Pepper takes ownership of a mailbox before passing it to the texture layer.

content/renderer/pepper/ppb_graphics_3d_impl.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CONTENT_RENDERER_PEPPER_PPB_GRAPHICS_3D_IMPL_H_
 #define CONTENT_RENDERER_PEPPER_PPB_GRAPHICS_3D_IMPL_H_
 
 #include <stdint.h>
 
 #include <memory>
@@ skipping 30 matching lines @@
   PP_Bool SetGetBuffer(int32_t transfer_buffer_id) override;
   scoped_refptr<gpu::Buffer> CreateTransferBuffer(uint32_t size,
                                                   int32_t* id) override;
   PP_Bool DestroyTransferBuffer(int32_t id) override;
   PP_Bool Flush(int32_t put_offset) override;
   gpu::CommandBuffer::State WaitForTokenInRange(int32_t start,
                                                 int32_t end) override;
   gpu::CommandBuffer::State WaitForGetOffsetInRange(int32_t start,
                                                     int32_t end) override;
   void EnsureWorkVisible() override;
+  void TakeFrontBuffer() override;
+  void ReturnFrontBuffer(const gpu::Mailbox& mailbox,
+                         const gpu::SyncToken& sync_token,
+                         bool is_lost);
 
   // Binds/unbinds the graphics of this context with the associated instance.
   // Returns true if binding/unbinding is successful.
   bool BindToInstance(bool bind);
 
   // Returns true if the backing texture is always opaque.
   bool IsOpaque();
 
   // Notifications about the view's progress painting.  See PluginInstance.
   // These messages are used to send Flush callbacks to the plugin.
   void ViewInitiatedPaint();
 
-  void GetBackingMailbox(gpu::Mailbox* mailbox, gpu::SyncToken* sync_token) {
-    *mailbox = mailbox_;
-    *sync_token = sync_token_;
-  }
-
   gpu::CommandBufferProxyImpl* GetCommandBufferProxy();
 
   gpu::GpuChannelHost* channel() { return channel_.get(); }
 
  protected:
   ~PPB_Graphics3D_Impl() override;
   // ppapi::PPB_Graphics3D_Shared overrides.
   gpu::CommandBuffer* GetCommandBuffer() override;
   gpu::GpuControl* GetGpuControl() override;
   int32_t DoSwapBuffers(const gpu::SyncToken& sync_token) override;
 
  private:
   explicit PPB_Graphics3D_Impl(PP_Instance instance);
 
   bool InitRaw(PPB_Graphics3D_API* share_context,
                const int32_t* attrib_list,
                gpu::Capabilities* capabilities,
                base::SharedMemoryHandle* shared_state_handle,
                gpu::CommandBufferId* command_buffer_id);
 
   // GpuControlClient implementation.
   void OnGpuControlLostContext() final;
   void OnGpuControlErrorMessage(const char* msg, int id) final;
 
   // Other notifications from the GPU process.
   void OnSwapBuffers();
   // Notifications sent to plugin.
   void SendContextLost();
 
+  // Reuses a mailbox if one is available, otherwise makes a new one.
+  gpu::Mailbox GenerateMailbox();
+
+  // A front buffer that was recently taken from the command buffer. This should
+  // be immediately consumed by DoSwapBuffers().
+  gpu::Mailbox taken_front_buffer_;
+
+  // Mailboxes that are no longer in use.
+  std::vector<gpu::Mailbox> mailboxes_to_reuse_;
+
   // True if context is bound to instance.
   bool bound_to_instance_;
   // True when waiting for compositor to commit our backing texture.
   bool commit_pending_;
 
 #if DCHECK_IS_ON()
   bool lost_context_ = false;
 #endif
 
-  gpu::Mailbox mailbox_;
-  gpu::SyncToken sync_token_;
   bool has_alpha_;
   scoped_refptr<gpu::GpuChannelHost> channel_;
   std::unique_ptr<gpu::CommandBufferProxyImpl> command_buffer_;
 
   base::WeakPtrFactory<PPB_Graphics3D_Impl> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(PPB_Graphics3D_Impl);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_RENDERER_PEPPER_PPB_GRAPHICS_3D_IMPL_H_