation,
-            ctx,
         };
-        async move {
+        self.to_dispatch
-            self.to_dispatch
+            .send(DispatchRequest {
-                .send(DispatchRequest {
+                ctx,
-                    ctx,
+                span,
-                    request_id,
+                request_id,
-                    request,
+                request,
-                    response_completion,
+                response_completion,
-                })
+            })
-                .await
+            .await
-                .map_err(|mpsc::error::SendError(_)| {
+            .map_err(|mpsc::error::SendError(_)| RpcError::Disconnected)?;
-                    io::Error::from(io::ErrorKind::ConnectionReset)
+        response_guard.response().await
-                })?;
-            Ok(response)
-        }
-    }
-
-    /// Sends a request to the dispatch task to forward to the server, returning a [`Future`] that
-    /// resolves to the response.
-    pub async fn call(&self, ctx: context::Context, request: Req) -> io::Result<Resp> {
-        let dispatch_response = self.send(ctx, request).await?;
-        dispatch_response.await
     }
 }
 
 /// A server response that is completed by request dispatch when the corresponding response
 /// arrives off the wire.
-#[derive(Debug)]
+struct ResponseGuard<'a, Resp> {
-struct DispatchResponse<Resp> {
+    response: &'a mut oneshot::Receiver<Result<Response<Resp>, DeadlineExceededError>>,
-    response: oneshot::Receiver<Response<Resp>>,
+    cancellation: &'a RequestCancellation,
-    ctx: context::Context,
-    cancellation: Option<RequestCancellation>,
     request_id: u64,
 }
 
-impl<Resp> Future for DispatchResponse<Resp> {
+/// An error that can occur in the processing of an RPC. This is not request-specific errors but
-    type Output = io::Result<Resp>;
+/// rather cross-cutting errors that can always occur.
+#[derive(thiserror::Error, Debug)]
+pub enum RpcError {
+    /// The client disconnected from the server.
+    #[error("the client disconnected from the server")]
+    Disconnected,
+    /// The request exceeded its deadline.
+    #[error("the request exceeded its deadline")]
+    DeadlineExceeded,
+    /// The server aborted request processing.
+    #[error("the server aborted request processing")]
+    Server(#[from] ServerError),
+}
 
-    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<Resp>> {
+impl From<DeadlineExceededError> for RpcError {
-        let resp = ready!(self.response.poll_unpin(cx));
+    fn from(_: DeadlineExceededError) -> Self {
-        self.cancellation.take();
+        RpcError::DeadlineExceeded
-        Poll::Ready(match resp {
+    }
-            Ok(resp) => Ok(resp.message?),
+}
+
+impl<Resp> ResponseGuard<'_, Resp> {
+    async fn response(mut self) -> Result<Resp, RpcError> {
+        let response = (&mut self.response).await;
+        // Cancel drop logic once a response has been received.
+        mem::forget(self);
+        match response {
+            Ok(resp) => Ok(resp?.message?),
             Err(oneshot::error::RecvError { .. }) => {
                 // The oneshot is Canceled when the dispatch task ends. In that case,
                 // there's nothing listening on the other side, so there's no point in
                 // propagating cancellation.
-                Err(io::Error::from(io::ErrorKind::ConnectionReset))
+                Err(RpcError::Disconnected)
             }
-        })
+        }
     }
 }
 
 // Cancels the request when dropped, if not already complete.
-impl<Resp> Drop for DispatchResponse<Resp> {
+impl<Resp> Drop for ResponseGuard<'_, Resp> {
     fn drop(&mut self) {
-        if let Some(cancellation) = &mut self.cancellation {
+        // The receiver needs to be closed to handle the edge case that the request has not
-            // The receiver needs to be closed to handle the edge case that the request has not
+        // yet been received by the dispatch task. It is possible for the cancel message to
-            // yet been received by the dispatch task. It is possible for the cancel message to
+        // arrive before the request itself, in which case the request could get stuck in the
-            // arrive before the request itself, in which case the request could get stuck in the
+        // dispatch map forever if the server never responds (e.g. if the server dies while
-            // dispatch map forever if the server never responds (e.g. if the server dies while
+        // responding). Even if the server does respond, it will have unnecessarily done work
-            // responding). Even if the server does respond, it will have unnecessarily done work
+        // for a client no longer waiting for a response. To avoid this, the dispatch task
-            // for a client no longer waiting for a response. To avoid this, the dispatch task
+        // checks if the receiver is closed before inserting the request in the map. By
-            // checks if the receiver is closed before inserting the request in the map. By
+        // closing the receiver before sending the cancel message, it is guaranteed that if the
-            // closing the receiver before sending the cancel message, it is guaranteed that if the
+        // dispatch task misses an early-arriving cancellation message, then it will see the
-            // dispatch task misses an early-arriving cancellation message, then it will see the
+        // receiver as closed.
-            // receiver as closed.
+        self.response.close();
-            self.response.close();
+        self.cancellation.cancel(self.request_id);
-            cancellation.cancel(self.request_id);
-        }
     }
 }
 
@@ -258,6 +273,32 @@ pub struct RequestDispatch<Req, Resp, C> {
     config: Config,
 }
 
+/// Critical errors that result in a Channel disconnecting.
+#[derive(thiserror::Error, Debug)]
+pub enum ChannelError<E>
+where
+    E: Error + Send + Sync + 'static,
+{
+    /// Could not read from the transport.
+    #[error("could not read from the transport")]
+    Read(#[source] E),
+    /// Could not ready the transport for writes.
+    #[error("could not ready the transport for writes")]
+    Ready(#[source] E),
+    /// Could not write to the transport.
+    #[error("could not write to the transport")]
+    Write(#[source] E),
+    /// Could not flush the transport.
+    #[error("could not flush the transport")]
+    Flush(#[source] E),
+    /// Could not close the write end of the transport.
+    #[error("could not close the write end of the transport")]
+    Close(#[source] E),
+    /// Could not poll expired requests.
+    #[error("could not poll expired requests")]
+    Timer(#[source] tokio::time::error::Error),
+}
+
 impl<Req, Resp, C> RequestDispatch<Req, Resp, C>
 where
     C: Transport<ClientMessage<Req>, Response<Resp>>,
@@ -270,6 +311,42 @@ where
         self.as_mut().project().transport
     }
 
+    fn poll_ready<'a>(
+        self: &'a mut Pin<&mut Self>,
+        cx: &mut Context<'_>,
+    ) -> Poll<Result<(), ChannelError<C::Error>>> {
+        self.transport_pin_mut()
+            .poll_ready(cx)
+            .map_err(ChannelError::Ready)
+    }
+
+    fn start_send(
+        self: &mut Pin<&mut Self>,
+        message: ClientMessage<Req>,
+    ) -> Result<(), ChannelError<C::Error>> {
+        self.transport_pin_mut()
+            .start_send(message)
+            .map_err(ChannelError::Write)
+    }
+
+    fn poll_flush<'a>(
+        self: &'a mut Pin<&mut Self>,
+        cx: &mut Context<'_>,
+    ) -> Poll<Result<(), ChannelError<C::Error>>> {
+        self.transport_pin_mut()
+            .poll_flush(cx)
+            .map_err(ChannelError::Flush)
+    }
+
+    fn poll_close<'a>(
+        self: &'a mut Pin<&mut Self>,
+        cx: &mut Context<'_>,
+    ) -> Poll<Result<(), ChannelError<C::Error>>> {
+        self.transport_pin_mut()
+            .poll_close(cx)
+            .map_err(ChannelError::Close)
+    }
+
     fn canceled_requests_mut<'a>(self: &'a mut Pin<&mut Self>) -> &'a mut CanceledRequests {
         self.as_mut().project().canceled_requests
     }
@@ -280,17 +357,22 @@ where
         self.as_mut().project().pending_requests
     }
 
-    fn pump_read(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> PollIo<()> {
+    fn pump_read(
-        Poll::Ready(match ready!(self.transport_pin_mut().poll_next(cx)?) {
+        mut self: Pin<&mut Self>,
-            Some(response) => {
+        cx: &mut Context<'_>,
+    ) -> Poll<Option<Result<(), ChannelError<C::Error>>>> {
+        self.transport_pin_mut()
+            .poll_next(cx)
+            .map_err(ChannelError::Read)
+            .map_ok(|response| {
                 self.complete(response);
-                Some(Ok(()))
+            })
-            }
-            None => None,
-        })
     }
 
-    fn pump_write(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> PollIo<()> {
+    fn pump_write(
+        mut self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+    ) -> Poll<Option<Result<(), ChannelError<C::Error>>>> {
         enum ReceiverStatus {
             Pending,
             Closed,
@@ -311,7 +393,11 @@ where
         // Receiving Poll::Ready(None) when polling expired requests never indicates "Closed",
         // because there can temporarily be zero in-flight rquests. Therefore, there is no need to
         // track the status like is done with pending and cancelled requests.
-        if let Poll::Ready(Some(_)) = self.in_flight_requests().poll_expired(cx)? {
+        if let Poll::Ready(Some(_)) = self
+            .in_flight_requests()
+            .poll_expired(cx)
+            .map_err(ChannelError::Timer)?
+        {
             // Expired requests are considered complete; there is no compelling reason to send a
             // cancellation message to the server, since it will have already exhausted its
             // allotted processing time.
@@ -320,12 +406,12 @@ where
 
         match (pending_requests_status, canceled_requests_status) {
             (ReceiverStatus::Closed, ReceiverStatus::Closed) => {
-                ready!(self.transport_pin_mut().poll_flush(cx)?);
+                ready!(self.poll_close(cx)?);
                 Poll::Ready(None)
             }
             (ReceiverStatus::Pending, _) | (_, ReceiverStatus::Pending) => {
                 // No more messages to process, so flush any messages buffered in the transport.
-                ready!(self.transport_pin_mut().poll_flush(cx)?);
+                ready!(self.poll_flush(cx)?);
 
                 // Even if we fully-flush, we return Pending, because we have no more requests
                 // or cancellations right now.
@@ -341,9 +427,9 @@ where
     fn poll_next_request(
         mut self: Pin<&mut Self>,
         cx: &mut Context<'_>,
-    ) -> PollIo<DispatchRequest<Req, Resp>> {
+    ) -> Poll<Option<Result<DispatchRequest<Req, Resp>, ChannelError<C::Error>>>> {
         if self.in_flight_requests().len() >= self.config.max_in_flight_requests {
-            info!(
+            tracing::info!(
                 "At in-flight request capacity ({}/{}).",
                 self.in_flight_requests().len(),
                 self.config.max_in_flight_requests
@@ -360,10 +446,8 @@ where
             match ready!(self.pending_requests_mut().poll_recv(cx)) {
                 Some(request) => {
                     if request.response_completion.is_closed() {
-                        trace!(
+                        let _entered = request.span.enter();
-                            "[{}] Request canceled before being sent.",
+                        tracing::info!("AbortRequest");
-                            request.ctx.trace_id()
-                        );
                         continue;
                     }
 
@@ -381,14 +465,15 @@ where
     fn poll_next_cancellation(
         mut self: Pin<&mut Self>,
         cx: &mut Context<'_>,
-    ) -> PollIo<(context::Context, u64)> {
+    ) -> Poll<Option<Result<(context::Context, Span, u64), ChannelError<C::Error>>>> {
         ready!(self.ensure_writeable(cx)?);
 
         loop {
             match ready!(self.canceled_requests_mut().poll_next_unpin(cx)) {
                 Some(request_id) => {
-                    if let Some(ctx) = self.in_flight_requests().cancel_request(request_id) {
+                    if let Some((ctx, span)) = self.in_flight_requests().cancel_request(request_id)
-                        return Poll::Ready(Some(Ok((ctx, request_id))));
+                    {
+                        return Poll::Ready(Some(Ok((ctx, span, request_id))));
                     }
                 }
                 None => return Poll::Ready(None),
@@ -399,54 +484,73 @@ where
     /// Returns Ready if writing a message to the transport (i.e. via write_request or
     /// write_cancel) would not fail due to a full buffer. If the transport is not ready to be
     /// written to, flushes it until it is ready.
-    fn ensure_writeable<'a>(self: &'a mut Pin<&mut Self>, cx: &mut Context<'_>) -> PollIo<()> {
+    fn ensure_writeable<'a>(
-        while self.transport_pin_mut().poll_ready(cx)?.is_pending() {
+        self: &'a mut Pin<&mut Self>,
-            ready!(self.transport_pin_mut().poll_flush(cx)?);
+        cx: &mut Context<'_>,
+    ) -> Poll<Option<Result<(), ChannelError<C::Error>>>> {
+        while self.poll_ready(cx)?.is_pending() {
+            ready!(self.poll_flush(cx)?);
         }
         Poll::Ready(Some(Ok(())))
     }
 
-    fn poll_write_request<'a>(self: &'a mut Pin<&mut Self>, cx: &mut Context<'_>) -> PollIo<()> {
+    fn poll_write_request<'a>(
-        let dispatch_request = match ready!(self.as_mut().poll_next_request(cx)?) {
+        self: &'a mut Pin<&mut Self>,
+        cx: &mut Context<'_>,
+    ) -> Poll<Option<Result<(), ChannelError<C::Error>>>> {
+        let DispatchRequest {
+            ctx,
+            span,
+            request_id,
+            request,
+            response_completion,
+        } = match ready!(self.as_mut().poll_next_request(cx)?) {
             Some(dispatch_request) => dispatch_request,
             None => return Poll::Ready(None),
         };
+        let entered = span.enter();
         // poll_next_request only returns Ready if there is room to buffer another request.
         // Therefore, we can call write_request without fear of erroring due to a full
         // buffer.
-        let request_id = dispatch_request.request_id;
+        let request_id = request_id;
         let request = ClientMessage::Request(Request {
             id: request_id,
-            message: dispatch_request.request,
+            message: request,
             context: context::Context {
-                deadline: dispatch_request.ctx.deadline,
+                deadline: ctx.deadline,
-                trace_context: dispatch_request.ctx.trace_context,
+                trace_context: ctx.trace_context,
             },
         });
-        self.transport_pin_mut().start_send(request)?;
+        self.start_send(request)?;
+        let deadline = ctx.deadline;
+        tracing::info!(
+            tarpc.deadline = %humantime::format_rfc3339(deadline),
+            "SendRequest"
+        );
+        drop(entered);
+
         self.in_flight_requests()
-            .insert_request(
+            .insert_request(request_id, ctx, span, response_completion)
-                request_id,
-                dispatch_request.ctx,
-                dispatch_request.response_completion,
-            )
             .expect("Request IDs should be unique");
         Poll::Ready(Some(Ok(())))
     }
 
-    fn poll_write_cancel<'a>(self: &'a mut Pin<&mut Self>, cx: &mut Context<'_>) -> PollIo<()> {
+    fn poll_write_cancel<'a>(
-        let (context, request_id) = match ready!(self.as_mut().poll_next_cancellation(cx)?) {
+        self: &'a mut Pin<&mut Self>,
-            Some((context, request_id)) => (context, request_id),
+        cx: &mut Context<'_>,
+    ) -> Poll<Option<Result<(), ChannelError<C::Error>>>> {
+        let (context, span, request_id) = match ready!(self.as_mut().poll_next_cancellation(cx)?) {
+            Some(triple) => triple,
             None => return Poll::Ready(None),
         };
+        let _entered = span.enter();
 
