DCHECK that listener thread != IO thread

ipc/ipc_sync_channel.cc

 // 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.
 
 #include "ipc/ipc_sync_channel.h"
 
 #include "base/bind.h"
 #include "base/debug/trace_event.h"
 #include "base/lazy_instance.h"
 #include "base/location.h"
@@ skipping 394 matching lines @@
 }
 
 SyncChannel::SyncChannel(
     const IPC::ChannelHandle& channel_handle,
     Channel::Mode mode,
     Listener* listener,
     base::SingleThreadTaskRunner* ipc_task_runner,
     bool create_pipe_now,
     WaitableEvent* shutdown_event)
     : ChannelProxy(new SyncContext(listener, ipc_task_runner, shutdown_event)) {
+  // The current (listener) thread must be distinct from the IPC thread, or else
+  // sending synchronous messages will deadlock.
+  DCHECK_NE(ipc_task_runner, base::ThreadTaskRunnerHandle::Get());
   ChannelProxy::Init(channel_handle, mode, create_pipe_now);
   StartWatching();
 }
 
 SyncChannel::SyncChannel(
     Listener* listener,
     base::SingleThreadTaskRunner* ipc_task_runner,
     WaitableEvent* shutdown_event)
     : ChannelProxy(new SyncContext(listener, ipc_task_runner, shutdown_event)) {
+  // The current (listener) thread must be distinct from the IPC thread, or else
+  // sending synchronous messages will deadlock.
+  DCHECK_NE(ipc_task_runner, base::ThreadTaskRunnerHandle::Get());
   StartWatching();
 }
 
 SyncChannel::~SyncChannel() {
 }
 
 void SyncChannel::SetRestrictDispatchChannelGroup(int group) {
   sync_context()->set_restrict_dispatch_group(group);
 }
 
@@ skipping 116 matching lines @@
   // manual reset since the object watcher might otherwise reset the event
   // when we're doing a WaitMany.
   dispatch_watcher_callback_ =
       base::Bind(&SyncChannel::OnWaitableEventSignaled,
                   base::Unretained(this));
   dispatch_watcher_.StartWatching(sync_context()->GetDispatchEvent(),
                                   dispatch_watcher_callback_);
 }
 
 }  // namespace IPC