gpu: Allow fences to check whether a flush has occurred

gpu/command_buffer/service/texture_definition.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 "gpu/command_buffer/service/texture_definition.h"
 
 #include "gpu/command_buffer/service/texture_manager.h"
 #include "ui/gl/gl_image.h"
 #include "ui/gl/gl_implementation.h"
 #include "ui/gl/scoped_binders.h"
@@ skipping 260 matching lines @@
     if (it->client == client) {
       it->read_fence = make_linked_ptr(gfx::GLFence::Create());
       return;
     }
   }
   NOTREACHED();
 }
 
 void NativeImageBuffer::DidWrite(gfx::GLImage* client) {
   base::AutoLock lock(lock_);
-  // TODO(sievers): crbug.com/352419
-  // This is super-risky. We need to somehow find out about when the current
-  // context gets flushed, so that we will only ever wait on the write fence
-  // (esp. from another context) if it was flushed and is guaranteed to clear.
-  // On the other hand, proactively flushing here is not feasible in terms
-  // of perf when there are multiple draw calls per frame.
+  // Sharing semantics require the client to flush in order to make changes
+  // visible to other clients.
   write_fence_.reset(gfx::GLFence::CreateWithoutFlush());
   write_client_ = client;
   for (std::list<ClientInfo>::iterator it = client_infos_.begin();
        it != client_infos_.end();
        it++) {
     it->needs_wait_before_read = true;
   }
 }
 
 TextureDefinition::LevelInfo::LevelInfo(GLenum target,
@@ skipping 138 matching lines @@
 
   // All structural changes should have orphaned the texture.
   if (image_buffer_ && !texture->GetLevelImage(texture->target(), 0))
     return false;
 
   return true;
 }
 
 }  // namespace gles2
 }  // namespace gpu