-        let trace_id = *context.trace_id();
         let cancel = ClientMessage::Cancel {
             trace_context: context.trace_context,
             request_id,
         };
-        self.transport_pin_mut().start_send(cancel)?;
+        self.start_send(cancel)?;
-        trace!("[{}] Cancel message sent.", trace_id);
+        tracing::info!("CancelRequest");
         Poll::Ready(Some(Ok(())))
     }
 
@@ -460,28 +564,24 @@ impl<Req, Resp, C> Future for RequestDispatch<Req, Resp, C>
 where
     C: Transport<ClientMessage<Req>, Response<Resp>>,
 {
-    type Output = anyhow::Result<()>;
+    type Output = Result<(), ChannelError<C::Error>>;
 
-    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<anyhow::Result<()>> {
+    fn poll(
+        mut self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+    ) -> Poll<Result<(), ChannelError<C::Error>>> {
         loop {
-            match (
+            match (self.as_mut().pump_read(cx)?, self.as_mut().pump_write(cx)?) {
-                self.as_mut()
-                    .pump_read(cx)
-                    .context("failed to read from transport")?,
-                self.as_mut()
-                    .pump_write(cx)
-                    .context("failed to write to transport")?,
-            ) {
                 (Poll::Ready(None), _) => {
-                    info!("Shutdown: read half closed, so shutting down.");
+                    tracing::info!("Shutdown: read half closed, so shutting down.");
                     return Poll::Ready(Ok(()));
                 }
                 (read, Poll::Ready(None)) => {
-                    if self.in_flight_requests().is_empty() {
+                    if self.in_flight_requests.is_empty() {
-                        info!("Shutdown: write half closed, and no requests in flight.");
+                        tracing::info!("Shutdown: write half closed, and no requests in flight.");
                         return Poll::Ready(Ok(()));
                     }
-                    info!(
+                    tracing::info!(
                         "Shutdown: write half closed, and {} requests in flight.",
                         self.in_flight_requests().len()
                     );
@@ -502,9 +602,10 @@ where
 #[derive(Debug)]
 struct DispatchRequest<Req, Resp> {
     pub ctx: context::Context,
+    pub span: Span,
     pub request_id: u64,
     pub request: Req,
-    pub response_completion: oneshot::Sender<Response<Resp>>,
+    pub response_completion: oneshot::Sender<Result<Response<Resp>, DeadlineExceededError>>,
 }
 
 /// Sends request cancellation signals.
@@ -518,16 +619,14 @@ struct CanceledRequests(mpsc::UnboundedReceiver<u64>);
 /// Returns a channel to send request cancellation messages.
 fn cancellations() -> (RequestCancellation, CanceledRequests) {
     // Unbounded because messages are sent in the drop fn. This is fine, because it's still
-    // bounded by the number of in-flight requests. Additionally, each request has a clone
+    // bounded by the number of in-flight requests.
-    // of the sender, so the bounded channel would have the same behavior,
-    // since it guarantees a slot.
     let (tx, rx) = mpsc::unbounded_channel();
     (RequestCancellation(tx), CanceledRequests(rx))
 }
 
 impl RequestCancellation {
     /// Cancels the request with ID `request_id`.
-    fn cancel(&mut self, request_id: u64) {
+    fn cancel(&self, request_id: u64) {
         let _ = self.0.send(request_id);
     }
 }
@@ -549,28 +648,58 @@ impl Stream for CanceledRequests {
 #[cfg(test)]
 mod tests {
     use super::{
-        cancellations, CanceledRequests, Channel, DispatchResponse, RequestCancellation,
+        cancellations, CanceledRequests, Channel, DispatchRequest, RequestCancellation,
-        RequestDispatch,
+        RequestDispatch, ResponseGuard,
     };
     use crate::{
-        client::{in_flight_requests::InFlightRequests, Config},
+        client::{
+            in_flight_requests::{DeadlineExceededError, InFlightRequests},
+            Config,
+        },
         context,
         transport::{self, channel::UnboundedChannel},
         ClientMessage, Response,
     };
+    use assert_matches::assert_matches;
     use futures::{prelude::*, task::*};
-    use std::{pin::Pin, sync::atomic::AtomicUsize, sync::Arc};
+    use std::{
+        convert::TryFrom,
+        pin::Pin,
+        sync::atomic::{AtomicUsize, Ordering},
+        sync::Arc,
+    };
     use tokio::sync::{mpsc, oneshot};
+    use tracing::Span;
+
+    #[tokio::test]
+    async fn response_completes_request_future() {
+        let (mut dispatch, mut _channel, mut server_channel) = set_up();
+        let cx = &mut Context::from_waker(&noop_waker_ref());
+        let (tx, mut rx) = oneshot::channel();
+
+        dispatch
+            .in_flight_requests
+            .insert_request(0, context::current(), Span::current(), tx)
+            .unwrap();
+        server_channel
+            .send(Response {
+                request_id: 0,
+                message: Ok("Resp".into()),
+            })
+            .await
+            .unwrap();
+        assert_matches!(dispatch.as_mut().poll(cx), Poll::Pending);
+        assert_matches!(rx.try_recv(), Ok(Ok(Response { request_id: 0, message: Ok(resp) })) if resp == "Resp");
+    }
 
     #[tokio::test]
     async fn dispatch_response_cancels_on_drop() {
         let (cancellation, mut canceled_requests) = cancellations();
-        let (_, response) = oneshot::channel();
+        let (_, mut response) = oneshot::channel();
-        drop(DispatchResponse::<u32> {
+        drop(ResponseGuard::<u32> {
-            response,
+            response: &mut response,
-            cancellation: Some(cancellation),
+            cancellation: &cancellation,
             request_id: 3,
-            ctx: context::current(),
         });
         // resp's drop() is run, which should send a cancel message.
         let cx = &mut Context::from_waker(&noop_waker_ref());
@@ -580,23 +709,22 @@ mod tests {
     #[tokio::test]
     async fn dispatch_response_doesnt_cancel_after_complete() {
         let (cancellation, mut canceled_requests) = cancellations();
-        let (tx, response) = oneshot::channel();
+        let (tx, mut response) = oneshot::channel();
-        tx.send(Response {
+        tx.send(Ok(Response {
             request_id: 0,
             message: Ok("well done"),
-        })
+        }))
         .unwrap();
-        {
+        // resp's drop() is run, but should not send a cancel message.
-            DispatchResponse {
+        ResponseGuard {
-                response,
+            response: &mut response,
-                cancellation: Some(cancellation),
+            cancellation: &cancellation,
-                request_id: 3,
+            request_id: 3,
-                ctx: context::current(),
-            }
-            .await
-            .unwrap();
-            // resp's drop() is run, but should not send a cancel message.
         }
+        .response()
+        .await
+        .unwrap();
+        drop(cancellation);
         let cx = &mut Context::from_waker(&noop_waker_ref());
         assert_eq!(canceled_requests.0.poll_recv(cx), Poll::Ready(None));
     }
@@ -604,12 +732,12 @@ mod tests {
     #[tokio::test]
     async fn stage_request() {
         let (mut dispatch, mut channel, _server_channel) = set_up();
-        let dispatch = Pin::new(&mut dispatch);
         let cx = &mut Context::from_waker(&noop_waker_ref());
+        let (tx, mut rx) = oneshot::channel();
 
-        let _resp = send_request(&mut channel, "hi").await;
+        let _resp = send_request(&mut channel, "hi", tx, &mut rx).await;
 
-        let req = dispatch.poll_next_request(cx).ready();
+        let req = dispatch.as_mut().poll_next_request(cx).ready();
         assert!(req.is_some());
 
         let req = req.unwrap();
@@ -621,10 +749,10 @@ mod tests {
     #[tokio::test]
     async fn stage_request_channel_dropped_doesnt_panic() {
         let (mut dispatch, mut channel, mut server_channel) = set_up();
-        let mut dispatch = Pin::new(&mut dispatch);
         let cx = &mut Context::from_waker(&noop_waker_ref());
+        let (tx, mut rx) = oneshot::channel();
 
-        let _ = send_request(&mut channel, "hi").await;
+        let _ = send_request(&mut channel, "hi", tx, &mut rx).await;
         drop(channel);
 
         assert!(dispatch.as_mut().poll(cx).is_ready());
@@ -642,61 +770,68 @@ mod tests {
     #[tokio::test]
     async fn stage_request_response_future_dropped_is_canceled_before_sending() {
         let (mut dispatch, mut channel, _server_channel) = set_up();
-        let dispatch = Pin::new(&mut dispatch);
         let cx = &mut Context::from_waker(&noop_waker_ref());
+        let (tx, mut rx) = oneshot::channel();
 
-        let _ = send_request(&mut channel, "hi").await;
+        let _ = send_request(&mut channel, "hi", tx, &mut rx).await;
 
         // Drop the channel so polling returns none if no requests are currently ready.
         drop(channel);
         // Test that a request future dropped before it's processed by dispatch will cause the request
         // to not be added to the in-flight request map.
-        assert!(dispatch.poll_next_request(cx).ready().is_none());
+        assert!(dispatch.as_mut().poll_next_request(cx).ready().is_none());
     }
 
     #[tokio::test]
     async fn stage_request_response_future_dropped_is_canceled_after_sending() {
         let (mut dispatch, mut channel, _server_channel) = set_up();
         let cx = &mut Context::from_waker(&noop_waker_ref());
-        let mut dispatch = Pin::new(&mut dispatch);
+        let (tx, mut rx) = oneshot::channel();
 
-        let req = send_request(&mut channel, "hi").await;
+        let req = send_request(&mut channel, "hi", tx, &mut rx).await;
 
         assert!(dispatch.as_mut().pump_write(cx).ready().is_some());
-        assert!(!dispatch.in_flight_requests().is_empty());
+        assert!(!dispatch.in_flight_requests.is_empty());
 
         // Test that a request future dropped after it's processed by dispatch will cause the request
         // to be removed from the in-flight request map.
         drop(req);
-        if let Poll::Ready(Some(_)) = dispatch.as_mut().poll_next_cancellation(cx).unwrap() {
+        assert_matches!(
-            // ok
+            dispatch.as_mut().poll_next_cancellation(cx),
-        } else {
+            Poll::Ready(Some(Ok(_)))
-            panic!("Expected request to be cancelled")
+        );
-        };
+        assert!(dispatch.in_flight_requests.is_empty());
-        assert!(dispatch.in_flight_requests().is_empty());
     }
 
     #[tokio::test]
     async fn stage_request_response_closed_skipped() {
         let (mut dispatch, mut channel, _server_channel) = set_up();
-        let dispatch = Pin::new(&mut dispatch);
         let cx = &mut Context::from_waker(&noop_waker_ref());
+        let (tx, mut rx) = oneshot::channel();
 
         // Test that a request future that's closed its receiver but not yet canceled its request --
         // i.e. still in `drop fn` -- will cause the request to not be added to the in-flight request
         // map.
-        let mut resp = send_request(&mut channel, "hi").await;
+        let resp = send_request(&mut channel, "hi", tx, &mut rx).await;
         resp.response.close();
 
-        assert!(dispatch.poll_next_request(cx).is_pending());
+        assert!(dispatch.as_mut().poll_next_request(cx).is_pending());
     }
 
     fn set_up() -> (
-        RequestDispatch<String, String, UnboundedChannel<Response<String>, ClientMessage<String>>>,
+        Pin<
+            Box<
+                RequestDispatch<
+                    String,
+                    String,
+                    UnboundedChannel<Response<String>, ClientMessage<String>>,
+                >,
+            >,
+        >,
         Channel<String, String>,
         UnboundedChannel<ClientMessage<String>, Response<String>>,
     ) {
-        let _ = env_logger::try_init();
+        let _ = tracing_subscriber::fmt().with_test_writer().try_init();
 
         let (to_dispatch, pending_requests) = mpsc::channel(1);
         let (cancel_tx, canceled_requests) = mpsc::unbounded_channel();
@@ -717,17 +852,31 @@ mod tests {
             next_request_id: Arc::new(AtomicUsize::new(0)),
         };
 
-        (dispatch, channel, server_channel)
+        (Box::pin(dispatch), channel, server_channel)
     }
 
-    async fn send_request(
+    async fn send_request<'a>(
-        channel: &mut Channel<String, String>,
+        channel: &'a mut Channel<String, String>,
         request: &str,
-    ) -> DispatchResponse<String> {
+        response_completion: oneshot::Sender<Result<Response<String>, DeadlineExceededError>>,
-        channel
+        response: &'a mut oneshot::Receiver<Result<Response<String>, DeadlineExceededError>>,
-            .send(context::current(), request.to_string())
+    ) -> ResponseGuard<'a, String> {
-            .await
+        let request_id =
-            .unwrap()
+            u64::try_from(channel.next_request_id.fetch_add(1, Ordering::Relaxed)).unwrap();
+        let request = DispatchRequest {
+            ctx: context::current(),
+            span: Span::current(),
+            request_id,
+            request: request.to_string(),
+            response_completion,
+        };
+        channel.to_dispatch.send(request).await.unwrap();
+
+        ResponseGuard {
+            response,
+            cancellation: &channel.cancellation,
+            request_id,
+        }
     }
 
     async fn send_response(

tarpc/src/client/in_flight_requests.rs

@@ -1,18 +1,16 @@
 use crate::{
     context,
     util::{Compact, TimeUntil},
-    PollIo, Response, ServerError,
+    Response,
 };
 use fnv::FnvHashMap;
-use futures::ready;
-use log::{debug, trace};
 use std::{
     collections::hash_map,
-    io,
     task::{Context, Poll},
 };
 use tokio::sync::oneshot;
 use tokio_util::time::delay_queue::{self, DelayQueue};
+use tracing::Span;
 
 /// Requests already written to the wire that haven't yet received responses.
 #[derive(Debug)]
@@ -30,10 +28,17 @@ impl<Resp> Default for InFlightRequests<Resp> {
     }
 }
 
+/// The request exceeded its deadline.
+#[derive(thiserror::Error, Debug)]
+#[non_exhaustive]
+#[error("the request exceeded its deadline")]
+pub struct DeadlineExceededError;
+
 #[derive(Debug)]
 struct RequestData<Resp> {
     ctx: context::Context,
-    response_completion: oneshot::Sender<Response<Resp>>,
+    span: Span,
+    response_completion: oneshot::Sender<Result<Response<Resp>, DeadlineExceededError>>,
     /// The key to remove the timer for the request's deadline.
     deadline_key: delay_queue::Key,
 }
@@ -59,20 +64,16 @@ impl<Resp> InFlightRequests<Resp> {
         &mut self,
         request_id: u64,
         ctx: context::Context,
-        response_completion: oneshot::Sender<Response<Resp>>,
+        span: Span,
+        response_completion: oneshot::Sender<Result<Response<Resp>, DeadlineExceededError>>,
     ) -> Result<(), AlreadyExistsError> {
         match self.request_data.entry(request_id) {
             hash_map::Entry::Vacant(vacant) => {
                 let timeout = ctx.deadline.time_until();
-                trace!(
-                    "[{}] Queuing request with timeout {:?}.",
-                    ctx.trace_id(),
-                    timeout,
-                );
-
                 let deadline_key = self.deadlines.insert(request_id, timeout);
                 vacant.insert(RequestData {
                     ctx,
+                    span,
                     response_completion,
                     deadline_key,
                 });
@@ -85,15 +86,15 @@ impl<Resp> InFlightRequests<Resp> {
     /// Removes a request without aborting. Returns true iff the request was found.
     pub fn complete_request(&mut self, response: Response<Resp>) -> bool {
         if let Some(request_data) = self.request_data.remove(&response.request_id) {
+            let _entered = request_data.span.enter();
+            tracing::info!("ReceiveResponse");
             self.request_data.compact(0.1);
-
-            trace!("[{}] Received response.", request_data.ctx.trace_id());
             self.deadlines.remove(&request_data.deadline_key);
-            request_data.complete(response);
+            let _ = request_data.response_completion.send(Ok(response));
             return true;
         }
 
