fix: Subscription message handling - serial queue and stop after failed event

AppSyncRealTimeClient/Connection/AppSyncConnection/AppSyncSubscriptionConnection+ErrorHandler.swift

@@ -32,6 +32,7 @@ extension AppSyncSubscriptionConnection {
             let connectionError = error as? ConnectionProviderError
         else {
             subscriptionItem.subscriptionEventHandler(.failed(error), subscriptionItem)
+            connectionProvider?.removeListener(identifier: subscriptionItem.identifier)
             return
         }
 
@@ -43,6 +44,7 @@ extension AppSyncSubscriptionConnection {
             }
         } else {
             subscriptionItem.subscriptionEventHandler(.failed(error), subscriptionItem)
+            connectionProvider?.removeListener(identifier: subscriptionItem.identifier)
         }
     }
 

AppSyncRealTimeClient/Connection/AppSyncConnection/AppSyncSubscriptionConnection.swift

@@ -28,6 +28,10 @@ public class AppSyncSubscriptionConnection: SubscriptionConnection, RetryableCon
     /// Retry logic to handle
     var retryHandler: ConnectionRetryHandler?
 
+    let serialQueue = DispatchQueue(
+        label: "com.amazonaws.AppSyncSubscriptionConnection.serialQueue"
+    )
+
     public init(provider: ConnectionProvider) {
         self.connectionProvider = provider
     }
@@ -84,6 +88,7 @@ public class AppSyncSubscriptionConnection: SubscriptionConnection, RetryableCon
                 AppSyncLogger.debug("[AppSyncSubscriptionConnection] \(#function): Self is nil, listener is not called.")
                 return
             }
+            self.serialQueue.async {
                 switch event {
                 case .connection(let state):
                     self.handleConnectionEvent(connectionState: state)
@@ -94,6 +99,7 @@ public class AppSyncSubscriptionConnection: SubscriptionConnection, RetryableCon
                 }
             }
         }
+    }
 
     public func addRetryHandler(handler: ConnectionRetryHandler) {
         retryHandler = handler