Bug 1510058 - Work around mWidget becoming null during DidComposite, and add a diagnostic assert that will hopefully root out the culprit. r=jrmuizel

Differential Revision: https://phabricator.services.mozilla.com/D13309

--HG--
extra : moz-landing-system : lando

gfx/layers/client/ClientLayerManager.cpp

@@ -58,6 +58,7 @@ ClientLayerManager::ClientLayerManager(nsIWidget* aWidget)
   , mCompositorMightResample(false)
   , mNeedsComposite(false)
   , mQueuedAsyncPaints(false)
+  , mNotifyingWidgetListener(false)
   , mPaintSequenceNumber(0)
   , mForwarder(new ShadowLayerForwarder(this))
 {
@@ -85,6 +86,9 @@ ClientLayerManager::~ClientLayerManager()
 void
 ClientLayerManager::Destroy()
 {
+  MOZ_DIAGNOSTIC_ASSERT(!mNotifyingWidgetListener,
+    "Try to avoid destroying widgets and layer managers during DidCompositeWindow, if you can");
+
   // It's important to call ClearCachedResource before Destroy because the
   // former will early-return if the later has already run.
   ClearCachedResources();
@@ -495,12 +499,19 @@ ClientLayerManager::DidComposite(TransactionId aTransactionId,
   if (aTransactionId.IsValid()) {
     nsIWidgetListener *listener = mWidget->GetWidgetListener();
     if (listener) {
+      mNotifyingWidgetListener = true;
       listener->DidCompositeWindow(aTransactionId, aCompositeStart, aCompositeEnd);
+      mNotifyingWidgetListener = false;
     }
+    // DidCompositeWindow might have called Destroy on us and nulled out mWidget,
+    // see bug 1510058.
+    // Re-check it here.
+    if (mWidget) {
       listener = mWidget->GetAttachedWidgetListener();
       if (listener) {
         listener->DidCompositeWindow(aTransactionId, aCompositeStart, aCompositeEnd);
       }
+    }
     if (mTransactionIdAllocator) {
       mTransactionIdAllocator->NotifyTransactionCompleted(aTransactionId);
     }

gfx/layers/client/ClientLayerManager.h

@@ -327,6 +327,7 @@ private:
   bool mCompositorMightResample;
   bool mNeedsComposite;
   bool mQueuedAsyncPaints;
+  bool mNotifyingWidgetListener;
 
   // An incrementing sequence number for paints.
   // Incremented in BeginTransaction(), but not for repeat transactions.