Use queue label as id in SequencedTaskChecker when not running on TaskQueue

webrtc/base/sequenced_task_checker_impl.cc

 /*
  *  Copyright (c) 2016 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
 #include "webrtc/base/sequenced_task_checker_impl.h"
 
+#if defined(WEBRTC_MAC)
+#include <dispatch/dispatch.h>
+#endif
+
 #include "webrtc/base/platform_thread.h"
 #include "webrtc/base/sequenced_task_checker.h"
 #include "webrtc/base/task_queue.h"
 
 namespace rtc {
 
 SequencedTaskCheckerImpl::SequencedTaskCheckerImpl()
     : attached_(true), valid_queue_(TaskQueue::Current()) {}
 
 SequencedTaskCheckerImpl::~SequencedTaskCheckerImpl() {}
 
 bool SequencedTaskCheckerImpl::CalledSequentially() const {
-  TaskQueue* current_queue = TaskQueue::Current();
+  QueueId current_queue = TaskQueue::Current();
+#if defined(WEBRTC_MAC)
+  // If we're not running on a TaskQueue, use the system dispatch queue
+  // label as an identifier.
+  if (current_queue == nullptr)
+    current_queue = dispatch_queue_get_label(DISPATCH_CURRENT_QUEUE_LABEL);
+#endif
   CritScope scoped_lock(&lock_);
   if (!attached_) {  // true if previously detached.
     valid_queue_ = current_queue;
     attached_ = true;
   }
   if (!valid_queue_)
     return thread_checker_.CalledOnValidThread();
   return valid_queue_ == current_queue;
 }
 
 void SequencedTaskCheckerImpl::Detach() {
   CritScope scoped_lock(&lock_);
   attached_ = false;
   valid_queue_ = nullptr;
   thread_checker_.DetachFromThread();
 }
 
 namespace internal {
 
 SequencedTaskCheckerScope::SequencedTaskCheckerScope(
     const SequencedTaskChecker* checker) {
   RTC_DCHECK(checker->CalledSequentially());
 }
 
 SequencedTaskCheckerScope::~SequencedTaskCheckerScope() {}
 
 }  // namespace internal
 }  // namespace rtc