Fix potential deadlock in TaskQueue's libevent PostTaskAndReply implementation

webrtc/base/task_queue_unittest.cc

Index: webrtc/base/task_queue_unittest.cc
diff --git a/webrtc/base/task_queue_unittest.cc b/webrtc/base/task_queue_unittest.cc
index 74433b91440117250fc72cc959326db0afa2f935..a263610acafbc1e226b10df4c73aadb18a97305c 100644
--- a/webrtc/base/task_queue_unittest.cc
+++ b/webrtc/base/task_queue_unittest.cc
@@ -206,6 +206,24 @@ TEST(TaskQueueTest, PostAndReplyLambda) {
   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";

[Taylor Brandstetter, 2017/02/22 00:24:27]

Out of curiosity, what's the purpose of having "static const char kPostQueue[]"
in each test, as opposed to just 'TaskQueue post_queue("PostQueue");'?

[tommi, 2017/02/22 12:43:54]

When I started writing these tests, the constant variable name was the same
(kQueueName) but I changed its value from test to test and I checked that the
name of the queue was the same as the constant specified (just so that the
literal string would only be in one place).  Then that pattern was repeated as
more tests were added.  For many of these tests today, that's not necessary.

+  static const char kReplyQueue[] = "ReplyQueue";
+  std::unique_ptr<TaskQueue> post_queue(new TaskQueue(kPostQueue));

[Taylor Brandstetter, 2017/02/22 00:24:28]

These don't need to be unique_ptrs

[tommi, 2017/02/22 12:43:54]

Done.  I made them unique_ptrs while debugging so that I could make the deletion
explicit rather than implicit and be able to put a breakpoint on either the
.reset() or the line immediately following.

+  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) {