Retry topic subscription on quota errors using exponential backoff (#6098)

Topic subscription requests might hit a quota, but we can retry these
failures with exponential backoff incase they are transient. Retries will only be attempted as long as the app lives and will eventually backoff to 8 hours, so even if an app is consistently at quota, these extra sporadic retries should be harmless.

Fixes #6032

Author: welishr

firebase-messaging/CHANGELOG.md

@@ -1,5 +1,6 @@
 # Unreleased
-
+* [changed] Retry Topic Subscribe/Unsubscribe operations with exponential
+  backoff if they hit a quota error.
 
 # 24.0.0
 * [changed] Switched Firelog to use the new TransportBackend.

firebase-messaging/src/main/java/com/google/firebase/messaging/GmsRpc.java

@@ -58,6 +58,12 @@ class GmsRpc {
   /** Another server error besides ERROR_SERVICE_NOT_AVAILABLE that we retry on. */
   static final String ERROR_INTERNAL_SERVER_ERROR = "INTERNAL_SERVER_ERROR";
 
+  /**
+   * A server error that represents hitting topic subscription quota. Trying again here may
+   * continue to fail, but as long as we use exponential backoff its okay to retry.
+   */
+  static final String TOO_MANY_SUBSCRIBERS = "TOO_MANY_SUBSCRIBERS";
+
   /** Heartbeat tag for firebase iid. */
   static final String FIREBASE_IID_HEARTBEAT_TAG = "fire-iid";
 

firebase-messaging/src/main/java/com/google/firebase/messaging/TopicsSubscriber.java

@@ -251,7 +251,8 @@ boolean performTopicOperation(TopicOperation topicOperation) throws IOException
     } catch (IOException e) {
       // Operation failed, retry failed only if errors from backend are server related error
       if (GmsRpc.ERROR_SERVICE_NOT_AVAILABLE.equals(e.getMessage())
-          || GmsRpc.ERROR_INTERNAL_SERVER_ERROR.equals(e.getMessage())) {
+          || GmsRpc.ERROR_INTERNAL_SERVER_ERROR.equals(e.getMessage())
+          || GmsRpc.TOO_MANY_SUBSCRIBERS.equals(e.getMessage())) {
         Log.e(TAG, "Topic operation failed: " + e.getMessage() + ". Will retry Topic operation.");
 
         return false; // will retry