Check if there is error on waiting for GL Fence.

ui/gl/gl_fence_arb.cc

 // Copyright 2014 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.
 
 #include "ui/gl/gl_fence_arb.h"
 
 #include "ui/gl/gl_bindings.h"
 #include "ui/gl/gl_context.h"
 
 namespace gfx {
@@ skipping 10 matching lines @@
 
 bool GLFenceARB::HasCompleted() {
   // Handle the case where FenceSync failed.
   if (!sync_)
     return true;
 
   DCHECK_EQ(GL_TRUE, glIsSync(sync_));
   // We could potentially use glGetSynciv here, but it doesn't work
   // on OSX 10.7 (always says the fence is not signaled yet).
   // glClientWaitSync works better, so let's use that instead.
-  return glClientWaitSync(sync_, 0, 0) != GL_TIMEOUT_EXPIRED;
+  GLenum result = glClientWaitSync(sync_, GL_SYNC_FLUSH_COMMANDS_BIT, 0);

[no sievers, 2014/10/31 18:49:32]

GL_SYNC_FLUSH_COMMANDS_BIT doesn't work here because we might call this function
with a different context current than the one we created the fence in.
Also it's not needed since we call glFlush() in the constructor.

+  DCHECK_NE(GL_WAIT_FAILED, result);

[no sievers, 2014/10/31 18:49:32]

The spec says it can only fail if you use an illegal flag, which we don't, or if
|sync| is not the name of a sync object which we handle in line 24.

So not sure why it would fail. Maybe some driver internal bug? I wonder if it
would also generate a GL error in that case which we should then clear here.

+  return result != GL_TIMEOUT_EXPIRED;
 }
 
 void GLFenceARB::ClientWait() {
   DCHECK_EQ(GL_TRUE, glIsSync(sync_));
   if (!flush_event_.get() || flush_event_->IsSignaled()) {
     glClientWaitSync(sync_, GL_SYNC_FLUSH_COMMANDS_BIT, GL_TIMEOUT_IGNORED);
   } else {
     LOG(ERROR) << "Trying to wait for uncommitted fence. Skipping...";
   }
 }
 
 void GLFenceARB::ServerWait() {
   DCHECK_EQ(GL_TRUE, glIsSync(sync_));
   if (!flush_event_.get() || flush_event_->IsSignaled()) {
     glWaitSync(sync_, 0, GL_TIMEOUT_IGNORED);
   } else {
     LOG(ERROR) << "Trying to wait for uncommitted fence. Skipping...";
   }
 }
 
 GLFenceARB::~GLFenceARB() {
   DCHECK_EQ(GL_TRUE, glIsSync(sync_));
   glDeleteSync(sync_);
 }
 
 }  // namespace gfx