-        debug!(
+        tracing::debug!(
             "No in-flight request found for request_id = {}.",
             response.request_id
         );
@@ -104,12 +105,11 @@ impl<Resp> InFlightRequests<Resp> {
 
     /// Cancels a request without completing (typically used when a request handle was dropped
     /// before the request completed).
-    pub fn cancel_request(&mut self, request_id: u64) -> Option<context::Context> {
+    pub fn cancel_request(&mut self, request_id: u64) -> Option<(context::Context, Span)> {
         if let Some(request_data) = self.request_data.remove(&request_id) {
             self.request_data.compact(0.1);
-            trace!("[{}] Cancelling request.", request_data.ctx.trace_id());
             self.deadlines.remove(&request_data.deadline_key);
-            Some(request_data.ctx)
+            Some((request_data.ctx, request_data.span))
         } else {
             None
         }
@@ -117,46 +117,21 @@ impl<Resp> InFlightRequests<Resp> {
 
     /// Yields a request that has expired, completing it with a TimedOut error.
     /// The caller should send cancellation messages for any yielded request ID.
-    pub fn poll_expired(&mut self, cx: &mut Context) -> PollIo<u64> {
+    pub fn poll_expired(
-        Poll::Ready(match ready!(self.deadlines.poll_expired(cx)) {
+        &mut self,
-            Some(Ok(expired)) => {
+        cx: &mut Context,
-                let request_id = expired.into_inner();
+    ) -> Poll<Option<Result<u64, tokio::time::error::Error>>> {
-                if let Some(request_data) = self.request_data.remove(&request_id) {
+        self.deadlines.poll_expired(cx).map_ok(|expired| {
-                    self.request_data.compact(0.1);
+            let request_id = expired.into_inner();
-                    request_data.complete(Self::deadline_exceeded_error(request_id));
+            if let Some(request_data) = self.request_data.remove(&request_id) {
-                }
+                let _entered = request_data.span.enter();
-                Some(Ok(request_id))
+                tracing::error!("DeadlineExceeded");
+                self.request_data.compact(0.1);
+                let _ = request_data
+                    .response_completion
+                    .send(Err(DeadlineExceededError));
             }
-            Some(Err(e)) => Some(Err(io::Error::new(io::ErrorKind::Other, e))),
+            request_id
-            None => None,
         })
     }
-
-    fn deadline_exceeded_error(request_id: u64) -> Response<Resp> {
-        Response {
-            request_id,
-            message: Err(ServerError {
-                kind: io::ErrorKind::TimedOut,
-                detail: Some("Client dropped expired request.".to_string()),
-            }),
-        }
-    }
-}
-
-/// When InFlightRequests is dropped, any outstanding requests are completed with a
-/// deadline-exceeded error.
-impl<Resp> Drop for InFlightRequests<Resp> {
-    fn drop(&mut self) {
-        let deadlines = &mut self.deadlines;
-        for (_, request_data) in self.request_data.drain() {
-            let expired = deadlines.remove(&request_data.deadline_key);
-            request_data.complete(Self::deadline_exceeded_error(expired.into_inner()));
-        }
-    }
-}
-
-impl<Resp> RequestData<Resp> {
-    fn complete(self, response: Response<Resp>) {
-        let _ = self.response_completion.send(response);
-    }
 }

tarpc/src/context.rs

@@ -8,8 +8,13 @@
 //! client to server and is used by the server to enforce response deadlines.
 
 use crate::trace::{self, TraceId};
+use opentelemetry::trace::TraceContextExt;
 use static_assertions::assert_impl_all;
-use std::time::{Duration, SystemTime};
+use std::{
+    convert::TryFrom,
+    time::{Duration, SystemTime},
+};
+use tracing_opentelemetry::OpenTelemetrySpanExt;
 
 /// A request context that carries request-scoped information like deadlines and trace information.
 /// It is sent from client to server and is used by the server to enforce response deadlines.
@@ -22,14 +27,6 @@ use std::time::{Duration, SystemTime};
 pub struct Context {
     /// When the client expects the request to be complete by. The server should cancel the request
     /// if it is not complete by this time.
-    #[cfg_attr(
-        feature = "serde1",
-        serde(serialize_with = "crate::util::serde::serialize_epoch_secs")
-    )]
-    #[cfg_attr(
-        feature = "serde1",
-        serde(deserialize_with = "crate::util::serde::deserialize_epoch_secs")
-    )]
     #[cfg_attr(feature = "serde1", serde(default = "ten_seconds_from_now"))]
     pub deadline: SystemTime,
     /// Uniquely identifies requests originating from the same source.
@@ -41,23 +38,65 @@ pub struct Context {
 
 assert_impl_all!(Context: Send, Sync);
 
-#[cfg(feature = "serde1")]
 fn ten_seconds_from_now() -> SystemTime {
     SystemTime::now() + Duration::from_secs(10)
 }
 
 /// Returns the context for the current request, or a default Context if no request is active.
-// TODO: populate Context with request-scoped data, with default fallbacks.
 pub fn current() -> Context {
-    Context {
+    Context::current()
-        deadline: SystemTime::now() + Duration::from_secs(10),
+}
-        trace_context: trace::Context::new_root(),
+
+#[derive(Clone)]
+struct Deadline(SystemTime);
+
+impl Default for Deadline {
+    fn default() -> Self {
+        Self(ten_seconds_from_now())
     }
 }
 
 impl Context {
+    /// Returns the context for the current request, or a default Context if no request is active.
+    pub fn current() -> Self {
+        let span = tracing::Span::current();
+        Self {
+            trace_context: trace::Context::try_from(&span)
+                .unwrap_or_else(|_| trace::Context::default()),
+            deadline: span
+                .context()
+                .get::<Deadline>()
+                .cloned()
+                .unwrap_or_default()
+                .0,
+        }
+    }
+
     /// Returns the ID of the request-scoped trace.
     pub fn trace_id(&self) -> &TraceId {
         &self.trace_context.trace_id
     }
 }
+
+/// An extension trait for [`tracing::Span`] for propagating tarpc Contexts.
+pub(crate) trait SpanExt {
+    /// Sets the given context on this span. Newly-created spans will be children of the given
+    /// context's trace context.
+    fn set_context(&self, context: &Context);
+}
+
+impl SpanExt for tracing::Span {
+    fn set_context(&self, context: &Context) {
+        self.set_parent(
+            opentelemetry::Context::new()
+                .with_remote_span_context(opentelemetry::trace::SpanContext::new(
+                    opentelemetry::trace::TraceId::from(context.trace_context.trace_id),
+                    opentelemetry::trace::SpanId::from(context.trace_context.span_id),
+                    opentelemetry::trace::TraceFlags::from(context.trace_context.sampling_decision),
+                    true,
+                    opentelemetry::trace::TraceState::default(),
+                ))
+                .with_value(Deadline(context.deadline)),
+        );
+    }
+}

tarpc/src/lib.rs

@@ -38,6 +38,14 @@
 //!   requests sent by the server that use the request context will propagate the request deadline.
 //!   For example, if a server is handling a request with a 10s deadline, does 2s of work, then
 //!   sends a request to another server, that server will see an 8s deadline.
+//! - Distributed tracing: tarpc is instrumented with
+//!   [tracing](https://github.com/tokio-rs/tracing) primitives extended with
+//!   [OpenTelemetry](https://opentelemetry.io/) traces. Using a compatible tracing subscriber like
+//!   [Jaeger](https://github.com/open-telemetry/opentelemetry-rust/tree/main/opentelemetry-jaeger),
+//!   each RPC can be traced through the client, server, amd other dependencies downstream of the
+//!   server. Even for applications not connected to a distributed tracing collector, the
+//!   instrumentation can also be ingested by regular loggers like
+//!   [env_logger](https://github.com/env-logger-rs/env_logger/).
 //! - Serde serialization: enabling the `serde1` Cargo feature will make service requests and
 //!   responses `Serialize + Deserialize`. It's entirely optional, though: in-memory transports can
 //!   be used, as well, so the price of serialization doesn't have to be paid when it's not needed.
@@ -46,7 +54,7 @@
 //! Add to your `Cargo.toml` dependencies:
 //!
 //! ```toml
-//! tarpc = "0.25"
+//! tarpc = "0.27"
 //! ```
 //!
 //! The `tarpc::service` attribute expands to a collection of items that form an rpc service.
@@ -59,8 +67,9 @@
 //! your `Cargo.toml`:
 //!
 //! ```toml
-//! futures = "1.0"
+//! anyhow = "1.0"
-//! tarpc = { version = "0.25", features = ["tokio1"] }
+//! futures = "0.3"
+//! tarpc = { version = "0.27", features = ["tokio1"] }
 //! tokio = { version = "1.0", features = ["macros"] }
 //! ```
 //!
@@ -79,9 +88,8 @@
 //! };
 //! use tarpc::{
 //!     client, context,
-//!     server::{self, Incoming},
+//!     server::{self, incoming::Incoming},
 //! };
-//! use std::io;
 //!
 //! // This is the service definition. It looks a lot like a trait definition.
 //! // It defines one RPC, hello, which takes one arg, name, and returns a String.
@@ -103,9 +111,8 @@
 //! # };
 //! # use tarpc::{
 //! #     client, context,
-//! #     server::{self, Incoming},
+//! #     server::{self, incoming::Incoming},
 //! # };
-//! # use std::io;
 //! # // This is the service definition. It looks a lot like a trait definition.
 //! # // It defines one RPC, hello, which takes one arg, name, and returns a String.
 //! # #[tarpc::service]
@@ -145,7 +152,6 @@
 //! #     client, context,
 //! #     server::{self, Channel},
 //! # };
-//! # use std::io;
 //! # // This is the service definition. It looks a lot like a trait definition.
 //! # // It defines one RPC, hello, which takes one arg, name, and returns a String.
 //! # #[tarpc::service]
@@ -169,7 +175,7 @@
 //! # fn main() {}
 //! # #[cfg(feature = "tokio1")]
 //! #[tokio::main]
-//! async fn main() -> io::Result<()> {
+//! async fn main() -> anyhow::Result<()> {
 //!     let (client_transport, server_transport) = tarpc::transport::channel::unbounded();
 //!
 //!     let server = server::BaseChannel::with_defaults(server_transport);
@@ -177,7 +183,7 @@
 //!
 //!     // WorldClient is generated by the #[tarpc::service] attribute. It has a constructor `new`
 //!     // that takes a config and any Transport as input.
-//!     let mut client = WorldClient::new(client::Config::default(), client_transport).spawn()?;
+//!     let mut client = WorldClient::new(client::Config::default(), client_transport).spawn();
 //!
 //!     // The client has an RPC method for each RPC defined in the annotated trait. It takes the same
 //!     // args as defined, with the addition of a Context, which is always the first arg. The Context
@@ -199,6 +205,7 @@
 #![cfg_attr(docsrs, feature(doc_cfg))]
 
 #[cfg(feature = "serde1")]
+#[doc(hidden)]
 pub use serde;
 
 #[cfg(feature = "serde-transport")]
@@ -303,7 +310,7 @@ pub use crate::transport::sealed::Transport;
 
 use anyhow::Context as _;
 use futures::task::*;
-use std::{fmt::Display, io, time::SystemTime};
+use std::{error::Error, fmt::Display, io, time::SystemTime};
 
 /// A message from a client to a server.
 #[derive(Debug)]
@@ -355,8 +362,9 @@ pub struct Response<T> {
     pub message: Result<T, ServerError>,
 }
 
-/// An error response from a server to a client.
+/// An error indicating the server aborted the request early, e.g., due to request throttling.
-#[derive(Clone, Debug, PartialEq, Eq, Hash)]
+#[derive(thiserror::Error, Clone, Debug, PartialEq, Eq, Hash)]
+#[error("{kind:?}: {detail}")]
 #[non_exhaustive]
 #[cfg_attr(feature = "serde1", derive(serde::Serialize, serde::Deserialize))]
 pub struct ServerError {
@@ -371,13 +379,7 @@ pub struct ServerError {
     /// The type of error that occurred to fail the request.
     pub kind: io::ErrorKind,
     /// A message describing more detail about the error that occurred.
-    pub detail: Option<String>,
+    pub detail: String,
-}
-
-impl From<ServerError> for io::Error {
-    fn from(e: ServerError) -> io::Error {
-        io::Error::new(e.kind, e.detail.unwrap_or_default())
-    }
 }
 
 impl<T> Request<T> {
@@ -387,7 +389,6 @@ impl<T> Request<T> {
     }
 }
 
-pub(crate) type PollIo<T> = Poll<Option<io::Result<T>>>;
 pub(crate) trait PollContext<T> {
     fn context<C>(self, context: C) -> Poll<Option<anyhow::Result<T>>>
     where
@@ -399,7 +400,10 @@ pub(crate) trait PollContext<T> {
         F: FnOnce() -> C;
 }
 
-impl<T> PollContext<T> for PollIo<T> {
+impl<T, E> PollContext<T> for Poll<Option<Result<T, E>>>
+where
+    E: Error + Send + Sync + 'static,
+{
     fn context<C>(self, context: C) -> Poll<Option<anyhow::Result<T>>>
     where
         C: Display + Send + Sync + 'static,

tarpc/src/serde_transport.rs

@@ -42,14 +42,10 @@ where
     type Item = io::Result<Item>;
 
     fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<io::Result<Item>>> {
-        match self.project().inner.poll_next(cx) {
+        self.project()
-            Poll::Pending => Poll::Pending,
+            .inner
-            Poll::Ready(None) => Poll::Ready(None),
+            .poll_next(cx)
-            Poll::Ready(Some(Ok::<_, CodecError>(next))) => Poll::Ready(Some(Ok(next))),
+            .map_err(|e| io::Error::new(io::ErrorKind::Other, e))
-            Poll::Ready(Some(Err::<_, CodecError>(e))) => {
-                Poll::Ready(Some(Err(io::Error::new(io::ErrorKind::Other, e))))
-            }
-        }
     }
 }
 
