Fix flickering.

Author: chinmaygarde

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

@@ -123,7 +123,7 @@ bool AHBSwapchainImplVK::Present(
     if (!thiz) {
       return;
     }
-    thiz->pool_->Push(texture);
+    thiz->OnTextureSetOnSurfaceControl(texture);
   });
 }
 
@@ -145,4 +145,15 @@ std::shared_ptr<ExternalFenceVK> AHBSwapchainImplVK::SubmitCompletionSignal()
   return fence;
 }
 
+void AHBSwapchainImplVK::OnTextureSetOnSurfaceControl(
+    std::shared_ptr<AHBTextureSourceVK> texture) {
+  // The transaction completion indicates that the surface control now
+  // references the hardware buffer. We can recycle the previous set buffer
+  // safely.
+  Lock lock(currently_displayed_texture_mutex_);
+  auto old_texture = currently_displayed_texture_;
+  currently_displayed_texture_ = std::move(texture);
+  pool_->Push(std::move(old_texture));
+}
+
 }  // namespace impeller

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

@@ -7,6 +7,7 @@
 
 #include <memory>
 
+#include "impeller/base/thread.h"
 #include "impeller/renderer/backend/vulkan/android/ahb_texture_source_vk.h"
 #include "impeller/renderer/backend/vulkan/swapchain/ahb/ahb_texture_pool_vk.h"
 #include "impeller/renderer/backend/vulkan/swapchain/ahb/external_fence_vk.h"
@@ -90,6 +91,11 @@ class AHBSwapchainImplVK final
   android::HardwareBufferDescriptor desc_;
   std::shared_ptr<AHBTexturePoolVK> pool_;
   std::shared_ptr<SwapchainTransientsVK> transients_;
+  // In C++20, this mutex can be replaced by the shared pointer specialization
+  // of std::atomic.
+  Mutex currently_displayed_texture_mutex_;
+  std::shared_ptr<AHBTextureSourceVK> currently_displayed_texture_
+      IPLR_GUARDED_BY(currently_displayed_texture_mutex_);
   bool is_valid_ = false;
 
   explicit AHBSwapchainImplVK(
@@ -101,6 +107,9 @@ class AHBSwapchainImplVK final
   bool Present(const std::shared_ptr<AHBTextureSourceVK>& texture);
 
   std::shared_ptr<ExternalFenceVK> SubmitCompletionSignal() const;
+
+  void OnTextureSetOnSurfaceControl(
+      std::shared_ptr<AHBTextureSourceVK> texture);
 };
 
 }  // namespace impeller

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

@@ -38,6 +38,9 @@ std::shared_ptr<AHBTextureSourceVK> AHBTexturePoolVK::Pop() {
 }
 
 void AHBTexturePoolVK::Push(std::shared_ptr<AHBTextureSourceVK> texture) {
+  if (!texture) {
+    return;
+  }
   Lock lock(pool_mutex_);
   pool_.push_back(PoolEntry{std::move(texture)});
   PerformGCLocked();

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

@@ -25,6 +25,8 @@ namespace impeller {
 ///             new entry is created. The only case where a valid texture source
 ///             cannot be obtained is due to resource exhaustion.
 ///
+///             Pools are thread-safe.
+///
 class AHBTexturePoolVK {
  public:
   //----------------------------------------------------------------------------
@@ -65,6 +67,8 @@ class AHBTexturePoolVK {
   /// @brief      Pops an texture source from the pool. If the pool is empty, a
   ///             new texture source is created and returned.
   ///
+  ///             This operation is thread-safe.
+  ///
   /// @return     A texture source that can be used as a swapchain image. This
   ///             can be nullptr in case of resource exhaustion.
   ///
@@ -74,6 +78,8 @@ class AHBTexturePoolVK {
   /// @brief      Push a popped texture back into the pool. This also performs a
   ///             GC.
   ///
+  ///             This operation is thread-safe.
+  ///
   /// @warning    Only a texture source obtained from the same pool can be
   ///             returned to it. It is user error to mix and match texture
   ///             sources from different pools.