Fix potential deadlock in TaskQueue's libevent PostTaskAndReply implementation

webrtc/base/task_queue_unittest.cc

 /*
  *  Copyright 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.
  */
 
@@ skipping 188 matching lines @@
   TaskQueue reply_queue(kReplyQueue);
 
   Event event(false, false);
   bool my_flag = false;
   post_queue.PostTaskAndReply([&my_flag]() { my_flag = true; },
                               [&event]() { event.Set(); }, &reply_queue);
   EXPECT_TRUE(event.Wait(1000));
   EXPECT_TRUE(my_flag);
 }
 
+// This test covers a particular bug that we had in the libevent implementation
+// where we could hit a deadlock while trying to post a reply task to a queue
+// that was being deleted.  The test isn't guaranteed to hit that case but it's
+// written in a way that makes it likely and by running with --gtest_repeat=1000
+// the bug would occur. Alas, now it should be fixed.
+TEST(TaskQueueTest, PostAndReplyDeadlock) {
+  static const char kPostQueue[] = "PostQueue";
+  static const char kReplyQueue[] = "ReplyQueue";
+  std::unique_ptr<TaskQueue> post_queue(new TaskQueue(kPostQueue));
+  std::unique_ptr<TaskQueue> reply_queue(new TaskQueue(kReplyQueue));
+
+  Event event(false, false);
+  post_queue->PostTaskAndReply([&event]() { event.Set(); }, []() {},
+                               reply_queue.get());
+  EXPECT_TRUE(event.Wait(1000));
+  reply_queue.reset();
+}
+
 void TestPostTaskAndReply(TaskQueue* work_queue,
                           const char* work_queue_name,
                           Event* event) {
   ASSERT_FALSE(work_queue->IsCurrent());
   work_queue->PostTaskAndReply(
       Bind(&CheckCurrent, work_queue_name, nullptr, work_queue),
       NewClosure([event]() { event->Set(); }));
 }
 
 // Does a PostTaskAndReply from within a task to post and reply to the current
@@ skipping 33 matching lines @@
       queue.PostTask(NewClosure([&tasks_executed]() { ++tasks_executed; },
                                 [&tasks_cleaned_up]() { ++tasks_cleaned_up; }));
     event.Set();  // Unblock the first task.
   }
 
   EXPECT_GE(tasks_cleaned_up, tasks_executed);
   EXPECT_EQ(kTaskCount, tasks_cleaned_up);
 }
 
 }  // namespace rtc