Fabric: Proper thread synchronization in MainRunLoopEventBeat

Summary:
To dispatch events synchonously on the main thread we still have to block the Message Queue.
We actually dispatch events on the Message Queue while the main thread is waiting.

Reviewed By: mdvacca

Differential Revision: D9799853

fbshipit-source-id: 8033be36f27371ad2f1dc7210d564fbca1174910

React/Fabric/RCTScheduler.mm

@@ -58,8 +58,8 @@ private:
 
     SharedContextContainer contextContainer = std::make_shared<ContextContainer>();
 
-    EventBeatFactory synchronousBeatFactory = []() {
-      return std::make_unique<MainRunLoopEventBeat>();
+    EventBeatFactory synchronousBeatFactory = [bridge]() {
+      return std::make_unique<MainRunLoopEventBeat>(bridge.jsMessageThread);
     };
 
     EventBeatFactory asynchronousBeatFactory = [bridge]() {

React/Fabric/Utils/MainRunLoopEventBeat.h

@@ -7,6 +7,7 @@
 
 #include <CoreFoundation/CoreFoundation.h>
 #include <CoreFoundation/CFRunLoop.h>
+#include <cxxreact/MessageQueueThread.h>
 #include <fabric/events/EventBeat.h>
 
 namespace facebook {
@@ -20,12 +21,15 @@ class MainRunLoopEventBeat final:
   public EventBeat {
 
 public:
-  MainRunLoopEventBeat();
+  MainRunLoopEventBeat(std::shared_ptr<MessageQueueThread> messageQueueThread);
   ~MainRunLoopEventBeat();
 
   void induce() const override;
 
 private:
+  void blockMessageQueueAndThenBeat() const;
+
+  std::shared_ptr<MessageQueueThread> messageQueueThread_;
   CFRunLoopObserverRef mainRunLoopObserver_;
 };
 

React/Fabric/Utils/MainRunLoopEventBeat.mm

@@ -5,12 +5,15 @@
 
 #import "MainRunLoopEventBeat.h"
 
+#import <mutex>
 #import <React/RCTUtils.h>
 
 namespace facebook {
 namespace react {
 
-MainRunLoopEventBeat::MainRunLoopEventBeat() {
+MainRunLoopEventBeat::MainRunLoopEventBeat(std::shared_ptr<MessageQueueThread> messageQueueThread):
+  messageQueueThread_(std::move(messageQueueThread)) {
+
   mainRunLoopObserver_ =
     CFRunLoopObserverCreateWithHandler(
       NULL /* allocator */,
@@ -18,7 +21,11 @@ MainRunLoopEventBeat::MainRunLoopEventBeat() {
       true /* repeats */,
       0 /* order */,
       ^(CFRunLoopObserverRef observer, CFRunLoopActivity activity) {
-        this->beat();
+        if (!this->isRequested_) {
+          return;
+        }
+
+        this->blockMessageQueueAndThenBeat();
       }
   );
 
@@ -38,9 +45,33 @@ void MainRunLoopEventBeat::induce() const {
   }
 
   RCTExecuteOnMainQueue(^{
-    this->beat();
+    this->blockMessageQueueAndThenBeat();
   });
 }
 
+void MainRunLoopEventBeat::blockMessageQueueAndThenBeat() const {
+  // Note: We need the third mutex to get back to the main thread before
+  // the lambda is finished (because all mutexes are allocated on the stack).
+
+  std::mutex mutex1;
+  std::mutex mutex2;
+  std::mutex mutex3;
+
+  mutex1.lock();
+  mutex2.lock();
+  mutex3.lock();
+
+  messageQueueThread_->runOnQueue([&]() {
+    mutex1.unlock();
+    mutex2.lock();
+    mutex3.unlock();
+  });
+
+  mutex1.lock();
+  beat();
+  mutex2.unlock();
+  mutex3.lock();
+}
+
 } // namespace react
 } // namespace facebook