@@ -65,7 +61,10 @@ where
     type Error = io::Error;
 
     fn poll_ready(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
-        convert(self.project().inner.poll_ready(cx))
+        self.project()
+            .inner
+            .poll_ready(cx)
+            .map_err(|e| io::Error::new(io::ErrorKind::Other, e))
     }
 
     fn start_send(self: Pin<&mut Self>, item: SinkItem) -> io::Result<()> {
@@ -76,20 +75,20 @@ where
     }
 
     fn poll_flush(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
-        convert(self.project().inner.poll_flush(cx))
+        self.project()
+            .inner
+            .poll_flush(cx)
+            .map_err(|e| io::Error::new(io::ErrorKind::Other, e))
     }
 
     fn poll_close(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
-        convert(self.project().inner.poll_close(cx))
+        self.project()
+            .inner
+            .poll_close(cx)
+            .map_err(|e| io::Error::new(io::ErrorKind::Other, e))
     }
 }
 
-fn convert<E: Into<Box<dyn Error + Send + Sync>>>(
-    poll: Poll<Result<(), E>>,
-) -> Poll<io::Result<()>> {
-    poll.map(|ready| ready.map_err(|e| io::Error::new(io::ErrorKind::Other, e)))
-}
-
 /// Constructs a new transport from a framed transport and a serialization codec.
 pub fn new<S, Item, SinkItem, Codec>(
     framed_io: Framed<S, LengthDelimitedCodec>,

tarpc/src/server/in_flight_requests.rs

@@ -1,19 +1,13 @@
-use crate::{
+use crate::util::{Compact, TimeUntil};
-    util::{Compact, TimeUntil},
-    PollIo,
-};
 use fnv::FnvHashMap;
-use futures::{
+use futures::future::{AbortHandle, AbortRegistration};
-    future::{AbortHandle, AbortRegistration},
-    ready,
-};
 use std::{
     collections::hash_map,
-    io,
     task::{Context, Poll},
     time::SystemTime,
 };
 use tokio_util::time::delay_queue::{self, DelayQueue};
+use tracing::Span;
 
 /// A data structure that tracks in-flight requests. It aborts requests,
 /// either on demand or when a request deadline expires.
@@ -23,13 +17,15 @@ pub struct InFlightRequests {
     deadlines: DelayQueue<u64>,
 }
 
-#[derive(Debug)]
 /// Data needed to clean up a single in-flight request.
+#[derive(Debug)]
 struct RequestData {
     /// Aborts the response handler for the associated request.
     abort_handle: AbortHandle,
     /// The key to remove the timer for the request's deadline.
     deadline_key: delay_queue::Key,
+    /// The client span.
+    span: Span,
 }
 
 /// An error returned when a request attempted to start with the same ID as a request already
@@ -48,6 +44,7 @@ impl InFlightRequests {
         &mut self,
         request_id: u64,
         deadline: SystemTime,
+        span: Span,
     ) -> Result<AbortRegistration, AlreadyExistsError> {
         match self.request_data.entry(request_id) {
             hash_map::Entry::Vacant(vacant) => {
@@ -57,6 +54,7 @@ impl InFlightRequests {
                 vacant.insert(RequestData {
                     abort_handle,
                     deadline_key,
+                    span,
                 });
                 Ok(abort_registration)
             }
@@ -66,12 +64,17 @@ impl InFlightRequests {
 
     /// Cancels an in-flight request. Returns true iff the request was found.
     pub fn cancel_request(&mut self, request_id: u64) -> bool {
-        if let Some(request_data) = self.request_data.remove(&request_id) {
+        if let Some(RequestData {
+            span,
+            abort_handle,
+            deadline_key,
+        }) = self.request_data.remove(&request_id)
+        {
+            let _entered = span.enter();
             self.request_data.compact(0.1);
-
+            abort_handle.abort();
-            request_data.abort_handle.abort();
+            self.deadlines.remove(&deadline_key);
-            self.deadlines.remove(&request_data.deadline_key);
+            tracing::info!("ReceiveCancel");
-
             true
         } else {
             false
@@ -80,30 +83,37 @@ impl InFlightRequests {
 
     /// Removes a request without aborting. Returns true iff the request was found.
     /// This method should be used when a response is being sent.
-    pub fn remove_request(&mut self, request_id: u64) -> bool {
+    pub fn remove_request(&mut self, request_id: u64) -> Option<Span> {
         if let Some(request_data) = self.request_data.remove(&request_id) {
             self.request_data.compact(0.1);
-
             self.deadlines.remove(&request_data.deadline_key);
-
+            Some(request_data.span)
-            true
         } else {
-            false
+            None
         }
     }
 
     /// Yields a request that has expired, aborting any ongoing processing of that request.
-    pub fn poll_expired(&mut self, cx: &mut Context) -> PollIo<u64> {
+    pub fn poll_expired(
-        Poll::Ready(match ready!(self.deadlines.poll_expired(cx)) {
+        &mut self,
-            Some(Ok(expired)) => {
+        cx: &mut Context,
-                if let Some(request_data) = self.request_data.remove(expired.get_ref()) {
+    ) -> Poll<Option<Result<u64, tokio::time::error::Error>>> {
-                    self.request_data.compact(0.1);
+        if self.deadlines.is_empty() {
-                    request_data.abort_handle.abort();
+            // TODO(https://github.com/tokio-rs/tokio/issues/4161)
-                }
+            // This is a workaround for DelayQueue not always treating this case correctly.
-                Some(Ok(expired.into_inner()))
+            return Poll::Ready(None);
+        }
+        self.deadlines.poll_expired(cx).map_ok(|expired| {
+            if let Some(RequestData {
+                abort_handle, span, ..
+            }) = self.request_data.remove(expired.get_ref())
+            {
+                let _entered = span.enter();
+                self.request_data.compact(0.1);
+                abort_handle.abort();
+                tracing::error!("DeadlineExceeded");
             }
-            Some(Err(e)) => Some(Err(io::Error::new(io::ErrorKind::Other, e))),
+            expired.into_inner()
-            None => None,
         })
     }
 }
@@ -118,75 +128,96 @@ impl Drop for InFlightRequests {
 }
 
 #[cfg(test)]
-use {
+mod tests {
-    assert_matches::assert_matches,
+    use super::*;
-    futures::{
+
+    use assert_matches::assert_matches;
+    use futures::{
         future::{pending, Abortable},
         FutureExt,
-    },
+    };
-    futures_test::task::noop_context,
+    use futures_test::task::noop_context;
-};
 
-#[tokio::test]
+    #[tokio::test]
-async fn start_request_increases_len() {
+    async fn start_request_increases_len() {
-    let mut in_flight_requests = InFlightRequests::default();
+        let mut in_flight_requests = InFlightRequests::default();
-    assert_eq!(in_flight_requests.len(), 0);
+        assert_eq!(in_flight_requests.len(), 0);
-    in_flight_requests
+        in_flight_requests
-        .start_request(0, SystemTime::now())
+            .start_request(0, SystemTime::now(), Span::current())
-        .unwrap();
+            .unwrap();
-    assert_eq!(in_flight_requests.len(), 1);
+        assert_eq!(in_flight_requests.len(), 1);
-}
+    }
-
+
-#[tokio::test]
+    #[tokio::test]
-async fn polling_expired_aborts() {
+    async fn polling_expired_aborts() {
-    let mut in_flight_requests = InFlightRequests::default();
+        let mut in_flight_requests = InFlightRequests::default();
-    let abort_registration = in_flight_requests
+        let abort_registration = in_flight_requests
-        .start_request(0, SystemTime::now())
+            .start_request(0, SystemTime::now(), Span::current())
-        .unwrap();
+            .unwrap();
-    let mut abortable_future = Box::new(Abortable::new(pending::<()>(), abort_registration));
+        let mut abortable_future = Box::new(Abortable::new(pending::<()>(), abort_registration));
-
+
-    tokio::time::pause();
+        tokio::time::pause();
-    tokio::time::advance(std::time::Duration::from_secs(1000)).await;
+        tokio::time::advance(std::time::Duration::from_secs(1000)).await;
-
+
-    assert_matches!(
+        assert_matches!(
-        in_flight_requests.poll_expired(&mut noop_context()),
+            in_flight_requests.poll_expired(&mut noop_context()),
-        Poll::Ready(Some(Ok(_)))
+            Poll::Ready(Some(Ok(_)))
-    );
+        );
-    assert_matches!(
+        assert_matches!(
-        abortable_future.poll_unpin(&mut noop_context()),
+            abortable_future.poll_unpin(&mut noop_context()),
-        Poll::Ready(Err(_))
+            Poll::Ready(Err(_))
-    );
+        );
-    assert_eq!(in_flight_requests.len(), 0);
+        assert_eq!(in_flight_requests.len(), 0);
-}
+    }
-
+
-#[tokio::test]
+    #[tokio::test]
-async fn cancel_request_aborts() {
+    async fn cancel_request_aborts() {
-    let mut in_flight_requests = InFlightRequests::default();
+        let mut in_flight_requests = InFlightRequests::default();
-    let abort_registration = in_flight_requests
+        let abort_registration = in_flight_requests
-        .start_request(0, SystemTime::now())
+            .start_request(0, SystemTime::now(), Span::current())
-        .unwrap();
+            .unwrap();
-    let mut abortable_future = Box::new(Abortable::new(pending::<()>(), abort_registration));
+        let mut abortable_future = Box::new(Abortable::new(pending::<()>(), abort_registration));
-
+
-    assert_eq!(in_flight_requests.cancel_request(0), true);
+        assert_eq!(in_flight_requests.cancel_request(0), true);
-    assert_matches!(
+        assert_matches!(
-        abortable_future.poll_unpin(&mut noop_context()),
+            abortable_future.poll_unpin(&mut noop_context()),
-        Poll::Ready(Err(_))
+            Poll::Ready(Err(_))
-    );
+        );
-    assert_eq!(in_flight_requests.len(), 0);
+        assert_eq!(in_flight_requests.len(), 0);
-}
+    }
-
+
-#[tokio::test]
+    #[tokio::test]
-async fn remove_request_doesnt_abort() {
+    async fn remove_request_doesnt_abort() {
-    let mut in_flight_requests = InFlightRequests::default();
+        let mut in_flight_requests = InFlightRequests::default();
-    let abort_registration = in_flight_requests
+        assert!(in_flight_requests.deadlines.is_empty());
-        .start_request(0, SystemTime::now())
+
-        .unwrap();
+        let abort_registration = in_flight_requests
-    let mut abortable_future = Box::new(Abortable::new(pending::<()>(), abort_registration));
+            .start_request(
-
+                0,
-    assert_eq!(in_flight_requests.remove_request(0), true);
+                SystemTime::now() + std::time::Duration::from_secs(10),
-    assert_matches!(
+                Span::current(),
-        abortable_future.poll_unpin(&mut noop_context()),
+            )
-        Poll::Pending
+            .unwrap();
-    );
+        let mut abortable_future = Box::new(Abortable::new(pending::<()>(), abort_registration));
-    assert_eq!(in_flight_requests.len(), 0);
+
+        // Precondition: Pending expiration
+        assert_matches!(
+            in_flight_requests.poll_expired(&mut noop_context()),
+            Poll::Pending
+        );
+        assert!(!in_flight_requests.deadlines.is_empty());
+
+        assert_matches!(in_flight_requests.remove_request(0), Some(_));
+        // Postcondition: No pending expirations
+        assert!(in_flight_requests.deadlines.is_empty());
+        assert_matches!(
+            in_flight_requests.poll_expired(&mut noop_context()),
+            Poll::Ready(None)
+        );
+        assert_matches!(
+            abortable_future.poll_unpin(&mut noop_context()),
+            Poll::Pending
+        );
+        assert_eq!(in_flight_requests.len(), 0);
+    }
 }

tarpc/src/server/incoming.rs

@@ -0,0 +1,49 @@
+use super::{
+    limits::{channels_per_key::MaxChannelsPerKey, requests_per_channel::MaxRequestsPerChannel},
+    Channel,
+};
+use futures::prelude::*;
+use std::{fmt, hash::Hash};
+
+#[cfg(feature = "tokio1")]
+use super::{tokio::TokioServerExecutor, Serve};
+
+/// An extension trait for [streams](futures::prelude::Stream) of [`Channels`](Channel).
+pub trait Incoming<C>
+where
+    Self: Sized + Stream<Item = C>,
+    C: Channel,
+{
+    /// Enforces channel per-key limits.
+    fn max_channels_per_key<K, KF>(self, n: u32, keymaker: KF) -> MaxChannelsPerKey<Self, K, KF>
+    where
+        K: fmt::Display + Eq + Hash + Clone + Unpin,
+        KF: Fn(&C) -> K,
+    {
+        MaxChannelsPerKey::new(self, n, keymaker)
+    }
+
+    /// Caps the number of concurrent requests per channel.
+    fn max_concurrent_requests_per_channel(self, n: usize) -> MaxRequestsPerChannel<Self> {
+        MaxRequestsPerChannel::new(self, n)
+    }
+
+    /// [Executes](Channel::execute) each incoming channel. Each channel will be handled
+    /// concurrently by spawning on tokio's default executor, and each request will be also
+    /// be spawned on tokio's default executor.
+    #[cfg(feature = "tokio1")]
+    #[cfg_attr(docsrs, doc(cfg(feature = "tokio1")))]
+    fn execute<S>(self, serve: S) -> TokioServerExecutor<Self, S>
+    where
+        S: Serve<C::Req, Resp = C::Resp>,
+    {
+        TokioServerExecutor::new(self, serve)
+    }
+}
+
+impl<S, C> Incoming<C> for S
+where
+    S: Sized + Stream<Item = C>,
+    C: Channel,
+{
+}

tarpc/src/server/limits.rs

@@ -0,0 +1,5 @@
+/// Provides functionality to limit the number of active channels.
+pub mod channels_per_key;
+
+/// Provides a [channel](crate::server::Channel) that limits the number of in-flight requests.
+pub mod requests_per_channel;

tarpc/src/server/limits/channels_per_key.rs

@@ -9,20 +9,23 @@ use crate::{
     util::Compact,
 };
 use fnv::FnvHashMap;
-use futures::{future::AbortRegistration, prelude::*, ready, stream::Fuse, task::*};
+use futures::{prelude::*, ready, stream::Fuse, task::*};
-use log::{debug, info, trace};
 use pin_project::pin_project;
 use std::sync::{Arc, Weak};
 use std::{
     collections::hash_map::Entry, convert::TryFrom, fmt, hash::Hash, marker::Unpin, pin::Pin,
-    time::SystemTime,
 };
 use tokio::sync::mpsc;
+use tracing::{debug, info, trace};
 
-/// A single-threaded filter that drops channels based on per-key limits.
+/// An [`Incoming`](crate::server::incoming::Incoming) stream that drops new channels based on
+/// per-key limits.
+///
+/// The decision to drop a Channel is made once at the time the Channel materializes. Once a
+/// Channel is yielded, it will not be prematurely dropped.
 #[pin_project]
 #[derive(Debug)]
-pub struct ChannelFilter<S, K, F>
+pub struct MaxChannelsPerKey<S, K, F>
 where
     K: Eq + Hash,
 {
@@ -35,7 +38,7 @@ where
     keymaker: F,
 }
 
-/// A channel that is tracked by a ChannelFilter.
+/// A channel that is tracked by [`MaxChannelsPerKey`].
 #[pin_project]
 #[derive(Debug)]
 pub struct TrackedChannel<C, K> {
@@ -103,6 +106,7 @@ where
 {
     type Req = C::Req;
     type Resp = C::Resp;
+    type Transport = C::Transport;
 
     fn config(&self) -> &server::Config {
         self.inner.config()
@@ -112,12 +116,8 @@ where
         self.inner.in_flight_requests()
     }
 
