Fix backpressure waits.

Author: chinmaygarde

impeller/renderer/backend/vulkan/swapchain/ahb/ahb_swapchain_vk.cc

@@ -134,8 +134,6 @@ std::shared_ptr<AHBSurfaceVK> AHBSwapchainVK::CreateNewSurface() {
 std::shared_ptr<Surface> AHBSwapchainVK::AcquireNextDrawable() {
   TRACE_EVENT0("impeller", __FUNCTION__);
 
-  Lock lock(mutex_);
-
   {
     TRACE_EVENT0("impeller", "CompositorBackpressure");
     // Accompanying signal is in the call to recycle the surface after the
@@ -146,6 +144,8 @@ std::shared_ptr<Surface> AHBSwapchainVK::AcquireNextDrawable() {
     }
   }
 
+  Lock lock(mutex_);
+
   if (auto surface = PopRecyclable()) {
     FML_LOG(IMPORTANT) << "Recycled surface.";
     return surface;
@@ -204,8 +204,10 @@ bool AHBSwapchainVK::PresentSurface(
 
 void AHBSwapchainVK::OnSurfaceDidCompleteBeingUsedByCompositor(
     std::shared_ptr<AHBSurfaceVK> surface) {
-  Lock lock(mutex_);
-  PushRecyclable(std::move(surface));
+  {
+    Lock lock(mutex_);
+    PushRecyclable(std::move(surface));
+  }
   drawable_count_sema_.Signal();
 }
 

impeller/renderer/backend/vulkan/swapchain/ahb/ahb_swapchain_vk.h

@@ -65,8 +65,8 @@ class AHBSwapchainVK final
   std::weak_ptr<Context> context_;
   std::shared_ptr<android::NativeWindow> native_window_;
   std::shared_ptr<android::SurfaceControl> surface_control_;
+  fml::Semaphore drawable_count_sema_;
   mutable Mutex mutex_;
-  fml::Semaphore drawable_count_sema_ IPLR_GUARDED_BY(mutex_);
   android::HardwareBufferDescriptor desc_ IPLR_GUARDED_BY(mutex_);
   std::deque<Recyclable> recyclable_ IPLR_GUARDED_BY(mutex_);
   bool is_valid_ = false;