Let IPC::SyncMessageFilter take advantage of thread-safe Send.

ipc/ipc_sync_message_filter.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 IPC_IPC_SYNC_MESSAGE_FILTER_H_
 #define IPC_IPC_SYNC_MESSAGE_FILTER_H_
 
 #include <set>
 
 #include "base/basictypes.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_vector.h"
 #include "base/synchronization/lock.h"
 #include "ipc/ipc_sender.h"
 #include "ipc/ipc_sync_message.h"
 #include "ipc/message_filter.h"
 
 namespace base {
 class SingleThreadTaskRunner;
 class WaitableEvent;
 }
 
 namespace IPC {
+class SyncChannel;
 
 // This MessageFilter allows sending synchronous IPC messages from a thread
 // other than the listener thread associated with the SyncChannel.  It does not
 // support fancy features that SyncChannel does, such as handling recursion or
 // receiving messages while waiting for a response.  Note that this object can
 // be used to send simultaneous synchronous messages from different threads.
 class IPC_EXPORT SyncMessageFilter : public MessageFilter, public Sender {
  public:
-  explicit SyncMessageFilter(base::WaitableEvent* shutdown_event);
-
   // MessageSender implementation.
   bool Send(Message* message) override;
 
   // MessageFilter implementation.
   void OnFilterAdded(Sender* sender) override;
   void OnChannelError() override;
   void OnChannelClosing() override;
   bool OnMessageReceived(const Message& message) override;
 
  protected:
+  SyncMessageFilter(base::WaitableEvent* shutdown_event,
+                    bool is_channel_send_thread_safe);
+
   ~SyncMessageFilter() override;
 
  private:
+  friend class SyncChannel;
+
   void SendOnIOThread(Message* message);
   // Signal all the pending sends as done, used in an error condition.
   void SignalAllEvents();
 
   // The channel to which this filter was added.
   Sender* sender_;
 
+  // Indicates if |sender_|'s Send method is thread-safe.
+  bool is_channel_send_thread_safe_;
+
   // The process's main thread.
   scoped_refptr<base::SingleThreadTaskRunner> listener_task_runner_;
 
   // The message loop where the Channel lives.
   scoped_refptr<base::SingleThreadTaskRunner> io_task_runner_;
 
   typedef std::set<PendingSyncMsg*> PendingSyncMessages;
   PendingSyncMessages pending_sync_messages_;
 
   // Messages waiting to be delivered after IO initialization.
   ScopedVector<Message> pending_messages_;
 
   // Locks data members above.
   base::Lock lock_;
 
   base::WaitableEvent* shutdown_event_;
 
   DISALLOW_COPY_AND_ASSIGN(SyncMessageFilter);
 };
 
 }  // namespace IPC
 
 #endif  // IPC_IPC_SYNC_MESSAGE_FILTER_H_