Make PartitionConsumer.Close idempotent

consumer.go

@@ -310,6 +310,7 @@ type partitionConsumer struct {
 
 	trigger, dying chan none
 	responseResult error
+	closeOnce      sync.Once
 
 	fetchSize int32
 	offset    int64
@@ -412,7 +413,9 @@ func (child *partitionConsumer) AsyncClose() {
 	// the dispatcher to exit its loop, which removes it from the consumer then closes its 'messages' and
 	// 'errors' channel (alternatively, if the child is already at the dispatcher for some reason, that will
 	// also just close itself)
-	close(child.dying)
+	child.closeOnce.Do(func() {
+		close(child.dying)
+	})
 }
 
 func (child *partitionConsumer) Close() error {