Bug 1340665 - Fix warnings on shutdown. r=billm

The code to make a ValidatingDispatcher break cycles when we're done with it
assumes that XPCOM is still up and running. This isn't the case during
shutdown. When XPCOM shuts down, we should avoid do_GetMainThread.

MozReview-Commit-ID: 5Gpko9FbFxl

--HG--
extra : rebase_source : 8c00c83b06e890802ade0d6df2a93f64dc8ea376

dom/base/TabGroup.cpp

@@ -167,7 +167,7 @@ TabGroup::Leave(nsPIDOMWindowOuter* aWindow)
   // out after the last window leaves.
   if (!mIsChrome && mWindows.IsEmpty()) {
     mLastWindowLeft = true;
-    Shutdown();
+    Shutdown(false);
   }
 }
 

xpcom/threads/Dispatcher.cpp

@@ -152,6 +152,7 @@ nsIEventTarget*
 ValidatingDispatcher::EventTargetFor(TaskCategory aCategory) const
 {
   MOZ_ASSERT(aCategory != TaskCategory::Count);
+  MOZ_ASSERT(mEventTargets[size_t(aCategory)]);
   return mEventTargets[size_t(aCategory)];
 }
 
@@ -160,6 +161,7 @@ ValidatingDispatcher::AbstractMainThreadForImpl(TaskCategory aCategory)
 {
   MOZ_RELEASE_ASSERT(NS_IsMainThread());
   MOZ_ASSERT(aCategory != TaskCategory::Count);
+  MOZ_ASSERT(mEventTargets[size_t(aCategory)]);
 
   if (!mAbstractThreads[size_t(aCategory)]) {
     mAbstractThreads[size_t(aCategory)] =
@@ -187,14 +189,14 @@ ValidatingDispatcher::CreateEventTargets(bool aNeedValidation)
 }
 
 void
-ValidatingDispatcher::Shutdown()
+ValidatingDispatcher::Shutdown(bool aXPCOMShutdown)
 {
   // There is a RefPtr cycle TabGroup -> DispatcherEventTarget -> TabGroup. To
   // avoid leaks, we need to break the chain somewhere. We shouldn't be using
   // the ThrottledEventQueue for this TabGroup when no windows belong to it,
   // so it's safe to null out the queue here.
   for (size_t i = 0; i < size_t(TaskCategory::Count); i++) {
-    mEventTargets[i] = do_GetMainThread();
+    mEventTargets[i] = aXPCOMShutdown ? nullptr : do_GetMainThread();
     mAbstractThreads[i] = nullptr;
   }
 }

xpcom/threads/Dispatcher.h

@@ -112,7 +112,10 @@ protected:
                            already_AddRefed<nsIRunnable>&& aRunnable);
 
   void CreateEventTargets(bool aNeedValidation);
-  void Shutdown();
+
+  // Shuts down this dispatcher. If aXPCOMShutdown is true, invalidates this
+  // dispatcher.
+  void Shutdown(bool aXPCOMShutdown);
 
   enum ValidationType {
     StartValidation,

xpcom/threads/SystemGroup.cpp

@@ -53,7 +53,7 @@ SystemGroupImpl::InitStatic()
 /* static */ void
 SystemGroupImpl::ShutdownStatic()
 {
-  sSingleton->Shutdown();
+  sSingleton->Shutdown(true);
   sSingleton = nullptr;
 }