address comment

src/objective-c/GRPCClient/GRPCCall.m

@@ -401,6 +401,7 @@ const char *kCFStreamVarName = "grpc_cfstream";
 }
 
 - (void)receiveNextMessages:(NSUInteger)numberOfMessages {
+  // branching based on _callOptions.enableFlowControl is handled inside _call
   GRPCCall *copiedCall = nil;
   @synchronized(self) {
     copiedCall = _call;
@@ -716,8 +717,13 @@ const char *kCFStreamVarName = "grpc_cfstream";
         @synchronized(strongSelf) {
           [strongSelf->_responseWriteable enqueueValue:data
                                      completionHandler:^{
+                                       __strong GRPCCall *strongSelf = weakSelf;
+                                       if (strongSelf) {
+                                         @synchronized (strongSelf) {
                                            strongSelf->_pendingCoreRead = NO;
                                            [strongSelf maybeStartNextRead];
+                                         }
+                                       }
                                      }];
         }
       }

src/objective-c/GRPCClient/GRPCCallOptions.h

@@ -243,8 +243,13 @@ typedef NS_ENUM(NSUInteger, GRPCTransportType) {
 /**
  * Enable flow control of a gRPC call. The option is default to NO. If set to YES, writeData: method
  * should only be called at most once before a didWriteData callback is issued, and
- * receiveNextMessage: must be called each time before gRPC call issues a didReceiveMessage
+ * receiveNextMessage: must be called each time before gRPC call can issues a didReceiveMessage
  * callback.
+ *
+ * If writeData: method is called more than once before issuance of a didWriteData callback, gRPC
+ * will continue to queue the message and write them to gRPC core in order. However, the user
+ * assumes their own responsibility of flow control by keeping tracking of the pending writes in
+ * the call.
  */
 @property(readwrite) BOOL enableFlowControl;