Clear CanvasShutdownObserver::mCanvas when the canvas goes away.

This is fallout from Bug 1167235 - Use a fast method of double buffering for canvas.

It is possible for the CanvasRenderingContext2D to be destroyed while we're in the
middle of the call to nsObserverService::NotifyObservers() for shutdown.
This leaves the shutdown observer with a dangling pointer to the canvas, so this
patch explicitly clears the pointer when the context goes away.

1 files changed, 11 insertions, 2 deletions

diff --git a/dom/canvas/CanvasRenderingContext2D.cpp b/dom/canvas/CanvasRenderingContext2D.cpp
index 18af28e9f7..e3406fc021 100644
--- a/dom/canvas/CanvasRenderingContext2D.cpp
+++ b/dom/canvas/CanvasRenderingContext2D.cpp
@@ -783,6 +783,15 @@ public:
   : mCanvas(aCanvas)
   {}
 
+  void OnShutdown() {
+    if(!mCanvas) {
+      return;
+    }
+    
+    mCanvas = nullptr;
+    nsContentUtils::UnregisterShutdownObserver(this);
+  }
+  
   NS_DECL_ISUPPORTS
   NS_DECL_NSIOBSERVER
 private:
@@ -800,7 +809,7 @@ CanvasShutdownObserver::Observe(nsISupports* aSubject,
 {
   if (mCanvas && strcmp(aTopic, NS_XPCOM_SHUTDOWN_OBSERVER_ID) == 0) {
     mCanvas->OnShutdown();
-    nsContentUtils::UnregisterShutdownObserver(this);
+    OnShutdown();
   }
 
   return NS_OK;
@@ -1218,7 +1227,7 @@ void
 CanvasRenderingContext2D::RemoveShutdownObserver()
 {
   if (mShutdownObserver) {
-    nsContentUtils::UnregisterShutdownObserver(mShutdownObserver);
+    mShutdownObserver->OnShutdown();
     mShutdownObserver = nullptr;
   }
 }