Authenticator should handle the server not returning a refresh_token.

Currently the authenticator will panic when trying to refresh an expired
token that does not have a refresh token. This change handles it so that
the authenticator will only attempt a refresh when a refresh_token
exists, and otherwise will attempt to retrieve a fresh token.

src/authenticator.rs

@@ -196,17 +196,16 @@ where
         let appsecret = gettoken.application_secret();
         loop {
             match store.get(scope_key, scopes) {
-                Ok(Some(t)) => {
-                    if !t.expired() {
-                        return Ok(t);
-                    }
-                    // Implement refresh flow.
-                    let rr = RefreshFlow::refresh_token(
-                        client,
-                        appsecret,
-                        &t.refresh_token.as_ref().unwrap(),
-                    )
-                    .await?;
+                Ok(Some(t)) if !t.expired() => {
+                    // unexpired token found
+                    return Ok(t);
+                }
+                Ok(Some(Token {
+                    refresh_token: Some(refresh_token),
+                    ..
+                })) => {
+                    // token is expired but has a refresh token.
+                    let rr = RefreshFlow::refresh_token(client, appsecret, &refresh_token).await?;
                     match rr {
                         RefreshResult::Error(ref e) => {
                             delegate.token_refresh_failed(
@@ -236,7 +235,12 @@ where
                         }
                     }
                 }
-                Ok(None) => {
+                Ok(None)
+                | Ok(Some(Token {
+                    refresh_token: None,
+                    ..
+                })) => {
+                    // no token in the cache or the token returned does not contain a refresh token.
                     let t = gettoken.token(scopes).await?;
                     if let Err(e) = store.set(scope_key, scopes, Some(t.clone())) {
                         match delegate.token_storage_failure(true, &e) {

src/installed.rs

@@ -255,7 +255,7 @@ where
         #[derive(Deserialize)]
         struct JSONTokenResponse {
             access_token: String,
-            refresh_token: String,
+            refresh_token: Option<String>,
             token_type: String,
             expires_in: Option<i64>,
         }
@@ -270,7 +270,7 @@ where
             }) => {
                 let mut token = Token {
                     access_token,
-                    refresh_token: Some(refresh_token),
+                    refresh_token,
                     token_type,
                     expires_in,
                     expires_in_timestamp: None,