-    fn start_request(
+    fn transport(&self) -> &Self::Transport {
-        mut self: Pin<&mut Self>,
+        self.inner.transport()
-        id: u64,
-        deadline: SystemTime,
-    ) -> Result<AbortRegistration, super::in_flight_requests::AlreadyExistsError> {
-        self.inner_pin_mut().start_request(id, deadline)
     }
 }
 
@@ -133,7 +133,7 @@ impl<C, K> TrackedChannel<C, K> {
     }
 }
 
-impl<S, K, F> ChannelFilter<S, K, F>
+impl<S, K, F> MaxChannelsPerKey<S, K, F>
 where
     K: Eq + Hash,
     S: Stream,
@@ -142,7 +142,7 @@ where
     /// Sheds new channels to stay under configured limits.
     pub(crate) fn new(listener: S, channels_per_key: u32, keymaker: F) -> Self {
         let (dropped_keys_tx, dropped_keys) = mpsc::unbounded_channel();
-        ChannelFilter {
+        MaxChannelsPerKey {
             listener: listener.fuse(),
             channels_per_key,
             dropped_keys,
@@ -153,7 +153,7 @@ where
     }
 }
 
-impl<S, K, F> ChannelFilter<S, K, F>
+impl<S, K, F> MaxChannelsPerKey<S, K, F>
 where
     S: Stream,
     K: fmt::Display + Eq + Hash + Clone + Unpin,
@@ -171,11 +171,10 @@ where
         let tracker = self.as_mut().increment_channels_for_key(key.clone())?;
 
         trace!(
-            "[{}] Opening channel ({}/{}) channels for key.",
+            channel_filter_key = %key,
-            key,
+            open_channels = Arc::strong_count(&tracker),
-            Arc::strong_count(&tracker),
+            max_open_channels = self.channels_per_key,
-            self.channels_per_key
+            "Opening channel");
-        );
 
         Ok(TrackedChannel {
             tracker,
@@ -200,9 +199,10 @@ where
                 let count = o.get().strong_count();
                 if count >= TryFrom::try_from(*self_.channels_per_key).unwrap() {
                     info!(
-                        "[{}] Opened max channels from key ({}/{}).",
+                        channel_filter_key = %key,
-                        key, count, self_.channels_per_key
+                        open_channels = count,
-                    );
+                        max_open_channels = *self_.channels_per_key,
+                        "At open channel limit");
                     Err(key)
                 } else {
                     Ok(o.get().upgrade().unwrap_or_else(|| {
@@ -233,7 +233,9 @@ where
         let self_ = self.project();
         match ready!(self_.dropped_keys.poll_recv(cx)) {
             Some(key) => {
-                debug!("All channels dropped for key [{}]", key);
+                debug!(
+                    channel_filter_key = %key,
+                    "All channels dropped");
                 self_.key_counts.remove(&key);
                 self_.key_counts.compact(0.1);
                 Poll::Ready(())
@@ -243,7 +245,7 @@ where
     }
 }
 
-impl<S, K, F> Stream for ChannelFilter<S, K, F>
+impl<S, K, F> Stream for MaxChannelsPerKey<S, K, F>
 where
     S: Stream,
     K: fmt::Display + Eq + Hash + Clone + Unpin,
@@ -346,7 +348,7 @@ fn channel_filter_increment_channels_for_key() {
         key: &'static str,
     }
     let (_, listener) = futures::channel::mpsc::unbounded();
-    let filter = ChannelFilter::new(listener, 2, |chan: &TestChannel| chan.key);
+    let filter = MaxChannelsPerKey::new(listener, 2, |chan: &TestChannel| chan.key);
     pin_mut!(filter);
     let tracker1 = filter.as_mut().increment_channels_for_key("key").unwrap();
     assert_eq!(Arc::strong_count(&tracker1), 1);
@@ -367,7 +369,7 @@ fn channel_filter_handle_new_channel() {
         key: &'static str,
     }
     let (_, listener) = futures::channel::mpsc::unbounded();
-    let filter = ChannelFilter::new(listener, 2, |chan: &TestChannel| chan.key);
+    let filter = MaxChannelsPerKey::new(listener, 2, |chan: &TestChannel| chan.key);
     pin_mut!(filter);
     let channel1 = filter
         .as_mut()
@@ -399,7 +401,7 @@ fn channel_filter_poll_listener() {
         key: &'static str,
     }
     let (new_channels, listener) = futures::channel::mpsc::unbounded();
-    let filter = ChannelFilter::new(listener, 2, |chan: &TestChannel| chan.key);
+    let filter = MaxChannelsPerKey::new(listener, 2, |chan: &TestChannel| chan.key);
     pin_mut!(filter);
 
     new_channels
@@ -435,7 +437,7 @@ fn channel_filter_poll_closed_channels() {
         key: &'static str,
     }
     let (new_channels, listener) = futures::channel::mpsc::unbounded();
-    let filter = ChannelFilter::new(listener, 2, |chan: &TestChannel| chan.key);
+    let filter = MaxChannelsPerKey::new(listener, 2, |chan: &TestChannel| chan.key);
     pin_mut!(filter);
 
     new_channels
@@ -463,7 +465,7 @@ fn channel_filter_stream() {
         key: &'static str,
     }
     let (new_channels, listener) = futures::channel::mpsc::unbounded();
-    let filter = ChannelFilter::new(listener, 2, |chan: &TestChannel| chan.key);
+    let filter = MaxChannelsPerKey::new(listener, 2, |chan: &TestChannel| chan.key);
     pin_mut!(filter);
 
     new_channels

tarpc/src/server/limits/requests_per_channel.rs

@@ -0,0 +1,349 @@
+// Copyright 2020 Google LLC
+//
+// Use of this source code is governed by an MIT-style
+// license that can be found in the LICENSE file or at
+// https://opensource.org/licenses/MIT.
+
+use crate::{
+    server::{Channel, Config},
+    Response, ServerError,
+};
+use futures::{prelude::*, ready, task::*};
+use pin_project::pin_project;
+use std::{io, pin::Pin};
+
+/// A [`Channel`] that limits the number of concurrent requests by throttling.
+///
+/// Note that this is a very basic throttling heuristic. It is easy to set a number that is too low
+/// for the resources available to the server. For production use cases, a more advanced throttler
+/// is likely needed.
+#[pin_project]
+#[derive(Debug)]
+pub struct MaxRequests<C> {
+    max_in_flight_requests: usize,
+    #[pin]
+    inner: C,
+}
+
+impl<C> MaxRequests<C> {
+    /// Returns the inner channel.
+    pub fn get_ref(&self) -> &C {
+        &self.inner
+    }
+}
+
+impl<C> MaxRequests<C>
+where
+    C: Channel,
+{
+    /// Returns a new `MaxRequests` that wraps the given channel and limits concurrent requests to
+    /// `max_in_flight_requests`.
+    pub fn new(inner: C, max_in_flight_requests: usize) -> Self {
+        MaxRequests {
+            max_in_flight_requests,
+            inner,
+        }
+    }
+}
+
+impl<C> Stream for MaxRequests<C>
+where
+    C: Channel,
+{
+    type Item = <C as Stream>::Item;
+
+    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context) -> Poll<Option<Self::Item>> {
+        while self.as_mut().in_flight_requests() >= *self.as_mut().project().max_in_flight_requests
+        {
+            ready!(self.as_mut().project().inner.poll_ready(cx)?);
+
+            match ready!(self.as_mut().project().inner.poll_next(cx)?) {
+                Some(r) => {
+                    let _entered = r.span.enter();
+                    tracing::info!(
+                        in_flight_requests = self.as_mut().in_flight_requests(),
+                        "ThrottleRequest",
+                    );
+
+                    self.as_mut().start_send(Response {
+                        request_id: r.request.id,
+                        message: Err(ServerError {
+                            kind: io::ErrorKind::WouldBlock,
+                            detail: "server throttled the request.".into(),
+                        }),
+                    })?;
+                }
+                None => return Poll::Ready(None),
+            }
+        }
+        self.project().inner.poll_next(cx)
+    }
+}
+
+impl<C> Sink<Response<<C as Channel>::Resp>> for MaxRequests<C>
+where
+    C: Channel,
+{
+    type Error = C::Error;
+
+    fn poll_ready(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Result<(), Self::Error>> {
+        self.project().inner.poll_ready(cx)
+    }
+
+    fn start_send(
+        self: Pin<&mut Self>,
+        item: Response<<C as Channel>::Resp>,
+    ) -> Result<(), Self::Error> {
+        self.project().inner.start_send(item)
+    }
+
+    fn poll_flush(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Result<(), Self::Error>> {
+        self.project().inner.poll_flush(cx)
+    }
+
+    fn poll_close(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Result<(), Self::Error>> {
+        self.project().inner.poll_close(cx)
+    }
+}
+
+impl<C> AsRef<C> for MaxRequests<C> {
+    fn as_ref(&self) -> &C {
+        &self.inner
+    }
+}
+
+impl<C> Channel for MaxRequests<C>
+where
+    C: Channel,
+{
+    type Req = <C as Channel>::Req;
+    type Resp = <C as Channel>::Resp;
+    type Transport = <C as Channel>::Transport;
+
+    fn in_flight_requests(&self) -> usize {
+        self.inner.in_flight_requests()
+    }
+
+    fn config(&self) -> &Config {
+        self.inner.config()
+    }
+
+    fn transport(&self) -> &Self::Transport {
+        self.inner.transport()
+    }
+}
+
+/// An [`Incoming`](crate::server::incoming::Incoming) stream of channels that enforce limits on
+/// the number of in-flight requests.
+#[pin_project]
+#[derive(Debug)]
+pub struct MaxRequestsPerChannel<S> {
+    #[pin]
+    inner: S,
+    max_in_flight_requests: usize,
+}
+
+impl<S> MaxRequestsPerChannel<S>
+where
+    S: Stream,
+    <S as Stream>::Item: Channel,
+{
+    pub(crate) fn new(inner: S, max_in_flight_requests: usize) -> Self {
+        Self {
+            inner,
+            max_in_flight_requests,
+        }
+    }
+}
+
+impl<S> Stream for MaxRequestsPerChannel<S>
+where
+    S: Stream,
+    <S as Stream>::Item: Channel,
+{
+    type Item = MaxRequests<<S as Stream>::Item>;
+
+    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context) -> Poll<Option<Self::Item>> {
+        match ready!(self.as_mut().project().inner.poll_next(cx)) {
+            Some(channel) => Poll::Ready(Some(MaxRequests::new(
+                channel,
+                *self.project().max_in_flight_requests,
+            ))),
+            None => Poll::Ready(None),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    use crate::server::{
+        testing::{self, FakeChannel, PollExt},
+        TrackedRequest,
+    };
+    use pin_utils::pin_mut;
+    use std::{
+        marker::PhantomData,
+        time::{Duration, SystemTime},
+    };
+    use tracing::Span;
+
+    #[tokio::test]
+    async fn throttler_in_flight_requests() {
+        let throttler = MaxRequests {
+            max_in_flight_requests: 0,
+            inner: FakeChannel::default::<isize, isize>(),
+        };
+
+        pin_mut!(throttler);
+        for i in 0..5 {
+            throttler
+                .inner
+                .in_flight_requests
+                .start_request(
+                    i,
+                    SystemTime::now() + Duration::from_secs(1),
+                    Span::current(),
+                )
+                .unwrap();
+        }
+        assert_eq!(throttler.as_mut().in_flight_requests(), 5);
+    }
+
+    #[test]
+    fn throttler_poll_next_done() {
+        let throttler = MaxRequests {
+            max_in_flight_requests: 0,
+            inner: FakeChannel::default::<isize, isize>(),
+        };
+
+        pin_mut!(throttler);
+        assert!(throttler.as_mut().poll_next(&mut testing::cx()).is_done());
+    }
+
+    #[test]
+    fn throttler_poll_next_some() -> io::Result<()> {
+        let throttler = MaxRequests {
+            max_in_flight_requests: 1,
+            inner: FakeChannel::default::<isize, isize>(),
+        };
+
+        pin_mut!(throttler);
+        throttler.inner.push_req(0, 1);
+        assert!(throttler.as_mut().poll_ready(&mut testing::cx()).is_ready());
+        assert_eq!(
+            throttler
+                .as_mut()
+                .poll_next(&mut testing::cx())?
+                .map(|r| r.map(|r| (r.request.id, r.request.message))),
+            Poll::Ready(Some((0, 1)))
+        );
+        Ok(())
+    }
+
+    #[test]
+    fn throttler_poll_next_throttled() {
+        let throttler = MaxRequests {
+            max_in_flight_requests: 0,
+            inner: FakeChannel::default::<isize, isize>(),
+        };
+
+        pin_mut!(throttler);
+        throttler.inner.push_req(1, 1);
+        assert!(throttler.as_mut().poll_next(&mut testing::cx()).is_done());
+        assert_eq!(throttler.inner.sink.len(), 1);
+        let resp = throttler.inner.sink.get(0).unwrap();
+        assert_eq!(resp.request_id, 1);
+        assert!(resp.message.is_err());
+    }
+
+    #[test]
+    fn throttler_poll_next_throttled_sink_not_ready() {
+        let throttler = MaxRequests {
+            max_in_flight_requests: 0,
+            inner: PendingSink::default::<isize, isize>(),
+        };
+        pin_mut!(throttler);
+        assert!(throttler.poll_next(&mut testing::cx()).is_pending());
+
+        struct PendingSink<In, Out> {
+            ghost: PhantomData<fn(Out) -> In>,
+        }
+        impl PendingSink<(), ()> {
+            pub fn default<Req, Resp>(
+            ) -> PendingSink<io::Result<TrackedRequest<Req>>, Response<Resp>> {
+                PendingSink { ghost: PhantomData }
+            }
+        }
+        impl<In, Out> Stream for PendingSink<In, Out> {
+            type Item = In;
+            fn poll_next(self: Pin<&mut Self>, _: &mut Context) -> Poll<Option<Self::Item>> {
+                unimplemented!()
+            }
+        }
+        impl<In, Out> Sink<Out> for PendingSink<In, Out> {
+            type Error = io::Error;
+            fn poll_ready(self: Pin<&mut Self>, _: &mut Context) -> Poll<Result<(), Self::Error>> {
+                Poll::Pending
+            }
+            fn start_send(self: Pin<&mut Self>, _: Out) -> Result<(), Self::Error> {
+                Err(io::Error::from(io::ErrorKind::WouldBlock))
+            }
+            fn poll_flush(self: Pin<&mut Self>, _: &mut Context) -> Poll<Result<(), Self::Error>> {
+                Poll::Pending
+            }
+            fn poll_close(self: Pin<&mut Self>, _: &mut Context) -> Poll<Result<(), Self::Error>> {
+                Poll::Pending
+            }
+        }
+        impl<Req, Resp> Channel for PendingSink<io::Result<TrackedRequest<Req>>, Response<Resp>> {
+            type Req = Req;
+            type Resp = Resp;
+            type Transport = ();
+            fn config(&self) -> &Config {
+                unimplemented!()
+            }
+            fn in_flight_requests(&self) -> usize {
+                0
+            }
+            fn transport(&self) -> &() {
+                &()
+            }
+        }
+    }
+
+    #[tokio::test]
+    async fn throttler_start_send() {
+        let throttler = MaxRequests {
+            max_in_flight_requests: 0,
+            inner: FakeChannel::default::<isize, isize>(),
+        };
+
+        pin_mut!(throttler);
+        throttler
+            .inner
+            .in_flight_requests
+            .start_request(
+                0,
+                SystemTime::now() + Duration::from_secs(1),
+                Span::current(),
+            )
+            .unwrap();
+        throttler
+            .as_mut()
+            .start_send(Response {
+                request_id: 0,
+                message: Ok(1),
+            })
+            .unwrap();
+        assert_eq!(throttler.inner.in_flight_requests.len(), 0);
+        assert_eq!(
+            throttler.inner.sink.get(0),
+            Some(&Response {
+                request_id: 0,
+                message: Ok(1),
+            })
+        );
+    }
+}

tarpc/src/server/testing.rs

@@ -6,15 +6,13 @@
 
 use crate::{
     context,
-    server::{Channel, Config},
+    server::{Channel, Config, TrackedRequest},
     Request, Response,
 };
-use futures::{future::AbortRegistration, task::*, Sink, Stream};
+use futures::{task::*, Sink, Stream};
 use pin_project::pin_project;
-use std::collections::VecDeque;
+use std::{collections::VecDeque, io, pin::Pin, time::SystemTime};
-use std::io;
+use tracing::Span;
-use std::pin::Pin;
-use std::time::SystemTime;
 
