Merge pull request #15086 from grpc/fix-channel-refcount

Fix _channel ref count issue

src/objective-c/GRPCClient/private/GRPCHost.m

@@ -108,7 +108,10 @@ static NSMutableDictionary *kHostCache;
                                    serverName:(NSString *)serverName
                                       timeout:(NSTimeInterval)timeout
                               completionQueue:(GRPCCompletionQueue *)queue {
-  GRPCChannel *channel;
+  // The __block attribute is to allow channel take refcount inside @synchronized block. Without
+  // this attribute, retain of channel object happens after objc_sync_exit in release builds, which
+  // may result in channel released before used. See grpc/#15033.
+  __block GRPCChannel *channel;
   // This is racing -[GRPCHost disconnect].
   @synchronized(self) {
     if (!_channel) {