Coroutines: Channel.offer can still be called after channel has been closed.

[EtienneTheScore]

Hi!

We use a wrapper around ApolloSubscriptionCall<T>.toChannel extension and we're having some crashes although we use it inside a CoroutineScope with an exception handler.

Error:

com.apollographql.apollo.internal.subscription.ApolloSubscriptionTerminatedException: 
  at com.myapp.extensions.ApolloExtensionsKt$toChannelHandleTerminate$2.onTerminated (ApolloExtensions.kt:39)
  at com.apollographql.apollo.internal.RealApolloSubscriptionCall$SubscriptionManagerCallback.onTerminated (RealApolloSubscriptionCall.java:168)
  at com.apollographql.apollo.internal.subscription.RealSubscriptionManager.onConnectionClosed (RealSubscriptionManager.java:284)
  at com.apollographql.apollo.internal.subscription.RealSubscriptionManager$SubscriptionTransportCallback$4.run (RealSubscriptionManager.java:457)
  at java.util.concurrent.ThreadPoolExecutor.runWorker (ThreadPoolExecutor.java:1167)
  at java.util.concurrent.ThreadPoolExecutor$Worker.run (ThreadPoolExecutor.java:641)
  at java.lang.Thread.run (Thread.java:764)

After investigation, we found that this is caused by some concurrency issues inside ApolloSubscriptionCall.Callback.

The method channel.offer(response) can still be called after the channel has been closed. And since the callback use a separate Thread, the exception isn't caught by the coroutine's exception handler, causing a crash.