 #[pin_project]
 pub(crate) struct FakeChannel<In, Out> {
@@ -64,12 +62,13 @@ impl<In, Resp> Sink<Response<Resp>> for FakeChannel<In, Response<Resp>> {
     }
 }
 
-impl<Req, Resp> Channel for FakeChannel<io::Result<Request<Req>>, Response<Resp>>
+impl<Req, Resp> Channel for FakeChannel<io::Result<TrackedRequest<Req>>, Response<Resp>>
 where
     Req: Unpin,
 {
     type Req = Req;
     type Resp = Resp;
+    type Transport = ();
 
     fn config(&self) -> &Config {
         &self.config
@@ -79,32 +78,31 @@ where
         self.in_flight_requests.len()
     }
 
-    fn start_request(
+    fn transport(&self) -> &() {
-        self: Pin<&mut Self>,
+        &()
-        id: u64,
-        deadline: SystemTime,
-    ) -> Result<AbortRegistration, super::in_flight_requests::AlreadyExistsError> {
-        self.project()
-            .in_flight_requests
-            .start_request(id, deadline)
     }
 }
 
-impl<Req, Resp> FakeChannel<io::Result<Request<Req>>, Response<Resp>> {
+impl<Req, Resp> FakeChannel<io::Result<TrackedRequest<Req>>, Response<Resp>> {
     pub fn push_req(&mut self, id: u64, message: Req) {
-        self.stream.push_back(Ok(Request {
+        let (_, abort_registration) = futures::future::AbortHandle::new_pair();
-            context: context::Context {
+        self.stream.push_back(Ok(TrackedRequest {
-                deadline: SystemTime::UNIX_EPOCH,
+            request: Request {
-                trace_context: Default::default(),
+                context: context::Context {
+                    deadline: SystemTime::UNIX_EPOCH,
+                    trace_context: Default::default(),
+                },
+                id,
+                message,
             },
-            id,
+            abort_registration,
-            message,
+            span: Span::none(),
         }));
     }
 }
 
 impl FakeChannel<(), ()> {
-    pub fn default<Req, Resp>() -> FakeChannel<io::Result<Request<Req>>, Response<Resp>> {
+    pub fn default<Req, Resp>() -> FakeChannel<io::Result<TrackedRequest<Req>>, Response<Resp>> {
         FakeChannel {
             stream: Default::default(),
             sink: Default::default(),

tarpc/src/server/throttle.rs

@@ -1,347 +0,0 @@
-// Copyright 2020 Google LLC
-//
-// Use of this source code is governed by an MIT-style
-// license that can be found in the LICENSE file or at
-// https://opensource.org/licenses/MIT.
-
-use super::{Channel, Config};
-use crate::{Response, ServerError};
-use futures::{future::AbortRegistration, prelude::*, ready, task::*};
-use log::debug;
-use pin_project::pin_project;
-use std::{io, pin::Pin, time::SystemTime};
-
-/// A [`Channel`] that limits the number of concurrent
-/// requests by throttling.
-#[pin_project]
-#[derive(Debug)]
-pub struct Throttler<C> {
-    max_in_flight_requests: usize,
-    #[pin]
-    inner: C,
-}
-
-impl<C> Throttler<C> {
-    /// Returns the inner channel.
-    pub fn get_ref(&self) -> &C {
-        &self.inner
-    }
-}
-
-impl<C> Throttler<C>
-where
-    C: Channel,
-{
-    /// Returns a new `Throttler` that wraps the given channel and limits concurrent requests to
-    /// `max_in_flight_requests`.
-    pub fn new(inner: C, max_in_flight_requests: usize) -> Self {
-        Throttler {
-            max_in_flight_requests,
-            inner,
-        }
-    }
-}
-
-impl<C> Stream for Throttler<C>
-where
-    C: Channel,
-{
-    type Item = <C as Stream>::Item;
-
-    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context) -> Poll<Option<Self::Item>> {
-        while self.as_mut().in_flight_requests() >= *self.as_mut().project().max_in_flight_requests
-        {
-            ready!(self.as_mut().project().inner.poll_ready(cx)?);
-
-            match ready!(self.as_mut().project().inner.poll_next(cx)?) {
-                Some(request) => {
-                    debug!(
-                        "[{}] Client has reached in-flight request limit ({}/{}).",
-                        request.context.trace_id(),
-                        self.as_mut().in_flight_requests(),
-                        self.as_mut().project().max_in_flight_requests,
-                    );
-
-                    self.as_mut().start_send(Response {
-                        request_id: request.id,
-                        message: Err(ServerError {
-                            kind: io::ErrorKind::WouldBlock,
-                            detail: Some("Server throttled the request.".into()),
-                        }),
-                    })?;
-                }
-                None => return Poll::Ready(None),
-            }
-        }
-        self.project().inner.poll_next(cx)
-    }
-}
-
-impl<C> Sink<Response<<C as Channel>::Resp>> for Throttler<C>
-where
-    C: Channel,
-{
-    type Error = io::Error;
-
-    fn poll_ready(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Result<(), Self::Error>> {
-        self.project().inner.poll_ready(cx)
-    }
-
-    fn start_send(self: Pin<&mut Self>, item: Response<<C as Channel>::Resp>) -> io::Result<()> {
-        self.project().inner.start_send(item)
-    }
-
-    fn poll_flush(self: Pin<&mut Self>, cx: &mut Context) -> Poll<io::Result<()>> {
-        self.project().inner.poll_flush(cx)
-    }
-
-    fn poll_close(self: Pin<&mut Self>, cx: &mut Context) -> Poll<io::Result<()>> {
-        self.project().inner.poll_close(cx)
-    }
-}
-
-impl<C> AsRef<C> for Throttler<C> {
-    fn as_ref(&self) -> &C {
-        &self.inner
-    }
-}
-
-impl<C> Channel for Throttler<C>
-where
-    C: Channel,
-{
-    type Req = <C as Channel>::Req;
-    type Resp = <C as Channel>::Resp;
-
-    fn in_flight_requests(&self) -> usize {
-        self.inner.in_flight_requests()
-    }
-
-    fn config(&self) -> &Config {
-        self.inner.config()
-    }
-
-    fn start_request(
-        self: Pin<&mut Self>,
-        id: u64,
-        deadline: SystemTime,
-    ) -> Result<AbortRegistration, super::in_flight_requests::AlreadyExistsError> {
-        self.project().inner.start_request(id, deadline)
-    }
-}
-
-/// A stream of throttling channels.
-#[pin_project]
-#[derive(Debug)]
-pub struct ThrottlerStream<S> {
-    #[pin]
-    inner: S,
-    max_in_flight_requests: usize,
-}
-
-impl<S> ThrottlerStream<S>
-where
-    S: Stream,
-    <S as Stream>::Item: Channel,
-{
-    pub(crate) fn new(inner: S, max_in_flight_requests: usize) -> Self {
-        Self {
-            inner,
-            max_in_flight_requests,
-        }
-    }
-}
-
-impl<S> Stream for ThrottlerStream<S>
-where
-    S: Stream,
-    <S as Stream>::Item: Channel,
-{
-    type Item = Throttler<<S as Stream>::Item>;
-
-    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context) -> Poll<Option<Self::Item>> {
-        match ready!(self.as_mut().project().inner.poll_next(cx)) {
-            Some(channel) => Poll::Ready(Some(Throttler::new(
-                channel,
-                *self.project().max_in_flight_requests,
-            ))),
-            None => Poll::Ready(None),
-        }
-    }
-}
-
-#[cfg(test)]
-use super::testing::{self, FakeChannel, PollExt};
-#[cfg(test)]
-use crate::Request;
-#[cfg(test)]
-use pin_utils::pin_mut;
-#[cfg(test)]
-use std::{marker::PhantomData, time::Duration};
-
-#[tokio::test]
-async fn throttler_in_flight_requests() {
-    let throttler = Throttler {
-        max_in_flight_requests: 0,
-        inner: FakeChannel::default::<isize, isize>(),
-    };
-
-    pin_mut!(throttler);
-    for i in 0..5 {
-        throttler
-            .inner
-            .in_flight_requests
-            .start_request(i, SystemTime::now() + Duration::from_secs(1))
-            .unwrap();
-    }
-    assert_eq!(throttler.as_mut().in_flight_requests(), 5);
-}
-
-#[tokio::test]
-async fn throttler_start_request() {
-    let throttler = Throttler {
-        max_in_flight_requests: 0,
-        inner: FakeChannel::default::<isize, isize>(),
-    };
-
-    pin_mut!(throttler);
-    throttler
-        .as_mut()
-        .start_request(1, SystemTime::now() + Duration::from_secs(1))
-        .unwrap();
-    assert_eq!(throttler.inner.in_flight_requests.len(), 1);
-}
-
-#[test]
-fn throttler_poll_next_done() {
-    let throttler = Throttler {
-        max_in_flight_requests: 0,
-        inner: FakeChannel::default::<isize, isize>(),
-    };
-
-    pin_mut!(throttler);
-    assert!(throttler.as_mut().poll_next(&mut testing::cx()).is_done());
-}
-
-#[test]
-fn throttler_poll_next_some() -> io::Result<()> {
-    let throttler = Throttler {
-        max_in_flight_requests: 1,
-        inner: FakeChannel::default::<isize, isize>(),
-    };
-
-    pin_mut!(throttler);
-    throttler.inner.push_req(0, 1);
-    assert!(throttler.as_mut().poll_ready(&mut testing::cx()).is_ready());
-    assert_eq!(
-        throttler
-            .as_mut()
-            .poll_next(&mut testing::cx())?
-            .map(|r| r.map(|r| (r.id, r.message))),
-        Poll::Ready(Some((0, 1)))
-    );
-    Ok(())
-}
-
-#[test]
-fn throttler_poll_next_throttled() {
-    let throttler = Throttler {
-        max_in_flight_requests: 0,
-        inner: FakeChannel::default::<isize, isize>(),
-    };
-
-    pin_mut!(throttler);
-    throttler.inner.push_req(1, 1);
-    assert!(throttler.as_mut().poll_next(&mut testing::cx()).is_done());
-    assert_eq!(throttler.inner.sink.len(), 1);
-    let resp = throttler.inner.sink.get(0).unwrap();
-    assert_eq!(resp.request_id, 1);
-    assert!(resp.message.is_err());
-}
-
-#[test]
-fn throttler_poll_next_throttled_sink_not_ready() {
-    let throttler = Throttler {
-        max_in_flight_requests: 0,
-        inner: PendingSink::default::<isize, isize>(),
-    };
-    pin_mut!(throttler);
-    assert!(throttler.poll_next(&mut testing::cx()).is_pending());
-
-    struct PendingSink<In, Out> {
-        ghost: PhantomData<fn(Out) -> In>,
-    }
-    impl PendingSink<(), ()> {
-        pub fn default<Req, Resp>() -> PendingSink<io::Result<Request<Req>>, Response<Resp>> {
-            PendingSink { ghost: PhantomData }
-        }
-    }
-    impl<In, Out> Stream for PendingSink<In, Out> {
-        type Item = In;
-        fn poll_next(self: Pin<&mut Self>, _: &mut Context) -> Poll<Option<Self::Item>> {
-            unimplemented!()
-        }
-    }
-    impl<In, Out> Sink<Out> for PendingSink<In, Out> {
-        type Error = io::Error;
-        fn poll_ready(self: Pin<&mut Self>, _: &mut Context) -> Poll<Result<(), Self::Error>> {
-            Poll::Pending
-        }
-        fn start_send(self: Pin<&mut Self>, _: Out) -> Result<(), Self::Error> {
-            Err(io::Error::from(io::ErrorKind::WouldBlock))
-        }
-        fn poll_flush(self: Pin<&mut Self>, _: &mut Context) -> Poll<Result<(), Self::Error>> {
-            Poll::Pending
-        }
-        fn poll_close(self: Pin<&mut Self>, _: &mut Context) -> Poll<Result<(), Self::Error>> {
-            Poll::Pending
-        }
-    }
-    impl<Req, Resp> Channel for PendingSink<io::Result<Request<Req>>, Response<Resp>> {
-        type Req = Req;
-        type Resp = Resp;
-        fn config(&self) -> &Config {
-            unimplemented!()
-        }
-        fn in_flight_requests(&self) -> usize {
-            0
-        }
-        fn start_request(
-            self: Pin<&mut Self>,
-            _id: u64,
-            _deadline: SystemTime,
-        ) -> Result<AbortRegistration, super::in_flight_requests::AlreadyExistsError> {
-            unimplemented!()
-        }
-    }
-}
-
-#[tokio::test]
-async fn throttler_start_send() {
-    let throttler = Throttler {
-        max_in_flight_requests: 0,
-        inner: FakeChannel::default::<isize, isize>(),
-    };
-
-    pin_mut!(throttler);
-    throttler
-        .inner
-        .in_flight_requests
-        .start_request(0, SystemTime::now() + Duration::from_secs(1))
-        .unwrap();
-    throttler
-        .as_mut()
-        .start_send(Response {
-            request_id: 0,
-            message: Ok(1),
-        })
-        .unwrap();
-    assert_eq!(throttler.inner.in_flight_requests.len(), 0);
-    assert_eq!(
-        throttler.inner.sink.get(0),
-        Some(&Response {
-            request_id: 0,
-            message: Ok(1),
-        })
-    );
-}

