[macOS] Do not block raster thread when shutting down

Author: knopp

shell/platform/darwin/macos/framework/Source/FlutterThreadSynchronizer.mm

@@ -79,6 +79,11 @@ - (void)performCommit:(CGSize)size notify:(nonnull dispatch_block_t)notify {
   fml::AutoResetWaitableEvent event;
   {
     std::unique_lock<std::mutex> lock(_mutex);
+    if (_shuttingDown) {
+      // FlutterView is shutting down and will not drain schedued blocks
+      // on main thread anymore.
+      return;
+    }
     fml::AutoResetWaitableEvent& e = event;
     _scheduledBlocks.push_back(^{
       notify();

shell/platform/darwin/macos/framework/Source/FlutterViewTest.mm

@@ -5,6 +5,7 @@
 #import "flutter/shell/platform/darwin/macos/framework/Source/FlutterView.h"
 
 #import <Metal/Metal.h>
+#include <thread>
 
 #import "flutter/testing/testing.h"
 
@@ -28,3 +29,16 @@ - (void)viewDidReshape:(nonnull NSView*)view {
                                              reshapeListener:listener];
   EXPECT_EQ([view layer:view.layer shouldInheritContentsScale:3.0 fromWindow:view.window], YES);
 }
+
+TEST(FlutterView, ThreadSynchronizerNotBlockingRasterThreadAfterShutdown) {
+  FlutterThreadSynchronizer* threadSynchronizer = [[FlutterThreadSynchronizer alloc] init];
+  [threadSynchronizer shutdown];
+
+  std::thread rasterThread([&threadSynchronizer] {
+    [threadSynchronizer performCommit:CGSizeMake(100, 100)
+                               notify:^{
+                               }];
+  });
+
+  rasterThread.join();
+}