Fix a rare crash when running simulate on macOS.

The bug was introduced in 2f0fb1e4ef. The glfwSwapBuffers call should occur on the main UI thread, not the CoreVideo DisplayLink thread.

PiperOrigin-RevId: 517172202
Change-Id: I2491f2194765019748198cd0cc61ca9263331b01

simulate/glfw_adapter.cc

@@ -175,14 +175,10 @@ bool GlfwAdapter::ShouldCloseWindow() const {
 void GlfwAdapter::SwapBuffers() {
 #ifdef __APPLE__
   if (core_video_.has_value()) {
-    core_video_->EnqueueSwap();
+    core_video_->WaitForDisplayRefresh();
-    core_video_->WaitForSwap();
-  } else {
-    Glfw().glfwSwapBuffers(window_);
   }
-#else
-  Glfw().glfwSwapBuffers(window_);
 #endif
+  Glfw().glfwSwapBuffers(window_);
 }
 
 void GlfwAdapter::ToggleFullscreen() {

simulate/glfw_corevideo.h

@@ -19,6 +19,7 @@
 #error "This header only works on macOS."
 #endif
 
+#include <atomic>
 #include <condition_variable>
 #include <mutex>
 
@@ -37,8 +38,8 @@ class GlfwCoreVideo {
  public:
   GlfwCoreVideo(GLFWwindow* window);
   ~GlfwCoreVideo();
-  void EnqueueSwap();
+
-  void WaitForSwap();
+  void WaitForDisplayRefresh();
   int DisplayLinkCallback();
   void UpdateDisplayLink();
 
@@ -46,7 +47,7 @@ class GlfwCoreVideo {
   GLFWwindow* window_;
   CVDisplayLinkRef display_link_;
 
-  bool second_buffer_has_content_;
+  std::atomic_bool waiting_;
   std::mutex mu_;
   std::condition_variable cond_;
 };

simulate/glfw_corevideo.mm

@@ -45,23 +45,16 @@ GlfwCoreVideo::~GlfwCoreVideo() {
   CVDisplayLinkRelease(display_link_);
 }
 
-void GlfwCoreVideo::EnqueueSwap() {
+void GlfwCoreVideo::WaitForDisplayRefresh() {
   std::unique_lock lock(mu_);
-  second_buffer_has_content_ = true;
+  waiting_.store(true);
-}
+  cond_.wait(lock, [this]() { return !this->waiting_; });
-
-void GlfwCoreVideo::WaitForSwap() {
-  if (second_buffer_has_content_) {
-    std::unique_lock lock(mu_);
-    cond_.wait(lock, [this]() { return !this->second_buffer_has_content_; });
-  }
 }
 
 int GlfwCoreVideo::DisplayLinkCallback() {
-  if (second_buffer_has_content_) {
+  if (waiting_.load()) {
     std::unique_lock lock(mu_);
-    Glfw().glfwSwapBuffers(window_);
+    waiting_.store(false);
-    second_buffer_has_content_ = false;
     cond_.notify_one();
   }
   return kCVReturnSuccess;