Bug 705371 - [CC] don't add JSContexts that have no children to the cycle collector graph, r=mccr8

content/base/src/nsFrameMessageManager.cpp

@@ -1019,6 +1019,16 @@ nsFrameScriptExecutor::Traverse(nsFrameScriptExecutor *tmp,
   }
 }
 
+// static
+void
+nsFrameScriptExecutor::Unlink(nsFrameScriptExecutor* aTmp)
+{
+  if (aTmp->mCx) {
+    JSAutoRequest ar(aTmp->mCx);
+    JS_SetGlobalObject(aTmp->mCx, nullptr);
+  }
+}
+
 NS_IMPL_ISUPPORTS1(nsScriptCacheCleaner, nsIObserver)
 
 nsFrameMessageManager* nsFrameMessageManager::sChildProcessManager = nullptr;

content/base/src/nsFrameMessageManager.h

@@ -265,6 +265,7 @@ protected:
   bool InitTabChildGlobalInternal(nsISupports* aScope);
   static void Traverse(nsFrameScriptExecutor *tmp,
                        nsCycleCollectionTraversalCallback &cb);
+  static void Unlink(nsFrameScriptExecutor* aTmp);
   nsCOMPtr<nsIXPConnectJSObjectHolder> mGlobal;
   JSContext* mCx;
   uint32_t mCxStackRefCnt;

content/base/src/nsInProcessTabChildGlobal.cpp

@@ -161,6 +161,7 @@ NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN_INHERITED(nsInProcessTabChildGlobal,
                                                 nsDOMEventTargetHelper)
   NS_IMPL_CYCLE_COLLECTION_UNLINK(mMessageManager)
   NS_IMPL_CYCLE_COLLECTION_UNLINK(mGlobal)
+  nsFrameScriptExecutor::Unlink(tmp);
 NS_IMPL_CYCLE_COLLECTION_UNLINK_END
 
 NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN_INHERITED(nsInProcessTabChildGlobal,

dom/base/nsJSEnvironment.cpp

@@ -1206,6 +1206,10 @@ NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(nsJSContext)
                "Trying to unlink a context with outstanding requests.");
   tmp->mIsInitialized = false;
   tmp->mGCOnDestruction = false;
+  if (tmp->mContext) {
+    JSAutoRequest ar(tmp->mContext);
+    JS_SetGlobalObject(tmp->mContext, nullptr);
+  }
   tmp->DestroyJSContext();
   NS_IMPL_CYCLE_COLLECTION_UNLINK(mGlobalObjectRef)
 NS_IMPL_CYCLE_COLLECTION_UNLINK_END

js/xpconnect/src/XPCJSRuntime.cpp

@@ -465,7 +465,12 @@ XPCJSRuntime::AddXPConnectRoots(nsCycleCollectionTraversalCallback &cb)
 
     JSContext *iter = nullptr, *acx;
     while ((acx = JS_ContextIterator(GetJSRuntime(), &iter))) {
-        cb.NoteNativeRoot(acx, nsXPConnect::JSContextParticipant());
+        // Add the context to the CC graph only if traversing it would
+        // end up doing something.
+        JSObject* global = JS_GetGlobalObject(acx);
+        if (global && xpc_IsGrayGCThing(global)) {
+            cb.NoteNativeRoot(acx, nsXPConnect::JSContextParticipant());
+        }
     }
 
     XPCAutoLock lock(mMapLock);

js/xpconnect/src/nsXPConnect.cpp

@@ -821,10 +821,6 @@ public:
     }
     static NS_METHOD UnlinkImpl(void *n)
     {
-        JSContext *cx = static_cast<JSContext*>(n);
-        JSAutoRequest ar(cx);
-        NS_ASSERTION(JS_GetGlobalObject(cx), "global object NULL before unlinking");
-        JS_SetGlobalObject(cx, NULL);
         return NS_OK;
     }
     static NS_METHOD UnrootImpl(void *n)