tarpc/src/server/tokio.rs

@@ -0,0 +1,111 @@
+use super::{Channel, Requests, Serve};
+use futures::{prelude::*, ready, task::*};
+use pin_project::pin_project;
+use std::pin::Pin;
+
+/// A future that drives the server by [spawning](tokio::spawn) a [`TokioChannelExecutor`](TokioChannelExecutor)
+/// for each new channel. Returned by
+/// [`Incoming::execute`](crate::server::incoming::Incoming::execute).
+#[pin_project]
+#[derive(Debug)]
+pub struct TokioServerExecutor<T, S> {
+    #[pin]
+    inner: T,
+    serve: S,
+}
+
+impl<T, S> TokioServerExecutor<T, S> {
+    pub(crate) fn new(inner: T, serve: S) -> Self {
+        Self { inner, serve }
+    }
+}
+
+/// A future that drives the server by [spawning](tokio::spawn) each [response
+/// handler](super::InFlightRequest::execute) on tokio's default executor. Returned by
+/// [`Channel::execute`](crate::server::Channel::execute).
+#[pin_project]
+#[derive(Debug)]
+pub struct TokioChannelExecutor<T, S> {
+    #[pin]
+    inner: T,
+    serve: S,
+}
+
+impl<T, S> TokioServerExecutor<T, S> {
+    fn inner_pin_mut<'a>(self: &'a mut Pin<&mut Self>) -> Pin<&'a mut T> {
+        self.as_mut().project().inner
+    }
+}
+
+impl<T, S> TokioChannelExecutor<T, S> {
+    fn inner_pin_mut<'a>(self: &'a mut Pin<&mut Self>) -> Pin<&'a mut T> {
+        self.as_mut().project().inner
+    }
+}
+
+// Send + 'static execution helper methods.
+
+impl<C> Requests<C>
+where
+    C: Channel,
+    C::Req: Send + 'static,
+    C::Resp: Send + 'static,
+{
+    /// Executes all requests using the given service function. Requests are handled concurrently
+    /// by [spawning](::tokio::spawn) each handler on tokio's default executor.
+    pub fn execute<S>(self, serve: S) -> TokioChannelExecutor<Self, S>
+    where
+        S: Serve<C::Req, Resp = C::Resp> + Send + 'static,
+    {
+        TokioChannelExecutor { inner: self, serve }
+    }
+}
+
+impl<St, C, Se> Future for TokioServerExecutor<St, Se>
+where
+    St: Sized + Stream<Item = C>,
+    C: Channel + Send + 'static,
+    C::Req: Send + 'static,
+    C::Resp: Send + 'static,
+    Se: Serve<C::Req, Resp = C::Resp> + Send + 'static + Clone,
+    Se::Fut: Send,
+{
+    type Output = ();
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> {
+        while let Some(channel) = ready!(self.inner_pin_mut().poll_next(cx)) {
+            tokio::spawn(channel.execute(self.serve.clone()));
+        }
+        tracing::info!("Server shutting down.");
+        Poll::Ready(())
+    }
+}
+
+impl<C, S> Future for TokioChannelExecutor<Requests<C>, S>
+where
+    C: Channel + 'static,
+    C::Req: Send + 'static,
+    C::Resp: Send + 'static,
+    S: Serve<C::Req, Resp = C::Resp> + Send + 'static + Clone,
+    S::Fut: Send,
+{
+    type Output = ();
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        while let Some(response_handler) = ready!(self.inner_pin_mut().poll_next(cx)) {
+            match response_handler {
+                Ok(resp) => {
+                    let server = self.serve.clone();
+                    tokio::spawn(async move {
+                        resp.execute(server).await;
+                    });
+                }
+                Err(e) => {
+                    tracing::warn!("Requests stream errored out: {}", e);
+                    break;
+                }
+            }
+        }
+        Poll::Ready(())
+    }
+}

tarpc/src/trace.rs

@@ -16,11 +16,14 @@
 //! This crate's design is based on [opencensus
 //! tracing](https://opencensus.io/core-concepts/tracing/).
 
+use opentelemetry::trace::TraceContextExt;
 use rand::Rng;
 use std::{
+    convert::TryFrom,
     fmt::{self, Formatter},
-    mem,
+    num::{NonZeroU128, NonZeroU64},
 };
+use tracing_opentelemetry::OpenTelemetrySpanExt;
 
 /// A context for tracing the execution of processes, distributed or otherwise.
 ///
@@ -36,33 +39,50 @@ pub struct Context {
     /// before making an RPC, and the span ID is sent to the server. The server is free to create
     /// its own spans, for which it sets the client's span as the parent span.
     pub span_id: SpanId,
-    /// An identifier of the span that originated the current span. For example, if a server sends
+    /// Indicates whether a sampler has already decided whether or not to sample the trace
-    /// an RPC in response to a client request that included a span, the server would create a span
+    /// associated with the Context. If `sampling_decision` is None, then a decision has not yet
-    /// for the RPC and set its parent to the span_id in the incoming request's context.
+    /// been made. Downstream samplers do not need to abide by "no sample" decisions--for example,
-    ///
+    /// an upstream client may choose to never sample, which may not make sense for the client's
-    /// If `parent_id` is `None`, then this is a root context.
+    /// dependencies. On the other hand, if an upstream process has chosen to sample this trace,
-    pub parent_id: Option<SpanId>,
+    /// then the downstream samplers are expected to respect that decision and also sample the
+    /// trace. Otherwise, the full trace would not be able to be reconstructed.
+    pub sampling_decision: SamplingDecision,
 }
 
 /// A 128-bit UUID identifying a trace. All spans caused by the same originating span share the
 /// same trace ID.
-#[derive(Debug, Default, PartialEq, Eq, Hash, Clone, Copy)]
+#[derive(Default, PartialEq, Eq, Hash, Clone, Copy)]
 #[cfg_attr(feature = "serde1", derive(serde::Serialize, serde::Deserialize))]
-pub struct TraceId(u128);
+pub struct TraceId(#[cfg_attr(feature = "serde1", serde(with = "u128_serde"))] u128);
 
 /// A 64-bit identifier of a span within a trace. The identifier is unique within the span's trace.
-#[derive(Debug, Default, PartialEq, Eq, Hash, Clone, Copy)]
+#[derive(Default, PartialEq, Eq, Hash, Clone, Copy)]
 #[cfg_attr(feature = "serde1", derive(serde::Serialize, serde::Deserialize))]
 pub struct SpanId(u64);
 
+/// Indicates whether a sampler has decided whether or not to sample the trace associated with the
+/// Context. Downstream samplers do not need to abide by "no sample" decisions--for example, an
+/// upstream client may choose to never sample, which may not make sense for the client's
+/// dependencies. On the other hand, if an upstream process has chosen to sample this trace, then
+/// the downstream samplers are expected to respect that decision and also sample the trace.
+/// Otherwise, the full trace would not be able to be reconstructed reliably.
+#[derive(Debug, PartialEq, Eq, Hash, Clone, Copy)]
+#[cfg_attr(feature = "serde1", derive(serde::Serialize, serde::Deserialize))]
+#[repr(u8)]
+pub enum SamplingDecision {
+    /// The associated span was sampled by its creating process. Child spans must also be sampled.
+    Sampled,
+    /// The associated span was not sampled by its creating process.
+    Unsampled,
+}
+
 impl Context {
-    /// Constructs a new root context. A root context is one with no parent span.
+    /// Constructs a new context with the trace ID and sampling decision inherited from the parent.
-    pub fn new_root() -> Self {
+    pub(crate) fn new_child(&self) -> Self {
-        let rng = &mut rand::thread_rng();
+        Self {
-        Context {
+            trace_id: self.trace_id,
-            trace_id: TraceId::random(rng),
+            span_id: SpanId::random(&mut rand::thread_rng()),
-            span_id: SpanId::random(rng),
+            sampling_decision: self.sampling_decision,
-            parent_id: None,
         }
     }
 }
@@ -71,17 +91,128 @@ impl TraceId {
     /// Returns a random trace ID that can be assumed to be globally unique if `rng` generates
     /// actually-random numbers.
     pub fn random<R: Rng>(rng: &mut R) -> Self {
-        TraceId(u128::from(rng.next_u64()) << mem::size_of::<u64>() | u128::from(rng.next_u64()))
+        TraceId(rng.gen::<NonZeroU128>().get())
+    }
+
+    /// Returns true iff the trace ID is 0.
+    pub fn is_none(&self) -> bool {
+        self.0 == 0
     }
 }
 
 impl SpanId {
     /// Returns a random span ID that can be assumed to be unique within a single trace.
     pub fn random<R: Rng>(rng: &mut R) -> Self {
-        SpanId(rng.next_u64())
+        SpanId(rng.gen::<NonZeroU64>().get())
+    }
+
+    /// Returns true iff the span ID is 0.
+    pub fn is_none(&self) -> bool {
+        self.0 == 0
     }
 }
 
+impl From<TraceId> for u128 {
+    fn from(trace_id: TraceId) -> Self {
+        trace_id.0
+    }
+}
+
+impl From<u128> for TraceId {
+    fn from(trace_id: u128) -> Self {
+        Self(trace_id)
+    }
+}
+
+impl From<SpanId> for u64 {
+    fn from(span_id: SpanId) -> Self {
+        span_id.0
+    }
+}
+
+impl From<u64> for SpanId {
+    fn from(span_id: u64) -> Self {
+        Self(span_id)
+    }
+}
+
+impl From<opentelemetry::trace::TraceId> for TraceId {
+    fn from(trace_id: opentelemetry::trace::TraceId) -> Self {
+        Self::from(trace_id.to_u128())
+    }
+}
+
+impl From<TraceId> for opentelemetry::trace::TraceId {
+    fn from(trace_id: TraceId) -> Self {
+        Self::from_u128(trace_id.into())
+    }
+}
+
+impl From<opentelemetry::trace::SpanId> for SpanId {
+    fn from(span_id: opentelemetry::trace::SpanId) -> Self {
+        Self::from(span_id.to_u64())
+    }
+}
+
+impl From<SpanId> for opentelemetry::trace::SpanId {
+    fn from(span_id: SpanId) -> Self {
+        Self::from_u64(span_id.0)
+    }
+}
+
+impl TryFrom<&tracing::Span> for Context {
+    type Error = NoActiveSpan;
+
+    fn try_from(span: &tracing::Span) -> Result<Self, NoActiveSpan> {
+        let context = span.context();
+        if context.has_active_span() {
+            Ok(Self::from(context.span()))
+        } else {
+            Err(NoActiveSpan)
+        }
+    }
+}
+
+impl From<opentelemetry::trace::SpanRef<'_>> for Context {
+    fn from(span: opentelemetry::trace::SpanRef<'_>) -> Self {
+        let otel_ctx = span.span_context();
+        Self {
+            trace_id: TraceId::from(otel_ctx.trace_id()),
+            span_id: SpanId::from(otel_ctx.span_id()),
+            sampling_decision: SamplingDecision::from(otel_ctx),
+        }
+    }
+}
+
+impl From<SamplingDecision> for opentelemetry::trace::TraceFlags {
+    fn from(decision: SamplingDecision) -> Self {
+        match decision {
+            SamplingDecision::Sampled => opentelemetry::trace::TraceFlags::SAMPLED,
+            SamplingDecision::Unsampled => opentelemetry::trace::TraceFlags::default(),
+        }
+    }
+}
+
+impl From<&opentelemetry::trace::SpanContext> for SamplingDecision {
+    fn from(context: &opentelemetry::trace::SpanContext) -> Self {
+        if context.is_sampled() {
+            SamplingDecision::Sampled
+        } else {
+            SamplingDecision::Unsampled
+        }
+    }
+}
+
+impl Default for SamplingDecision {
+    fn default() -> Self {
+        Self::Unsampled
+    }
+}
+
+/// Returned when a [`Context`] cannot be constructed from a [`Span`](tracing::Span).
+#[derive(Debug)]
+pub struct NoActiveSpan;
+
 impl fmt::Display for TraceId {
     fn fmt(&self, f: &mut Formatter) -> Result<(), fmt::Error> {
         write!(f, "{:02x}", self.0)?;
@@ -89,9 +220,42 @@ impl fmt::Display for TraceId {
     }
 }
 
+impl fmt::Debug for TraceId {
+    fn fmt(&self, f: &mut Formatter) -> Result<(), fmt::Error> {
+        write!(f, "{:02x}", self.0)?;
+        Ok(())
+    }
+}
+
 impl fmt::Display for SpanId {
     fn fmt(&self, f: &mut Formatter) -> Result<(), fmt::Error> {
         write!(f, "{:02x}", self.0)?;
         Ok(())
     }
 }
+
+impl fmt::Debug for SpanId {
+    fn fmt(&self, f: &mut Formatter) -> Result<(), fmt::Error> {
+        write!(f, "{:02x}", self.0)?;
+        Ok(())
+    }
+}
+
+#[cfg(feature = "serde1")]
+mod u128_serde {
+    pub fn serialize<S>(u: &u128, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: serde::Serializer,
+    {
+        serde::Serialize::serialize(&u.to_le_bytes(), serializer)
+    }
+
+    pub fn deserialize<'de, D>(deserializer: D) -> Result<u128, D::Error>
+    where
+        D: serde::Deserializer<'de>,
+    {
+        Ok(u128::from_le_bytes(serde::Deserialize::deserialize(
+            deserializer,
+        )?))
+    }
+}

tarpc/src/transport.rs

@@ -9,22 +9,32 @@
 //! The rpc crate is transport- and protocol-agnostic. Any transport that impls [`Transport`](sealed::Transport)
 //! can be plugged in, using whatever protocol it wants.
 
-use futures::prelude::*;
-use std::io;
-
 pub mod channel;
 
 pub(crate) mod sealed {
-    use super::*;
+    use futures::prelude::*;
+    use std::error::Error;
 
     /// A bidirectional stream ([`Sink`] + [`Stream`]) of messages.
-    pub trait Transport<SinkItem, Item>:
+    pub trait Transport<SinkItem, Item>
-        Stream<Item = io::Result<Item>> + Sink<SinkItem, Error = io::Error>
+    where
+        Self: Stream<Item = Result<Item, <Self as Sink<SinkItem>>::Error>>,
+        Self: Sink<SinkItem, Error = <Self as Transport<SinkItem, Item>>::TransportError>,
+        <Self as Sink<SinkItem>>::Error: Error,
     {
+        /// Associated type where clauses are not elaborated; this associated type allows users
+        /// bounding types by Transport to avoid having to explicitly add `T::Error: Error` to their
+        /// bounds.
+        type TransportError: Error + Send + Sync + 'static;
     }
 
-    impl<T, SinkItem, Item> Transport<SinkItem, Item> for T where
+    impl<T, SinkItem, Item, E> Transport<SinkItem, Item> for T
-        T: Stream<Item = io::Result<Item>> + Sink<SinkItem, Error = io::Error> + ?Sized
+    where
+        T: ?Sized,
+        T: Stream<Item = Result<Item, E>>,
+        T: Sink<SinkItem, Error = E>,
+        T::Error: Error + Send + Sync + 'static,
     {
+        type TransportError = E;
     }
 }

tarpc/src/transport/channel.rs

@@ -6,13 +6,19 @@
 
 //! Transports backed by in-memory channels.
 
