Bug 877534 - Use a shutdown listener to destroy the compositor from nsBaseWidget. r=roc

This prevents a case where the nsBaseWidget isn't destroyed until the cycle collector is shutdown,
and we are too late in the shutdown sequence to process the events that get queued during
Compositor teardown.

widget/xpwidgets/nsBaseWidget.cpp

@@ -118,8 +118,31 @@ nsBaseWidget::nsBaseWidget()
 #ifdef DEBUG
   debug_RegisterPrefCallbacks();
 #endif
+
+  mShutdownObserver = new WidgetShutdownObserver(this);
+  nsContentUtils::RegisterShutdownObserver(mShutdownObserver);
 }
 
+NS_IMPL_ISUPPORTS1(WidgetShutdownObserver, nsIObserver)
+
+NS_IMETHODIMP
+WidgetShutdownObserver::Observe(nsISupports *aSubject,
+                                const char *aTopic,
+                                const PRUnichar *aData)
+{
+  if (strcmp(aTopic, NS_XPCOM_SHUTDOWN_OBSERVER_ID) == 0) {
+    mWidget->Shutdown();
+    nsContentUtils::UnregisterShutdownObserver(this);
+  }
+ return NS_OK;
+}
+
+void
+nsBaseWidget::Shutdown()
+{
+  DestroyCompositor();
+  mShutdownObserver = nullptr;
+}
 
 static void DeferredDestroyCompositor(CompositorParent* aCompositorParent,
                               CompositorChild* aCompositorChild)
@@ -171,6 +194,10 @@ nsBaseWidget::~nsBaseWidget()
     mLayerManager = nullptr;
   }
 
+  if (mShutdownObserver) {
+    nsContentUtils::UnregisterShutdownObserver(mShutdownObserver);
+  }
+
   DestroyCompositor();
 
 #ifdef NOISY_WIDGET_LEAKS
@@ -893,6 +920,12 @@ void nsBaseWidget::CreateCompositor(int aWidth, int aHeight)
   // Recreating this is tricky, as we may still have an old and we need
   // to make sure it's properly destroyed by calling DestroyCompositor!
 
+  // If we've already received a shutdown notification, don't try
+  // create a new compositor.
+  if (!mShutdownObserver) {
+    return;
+  }
+
   mCompositorParent = NewCompositorParent(aWidth, aHeight);
   AsyncChannel *parentChannel = mCompositorParent->GetIPCChannel();
   LayerManager* lm = new ClientLayerManager(this);

widget/xpwidgets/nsBaseWidget.h

@@ -15,6 +15,7 @@
 #include "nsGUIEvent.h"
 #include "nsAutoPtr.h"
 #include "nsIRollupListener.h"
+#include "nsIObserver.h"
 #include <algorithm>
 class nsIContent;
 class nsAutoRollup;
@@ -38,6 +39,22 @@ namespace base {
 class Thread;
 }
 
+class nsBaseWidget;
+
+class WidgetShutdownObserver MOZ_FINAL : public nsIObserver
+{
+public:
+  WidgetShutdownObserver(nsBaseWidget* aWidget)
+    : mWidget(aWidget)
+  { }
+
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIOBSERVER
+
+private:
+  nsBaseWidget *mWidget;
+};
+
 /**
  * Common widget implementation used as base class for native
  * or crossplatform implementations of Widgets. 
@@ -254,6 +271,8 @@ public:
 
   static nsIRollupListener* GetActiveRollupListener();
 
+  void Shutdown();
+
 protected:
 
   virtual void            ResolveIconName(const nsAString &aIconName,
@@ -356,6 +375,7 @@ protected:
   nsRefPtr<LayerManager> mBasicLayerManager;
   nsRefPtr<CompositorChild> mCompositorChild;
   nsRefPtr<CompositorParent> mCompositorParent;
+  nsCOMPtr<WidgetShutdownObserver> mShutdownObserver;
   nscolor           mBackground;
   nscolor           mForeground;
   nsCursor          mCursor;