Allow overriding metadata url used during testing

src/application_default_credentials.rs

@@ -1,13 +1,25 @@
 use crate::error::Error;
 use crate::types::TokenInfo;
 
-pub struct ApplicationDefaultCredentialsFlowOpts;
+/// Provide options for the Application Default Credential Flow, mostly used for testing
+pub struct ApplicationDefaultCredentialsFlowOpts {
+    /// Used as base to build the url during token request from GCP metadata server
+    pub metadata_url: Option<String>,
+}
+impl Default for ApplicationDefaultCredentialsFlowOpts {
+    fn default() -> Self {
+        Self { metadata_url: None }
+    }
+}
+
+pub struct ApplicationDefaultCredentialsFlow {
+    metadata_url: String,
+}
 
-/// ServiceAccountFlow can fetch oauth tokens using a service account.
-pub struct ApplicationDefaultCredentialsFlow;
 impl ApplicationDefaultCredentialsFlow {
-    pub(crate) fn new(_opts: ApplicationDefaultCredentialsFlowOpts) -> Self {
-        ApplicationDefaultCredentialsFlow {}
+    pub(crate) fn new(opts: ApplicationDefaultCredentialsFlowOpts) -> Self {
+        let metadata_url = opts.metadata_url.unwrap_or_else(|| "http://metadata.google.internal/computeMetadata/v1/instance/service-accounts/default/token".to_string());
+        ApplicationDefaultCredentialsFlow { metadata_url }
     }
 
     pub(crate) async fn token<C, T>(
@@ -20,7 +32,7 @@ impl ApplicationDefaultCredentialsFlow {
         C: hyper::client::connect::Connect + Clone + Send + Sync + 'static,
     {
         let scope = crate::helper::join(scopes, ",");
-        let token_uri = format!("http://metadata.google.internal/computeMetadata/v1/instance/service-accounts/default/token?scopes={}", scope);
+        let token_uri = format!("{}?scopes={}", self.metadata_url, scope); // TODO: This feels jank, can it be done better?
         let request = hyper::Request::get(token_uri)
             .header("Metadata-Flavor", "Google")
             .body(hyper::Body::from(String::new())) // why body is needed?

src/authenticator.rs

@@ -274,14 +274,7 @@ impl ServiceAccountAuthenticator {
 /// ```
 pub struct ApplicationDefaultCredentialsAuthenticator;
 impl ApplicationDefaultCredentialsAuthenticator {
-    /// Use modified builder pattern to create an Authenticator that uses GCE instance metadata server
-    /// to provide tokens.
-    pub fn from_instance_metadata() -> ApplicationDefaultCredentialsFlowOpts {
-        ApplicationDefaultCredentialsFlowOpts {}
-    }
-
-    /// Use modified builder pattern to create an Authenticator that pulls default application credentials
-    /// service account file name from os environment variable.
+    /// Try to build ServiceAccountFlowOpts from the environment
     pub async fn from_environment() -> Result<ServiceAccountFlowOpts, std::env::VarError> {
         let service_account_key =
             crate::read_service_account_key(std::env::var("GOOGLE_APPLICATION_CREDENTIALS")?)
@@ -296,12 +289,17 @@ impl ApplicationDefaultCredentialsAuthenticator {
 
     /// Use the builder pattern to deduce which model of authenticator should be used:
     /// Service account one or GCE instance metadata kind
-    pub async fn builder() -> ApplicationDefaultCredentialsTypes<DefaultHyperClient> {
-        Self::with_client(DefaultHyperClient).await
+    pub async fn builder(
+        opts: ApplicationDefaultCredentialsFlowOpts,
+    ) -> ApplicationDefaultCredentialsTypes<DefaultHyperClient> {
+        Self::with_client(DefaultHyperClient, opts).await
     }
 
     /// Use the builder pattern to deduce which model of authenticator should be used and allow providing a hyper client
-    pub async fn with_client<C>(client: C) -> ApplicationDefaultCredentialsTypes<C>
+    pub async fn with_client<C>(
+        client: C,
+        opts: ApplicationDefaultCredentialsFlowOpts,
+    ) -> ApplicationDefaultCredentialsTypes<C>
     where
         C: HyperClientBuilder,
     {
@@ -311,12 +309,9 @@ impl ApplicationDefaultCredentialsAuthenticator {
 
                 ApplicationDefaultCredentialsTypes::ServiceAccount(builder)
             }
-            Err(_) => {
-                ApplicationDefaultCredentialsTypes::InstanceMetadata(AuthenticatorBuilder::new(
-                    ApplicationDefaultCredentialsAuthenticator::from_instance_metadata(),
-                    client,
-                ))
-            }
+            Err(_) => ApplicationDefaultCredentialsTypes::InstanceMetadata(
+                AuthenticatorBuilder::new(opts, client),
+            ),
         }
     }
 }

src/lib.rs

@@ -97,6 +97,7 @@ pub use crate::authenticator::{
 pub use crate::helper::*;
 pub use crate::installed::InstalledFlowReturnMethod;
 
+pub use crate::application_default_credentials::ApplicationDefaultCredentialsFlowOpts;
 pub use crate::service_account::ServiceAccountKey;
 
 #[doc(inline)]