-use crate::PollIo;
 use futures::{task::*, Sink, Stream};
 use pin_project::pin_project;
-use std::io;
+use std::{error::Error, pin::Pin};
-use std::pin::Pin;
 use tokio::sync::mpsc;
 
+/// Errors that occur in the sending or receiving of messages over a channel.
+#[derive(thiserror::Error, Debug)]
+pub enum ChannelError {
+    /// An error occurred sending over the channel.
+    #[error("an error occurred sending over the channel")]
+    Send(#[source] Box<dyn Error + Send + Sync + 'static>),
+}
+
 /// Returns two unbounded channel peers. Each [`Stream`] yields items sent through the other's
 /// [`Sink`].
 pub fn unbounded<SinkItem, Item>() -> (
@@ -36,28 +42,33 @@ pub struct UnboundedChannel<Item, SinkItem> {
 }
 
 impl<Item, SinkItem> Stream for UnboundedChannel<Item, SinkItem> {
-    type Item = Result<Item, io::Error>;
+    type Item = Result<Item, ChannelError>;
 
-    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> PollIo<Item> {
+    fn poll_next(
+        mut self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+    ) -> Poll<Option<Result<Item, ChannelError>>> {
         self.rx.poll_recv(cx).map(|option| option.map(Ok))
     }
 }
 
-impl<Item, SinkItem> Sink<SinkItem> for UnboundedChannel<Item, SinkItem> {
+const CLOSED_MESSAGE: &str = "the channel is closed and cannot accept new items for sending";
-    type Error = io::Error;
 
-    fn poll_ready(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<io::Result<()>> {
+impl<Item, SinkItem> Sink<SinkItem> for UnboundedChannel<Item, SinkItem> {
+    type Error = ChannelError;
+
+    fn poll_ready(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
         Poll::Ready(if self.tx.is_closed() {
-            Err(io::Error::from(io::ErrorKind::NotConnected))
+            Err(ChannelError::Send(CLOSED_MESSAGE.into()))
         } else {
             Ok(())
         })
     }
 
-    fn start_send(self: Pin<&mut Self>, item: SinkItem) -> io::Result<()> {
+    fn start_send(self: Pin<&mut Self>, item: SinkItem) -> Result<(), Self::Error> {
         self.tx
             .send(item)
-            .map_err(|_| io::Error::from(io::ErrorKind::NotConnected))
+            .map_err(|_| ChannelError::Send(CLOSED_MESSAGE.into()))
     }
 
     fn poll_flush(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
@@ -65,7 +76,7 @@ impl<Item, SinkItem> Sink<SinkItem> for UnboundedChannel<Item, SinkItem> {
         Poll::Ready(Ok(()))
     }
 
-    fn poll_close(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<io::Result<()>> {
+    fn poll_close(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
         // UnboundedSender can't initiate closure.
         Poll::Ready(Ok(()))
     }
@@ -93,52 +104,45 @@ pub struct Channel<Item, SinkItem> {
 }
 
 impl<Item, SinkItem> Stream for Channel<Item, SinkItem> {
-    type Item = Result<Item, io::Error>;
+    type Item = Result<Item, ChannelError>;
 
-    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> PollIo<Item> {
+    fn poll_next(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+    ) -> Poll<Option<Result<Item, ChannelError>>> {
         self.project().rx.poll_next(cx).map(|option| option.map(Ok))
     }
 }
 
 impl<Item, SinkItem> Sink<SinkItem> for Channel<Item, SinkItem> {
-    type Error = io::Error;
+    type Error = ChannelError;
 
-    fn poll_ready(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
+    fn poll_ready(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
         self.project()
             .tx
             .poll_ready(cx)
-            .map_err(convert_send_err_to_io)
+            .map_err(|e| ChannelError::Send(Box::new(e)))
     }
 
-    fn start_send(self: Pin<&mut Self>, item: SinkItem) -> io::Result<()> {
+    fn start_send(self: Pin<&mut Self>, item: SinkItem) -> Result<(), Self::Error> {
         self.project()
             .tx
             .start_send(item)
-            .map_err(convert_send_err_to_io)
+            .map_err(|e| ChannelError::Send(Box::new(e)))
     }
 
     fn poll_flush(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
         self.project()
             .tx
             .poll_flush(cx)
-            .map_err(convert_send_err_to_io)
+            .map_err(|e| ChannelError::Send(Box::new(e)))
     }
 
-    fn poll_close(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
+    fn poll_close(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
         self.project()
             .tx
             .poll_close(cx)
-            .map_err(convert_send_err_to_io)
+            .map_err(|e| ChannelError::Send(Box::new(e)))
-    }
-}
-
-fn convert_send_err_to_io(e: futures::channel::mpsc::SendError) -> io::Error {
-    if e.is_disconnected() {
-        io::Error::from(io::ErrorKind::NotConnected)
-    } else if e.is_full() {
-        io::Error::from(io::ErrorKind::WouldBlock)
-    } else {
-        io::Error::new(io::ErrorKind::Other, e)
     }
 }
 
@@ -147,17 +151,27 @@ fn convert_send_err_to_io(e: futures::channel::mpsc::SendError) -> io::Error {
 mod tests {
     use crate::{
         client, context,
-        server::{BaseChannel, Incoming},
+        server::{incoming::Incoming, BaseChannel},
-        transport,
+        transport::{
+            self,
+            channel::{Channel, UnboundedChannel},
+        },
     };
     use assert_matches::assert_matches;
     use futures::{prelude::*, stream};
-    use log::trace;
     use std::io;
+    use tracing::trace;
+
+    #[test]
+    fn ensure_is_transport() {
+        fn is_transport<SinkItem, Item, T: crate::Transport<SinkItem, Item>>() {}
+        is_transport::<(), (), UnboundedChannel<(), ()>>();
+        is_transport::<(), (), Channel<(), ()>>();
+    }
 
     #[tokio::test]
-    async fn integration() -> io::Result<()> {
+    async fn integration() -> anyhow::Result<()> {
-        let _ = env_logger::try_init();
+        let _ = tracing_subscriber::fmt::try_init();
 
         let (client_channel, server_channel) = transport::channel::unbounded();
         tokio::spawn(
@@ -173,10 +187,10 @@ mod tests {
                 }),
         );
 
-        let client = client::new(client::Config::default(), client_channel).spawn()?;
+        let client = client::new(client::Config::default(), client_channel).spawn();
 
-        let response1 = client.call(context::current(), "123".into()).await?;
+        let response1 = client.call(context::current(), "", "123".into()).await?;
-        let response2 = client.call(context::current(), "abc".into()).await?;
+        let response2 = client.call(context::current(), "", "abc".into()).await?;
 
         trace!("response1: {:?}, response2: {:?}", response1, response2);
 

tarpc/src/util/serde.rs

@@ -5,31 +5,7 @@
 // https://opensource.org/licenses/MIT.
 
 use serde::{Deserialize, Deserializer, Serialize, Serializer};
-use std::{
+use std::io;
-    io,
-    time::{Duration, SystemTime},
-};
-
-/// Serializes `system_time` as a `u64` equal to the number of seconds since the epoch.
-pub fn serialize_epoch_secs<S>(system_time: &SystemTime, serializer: S) -> Result<S::Ok, S::Error>
-where
-    S: Serializer,
-{
-    const ZERO_SECS: Duration = Duration::from_secs(0);
-    system_time
-        .duration_since(SystemTime::UNIX_EPOCH)
-        .unwrap_or(ZERO_SECS)
-        .as_secs() // Only care about second precision
-        .serialize(serializer)
-}
-
-/// Deserializes [`SystemTime`] from a `u64` equal to the number of seconds since the epoch.
-pub fn deserialize_epoch_secs<'de, D>(deserializer: D) -> Result<SystemTime, D::Error>
-where
-    D: Deserializer<'de>,
-{
-    Ok(SystemTime::UNIX_EPOCH + Duration::from_secs(u64::deserialize(deserializer)?))
-}
 
 /// Serializes [`io::ErrorKind`] as a `u32`.
 #[allow(clippy::trivially_copy_pass_by_ref)] // Exact fn signature required by serde derive

tarpc/tests/dataservice.rs

@@ -1,9 +1,8 @@
 use futures::prelude::*;
-use std::io;
 use tarpc::serde_transport;
 use tarpc::{
     client, context,
-    server::{BaseChannel, Incoming},
+    server::{incoming::Incoming, BaseChannel},
 };
 use tokio_serde::formats::Json;
 
@@ -33,7 +32,7 @@ impl ColorProtocol for ColorServer {
 }
 
 #[tokio::test]
-async fn test_call() -> io::Result<()> {
+async fn test_call() -> anyhow::Result<()> {
     let transport = tarpc::serde_transport::tcp::listen("localhost:56797", Json::default).await?;
     let addr = transport.local_addr();
     tokio::spawn(
@@ -45,7 +44,7 @@ async fn test_call() -> io::Result<()> {
     );
 
     let transport = serde_transport::tcp::connect(addr, Json::default).await?;
-    let client = ColorProtocolClient::new(client::Config::default(), transport).spawn()?;
+    let client = ColorProtocolClient::new(client::Config::default(), transport).spawn();
 
     let color = client
         .get_opposite_color(context::current(), TestData::White)

tarpc/tests/service_functional.rs

@@ -3,14 +3,11 @@ use futures::{
     future::{join_all, ready, Ready},
     prelude::*,
 };
-use std::{
+use std::time::{Duration, SystemTime};
-    io,
-    time::{Duration, SystemTime},
-};
 use tarpc::{
     client::{self},
     context,
-    server::{self, BaseChannel, Channel, Incoming},
+    server::{self, incoming::Incoming, BaseChannel, Channel},
     transport::channel,
 };
 use tokio::join;
@@ -39,8 +36,8 @@ impl Service for Server {
 }
 
 #[tokio::test]
-async fn sequential() -> io::Result<()> {
+async fn sequential() -> anyhow::Result<()> {
-    let _ = env_logger::try_init();
+    let _ = tracing_subscriber::fmt::try_init();
 
     let (tx, rx) = channel::unbounded();
 
@@ -50,7 +47,7 @@ async fn sequential() -> io::Result<()> {
             .execute(Server.serve()),
     );
 
-    let client = ServiceClient::new(client::Config::default(), tx).spawn()?;
+    let client = ServiceClient::new(client::Config::default(), tx).spawn();
 
     assert_matches!(client.add(context::current(), 1, 2).await, Ok(3));
     assert_matches!(
@@ -61,7 +58,7 @@ async fn sequential() -> io::Result<()> {
 }
 
 #[tokio::test]
-async fn dropped_channel_aborts_in_flight_requests() -> io::Result<()> {
+async fn dropped_channel_aborts_in_flight_requests() -> anyhow::Result<()> {
     #[tarpc_plugins::service]
     trait Loop {
         async fn r#loop();
@@ -82,16 +79,14 @@ async fn dropped_channel_aborts_in_flight_requests() -> io::Result<()> {
         }
     }
 
-    let _ = env_logger::try_init();
+    let _ = tracing_subscriber::fmt::try_init();
 
     let (tx, rx) = channel::unbounded();
 
     // Set up a client that initiates a long-lived request.
     // The request will complete in error when the server drops the connection.
     tokio::spawn(async move {
-        let client = LoopClient::new(client::Config::default(), tx)
+        let client = LoopClient::new(client::Config::default(), tx).spawn();
-            .spawn()
-            .unwrap();
 
         let mut ctx = context::current();
         ctx.deadline = SystemTime::now() + Duration::from_secs(60 * 60);
@@ -113,11 +108,11 @@ async fn dropped_channel_aborts_in_flight_requests() -> io::Result<()> {
 
 #[cfg(all(feature = "serde-transport", feature = "tcp"))]
 #[tokio::test]
-async fn serde() -> io::Result<()> {
+async fn serde() -> anyhow::Result<()> {
     use tarpc::serde_transport;
     use tokio_serde::formats::Json;
 
-    let _ = env_logger::try_init();
+    let _ = tracing_subscriber::fmt::try_init();
 
     let transport = tarpc::serde_transport::tcp::listen("localhost:56789", Json::default).await?;
     let addr = transport.local_addr();
@@ -130,7 +125,7 @@ async fn serde() -> io::Result<()> {
     );
 
     let transport = serde_transport::tcp::connect(addr, Json::default).await?;
-    let client = ServiceClient::new(client::Config::default(), transport).spawn()?;
+    let client = ServiceClient::new(client::Config::default(), transport).spawn();
 
     assert_matches!(client.add(context::current(), 1, 2).await, Ok(3));
     assert_matches!(
@@ -142,8 +137,8 @@ async fn serde() -> io::Result<()> {
 }
 
 #[tokio::test]
-async fn concurrent() -> io::Result<()> {
+async fn concurrent() -> anyhow::Result<()> {
-    let _ = env_logger::try_init();
+    let _ = tracing_subscriber::fmt::try_init();
 
     let (tx, rx) = channel::unbounded();
     tokio::spawn(
@@ -152,7 +147,7 @@ async fn concurrent() -> io::Result<()> {
             .execute(Server.serve()),
     );
 
-    let client = ServiceClient::new(client::Config::default(), tx).spawn()?;
+    let client = ServiceClient::new(client::Config::default(), tx).spawn();
 
     let req1 = client.add(context::current(), 1, 2);
     let req2 = client.add(context::current(), 3, 4);
@@ -166,8 +161,8 @@ async fn concurrent() -> io::Result<()> {
 }
 
 #[tokio::test]
-async fn concurrent_join() -> io::Result<()> {
+async fn concurrent_join() -> anyhow::Result<()> {
-    let _ = env_logger::try_init();
+    let _ = tracing_subscriber::fmt::try_init();
 
     let (tx, rx) = channel::unbounded();
     tokio::spawn(
@@ -176,7 +171,7 @@ async fn concurrent_join() -> io::Result<()> {
             .execute(Server.serve()),
     );
 
-    let client = ServiceClient::new(client::Config::default(), tx).spawn()?;
+    let client = ServiceClient::new(client::Config::default(), tx).spawn();
 
     let req1 = client.add(context::current(), 1, 2);
     let req2 = client.add(context::current(), 3, 4);
@@ -191,8 +186,8 @@ async fn concurrent_join() -> io::Result<()> {
 }
 
 #[tokio::test]
-async fn concurrent_join_all() -> io::Result<()> {
+async fn concurrent_join_all() -> anyhow::Result<()> {
-    let _ = env_logger::try_init();
+    let _ = tracing_subscriber::fmt::try_init();
 
     let (tx, rx) = channel::unbounded();
     tokio::spawn(
@@ -201,7 +196,7 @@ async fn concurrent_join_all() -> io::Result<()> {
             .execute(Server.serve()),
     );
 
-    let client = ServiceClient::new(client::Config::default(), tx).spawn()?;
+    let client = ServiceClient::new(client::Config::default(), tx).spawn();
 
     let req1 = client.add(context::current(), 1, 2);
     let req2 = client.add(context::current(), 3, 4);
@@ -214,7 +209,7 @@ async fn concurrent_join_all() -> io::Result<()> {
 }
 
 #[tokio::test]
-async fn counter() -> io::Result<()> {
+async fn counter() -> anyhow::Result<()> {
     #[tarpc::service]
     trait Counter {
         async fn count() -> u32;
@@ -241,7 +236,7 @@ async fn counter() -> io::Result<()> {
         }
     });
 
-    let client = CounterClient::new(client::Config::default(), tx).spawn()?;
+    let client = CounterClient::new(client::Config::default(), tx).spawn();
     assert_matches!(client.count(context::current()).await, Ok(1));
     assert_matches!(client.count(context::current()).await, Ok(2));