Make sure that "current" rtc::Thread instances are always current for TaskQueueBase.

This is a necessary part of fulfilling the TaskQueueBase
interface. If a thread does not register as the current TQ, yet offers
the TQ interface, TQ 'current' checks will not work as expected and
code that relies them (TaskQueueBase::Current() and IsCurrent())
will run in unexpected ways.

Bug: webrtc:11572
Change-Id: Iab747bc474e74e6ce4f9e914cfd5b0578b19d19c
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/175080
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Tommi <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31254}

rtc_base/thread.cc

@@ -296,6 +296,21 @@ void ThreadManager::SetCurrentThread(Thread* thread) {
     RTC_DLOG(LS_ERROR) << "SetCurrentThread: Overwriting an existing value?";
   }
 #endif  // RTC_DLOG_IS_ON
+
+  if (thread) {
+    thread->EnsureIsCurrentTaskQueue();
+  } else {
+    Thread* current = CurrentThread();
+    if (current) {
+      // The current thread is being cleared, e.g. as a result of
+      // UnwrapCurrent() being called or when a thread is being stopped
+      // (see PreRun()). This signals that the Thread instance is being detached
+      // from the thread, which also means that TaskQueue::Current() must not
+      // return a pointer to the Thread instance.
+      current->ClearCurrentTaskQueue();
+    }
+  }
+
   SetCurrentThreadInternal(thread);
 }
 
@@ -824,7 +839,6 @@ void* Thread::PreRun(void* pv) {
   Thread* thread = static_cast<Thread*>(pv);
   ThreadManager::Instance()->SetCurrentThread(thread);
   rtc::SetCurrentThreadName(thread->name_.c_str());
-  CurrentTaskQueueSetter set_current_task_queue(thread);
 #if defined(WEBRTC_MAC)
   ScopedAutoReleasePool pool;
 #endif
@@ -935,6 +949,17 @@ void Thread::InvokeInternal(const Location& posted_from,
   Send(posted_from, &handler);
 }
 
+// Called by the ThreadManager when being set as the current thread.
+void Thread::EnsureIsCurrentTaskQueue() {
+  task_queue_registration_ =
+      std::make_unique<TaskQueueBase::CurrentTaskQueueSetter>(this);
+}
+
+// Called by the ThreadManager when being set as the current thread.
+void Thread::ClearCurrentTaskQueue() {
+  task_queue_registration_.reset();
+}
+
 void Thread::QueuedTaskHandler::OnMessage(Message* msg) {
   RTC_DCHECK(msg);
   auto* data = static_cast<ScopedMessageData<webrtc::QueuedTask>*>(msg->pdata);