[Reland 1] Pepper takes ownership of a mailbox before passing it to the texture layer.

gpu/ipc/service/gpu_command_buffer_stub.cc

Index: gpu/ipc/service/gpu_command_buffer_stub.cc
diff --git a/gpu/ipc/service/gpu_command_buffer_stub.cc b/gpu/ipc/service/gpu_command_buffer_stub.cc
index b90275aae15abbc1372ac069b8c65cde9758b463..2172f4a9763461b1b2699546b41fb74714c41674 100644
--- a/gpu/ipc/service/gpu_command_buffer_stub.cc
+++ b/gpu/ipc/service/gpu_command_buffer_stub.cc
@@ -295,8 +295,9 @@ bool GpuCommandBufferStub::OnMessageReceived(const IPC::Message& message) {
                                     OnInitialize);
     IPC_MESSAGE_HANDLER_DELAY_REPLY(GpuCommandBufferMsg_SetGetBuffer,
                                     OnSetGetBuffer);
-    IPC_MESSAGE_HANDLER(GpuCommandBufferMsg_ProduceFrontBuffer,
-                        OnProduceFrontBuffer);
+    IPC_MESSAGE_HANDLER(GpuCommandBufferMsg_TakeFrontBuffer, OnTakeFrontBuffer);
+    IPC_MESSAGE_HANDLER(GpuCommandBufferMsg_ReturnFrontBuffer,
+                        OnReturnFrontBuffer);
     IPC_MESSAGE_HANDLER_DELAY_REPLY(GpuCommandBufferMsg_WaitForTokenInRange,
                                     OnWaitForTokenInRange);
     IPC_MESSAGE_HANDLER_DELAY_REPLY(GpuCommandBufferMsg_WaitForGetOffsetInRange,
@@ -714,14 +715,22 @@ void GpuCommandBufferStub::OnSetGetBuffer(int32_t shm_id,
   Send(reply_message);
 }
 
-void GpuCommandBufferStub::OnProduceFrontBuffer(const Mailbox& mailbox) {
-  TRACE_EVENT0("gpu", "GpuCommandBufferStub::OnProduceFrontBuffer");
+void GpuCommandBufferStub::OnTakeFrontBuffer(const Mailbox& mailbox) {
+  TRACE_EVENT0("gpu", "GpuCommandBufferStub::OnTakeFrontBuffer");
   if (!decoder_) {
-    LOG(ERROR) << "Can't produce front buffer before initialization.";
+    LOG(ERROR) << "Can't take front buffer before initialization.";
     return;
   }
 
-  decoder_->ProduceFrontBuffer(mailbox);
+  decoder_->TakeFrontBuffer(mailbox);
+}
+
+void GpuCommandBufferStub::OnReturnFrontBuffer(const Mailbox& mailbox,
+                                               const SyncToken& sync_token,
+                                               bool is_lost) {
+  OnWaitFenceSync(sync_token.namespace_id(), sync_token.command_buffer_id(),
+                  sync_token.release_count());

[piman, 2016/05/03 02:52:04]

Oh, duh, sorry I missed this. OnWaitFenceSync can't work, we need to go back to
the message loop.

Doing the round trip with SignalSyncToken works, but the extra latency means
that we're going to use more memory, because we'd probably squeeze an extra
SwapBuffers (or 2) before the actual ReturnFrontBuffer happens.

Really what we need is something akin to GpuCommandBufferStub::OnWaitFenceSync,
where we'd do ReturnFrontBuffer from the callback.

Another way is to introduce a WaitSyncToken IPC that behaves just like the
WaitSyncToken command (but could be done from the renderer side) - i.e. just
call OnWaitFenceSync. From the renderer you'd send WaitSyncToken then
ReturnFrontBuffer, that would ensure the ReturnFrontBuffer isn't processed until
the fence has passed.

WDYT?

[erikchen, 2016/05/03 17:24:14]

Ah, good point. I went with your second approach and defined a new IPC message
WaitSyncToken.

+  decoder_->ReturnFrontBuffer(mailbox, is_lost);
 }
 
 void GpuCommandBufferStub::OnParseError() {