Bug 1644009 - P13. Don't have AutoTaskGuard inherit from AutoTaskDispatcher. r=bholley.

This prevent being able to assert in the AutoTaskDispatcher that we are using it from the right thread/taskqueue as the SimpleTaskQueue object isn't thread-safe.

We want to assert that all nsIDirectTaskDispatcher methods are only ever accessed on the underlying thread. To do so require that the scope of AutoTaskDispatcher to terminate prior the AutoTaskGuard one.

Differential Revision: https://phabricator.services.mozilla.com/D79096

Author: Jean-Yves Avenard

xpcom/threads/TaskQueue.h

@@ -9,6 +9,7 @@
 
 #include <queue>
 
+#include "mozilla/Maybe.h"
 #include "mozilla/Monitor.h"
 #include "mozilla/MozPromise.h"
 #include "mozilla/RefPtr.h"
@@ -150,17 +151,16 @@ class TaskQueue : public AbstractThread, public nsIDirectTaskDispatcher {
   Atomic<PRThread*> mRunningThread;
 
   // RAII class that gets instantiated for each dispatched task.
-  class AutoTaskGuard : public AutoTaskDispatcher {
+  class AutoTaskGuard {
    public:
     explicit AutoTaskGuard(TaskQueue* aQueue)
-        : AutoTaskDispatcher(aQueue,
-                             /* aIsTailDispatcher = */ true),
-          mQueue(aQueue),
-          mLastCurrentThread(nullptr) {
+        : mQueue(aQueue), mLastCurrentThread(nullptr) {
       // NB: We don't hold the lock to aQueue here. Don't do anything that
       // might require it.
       MOZ_ASSERT(!mQueue->mTailDispatcher);
-      mQueue->mTailDispatcher = this;
+      mTaskDispatcher.emplace(aQueue,
+                              /* aIsTailDispatcher = */ true);
+      mQueue->mTailDispatcher = mTaskDispatcher.ptr();
 
       mLastCurrentThread = sCurrentThreadTLS.get();
       sCurrentThreadTLS.set(aQueue);
@@ -170,7 +170,8 @@ class TaskQueue : public AbstractThread, public nsIDirectTaskDispatcher {
     }
 
     ~AutoTaskGuard() {
-      DrainDirectTasks();
+      mTaskDispatcher->DrainDirectTasks();
+      mTaskDispatcher.reset();
 
       MOZ_ASSERT(mQueue->mRunningThread == PR_GetCurrentThread());
       mQueue->mRunningThread = nullptr;
@@ -180,6 +181,7 @@ class TaskQueue : public AbstractThread, public nsIDirectTaskDispatcher {
     }
 
    private:
+    Maybe<AutoTaskDispatcher> mTaskDispatcher;
     TaskQueue* mQueue;
     AbstractThread* mLastCurrentThread;
   };