Proposed ~TaskQueue() solution - GCD

webrtc/rtc_base/task_queue_gcd.cc

Index: webrtc/rtc_base/task_queue_gcd.cc
diff --git a/webrtc/rtc_base/task_queue_gcd.cc b/webrtc/rtc_base/task_queue_gcd.cc
index 45c5f09ebfc1f81a1ec18206bcd0c953f98579fb..fd221969407ada3aae9cc5e5768ebeb9f86bb0b0 100644
--- a/webrtc/rtc_base/task_queue_gcd.cc
+++ b/webrtc/rtc_base/task_queue_gcd.cc
@@ -16,6 +16,8 @@
 
 #include <string.h>
 
+#include <atomic>
+
 #include "webrtc/rtc_base/checks.h"
 #include "webrtc/rtc_base/logging.h"
 #include "webrtc/rtc_base/task_queue_posix.h"
@@ -53,8 +55,12 @@ struct TaskQueue::QueueContext {
     delete qc;
   }
 
+  // By posting this function synchronously, one can wait until the queue
+  // has been emptied.
+  static void VoidFunction(void* context) {}  // TODO(eladalon): !!! Rename?
+
   TaskQueue* const queue;
-  bool is_active;
+  std::atomic<bool> is_active;
 };
 
 struct TaskQueue::TaskContext {
@@ -134,9 +140,17 @@ TaskQueue::~TaskQueue() {
   // queue have been released, the queue will be deallocated by the system.
   // This is why we check the context before running tasks.
 
-  // Use dispatch_sync to set the context to null to guarantee that there's not
-  // a race between checking the context and using it from a task.
-  dispatch_sync_f(queue_, context_, &QueueContext::SetNotActive);
+  // Prevent pending tasks from running.
+  QueueContext::SetNotActive(context_);
+
+  // Wait until all previous tasks' destructors have been invoked.
+  dispatch_sync_f(queue_, context_, &QueueContext::VoidFunction);
+
+  // TODO(eladalon): !!! It would be good to also RTC_DCHECK here to make sure
+  // no additional tasks were posted in the meantime. Posting tasks during
+  // tear-down would be an error of poster. Additionally, the tasks' destructors
+  // would not get called.
+
   dispatch_release(queue_);
 }