Bug 780770, nsThread::SetMainThreadObserver, r=bz+bholley

js/xpconnect/src/nsXPConnect.cpp

@@ -39,6 +39,7 @@
 #include "nsWrapperCacheInlines.h"
 #include "nsDOMMutationObserver.h"
 #include "nsICycleCollectorListener.h"
+#include "nsThread.h"
 
 using namespace mozilla::dom;
 using namespace xpc;
@@ -157,13 +158,12 @@ nsXPConnect::GetXPConnect()
         // balanced by explicit call to ReleaseXPConnectSingleton()
         NS_ADDREF(gSelf);
 
-        // Add XPConnect as an thread observer.
+        // Set XPConnect as the main thread observer.
         //
         // The cycle collector sometimes calls GetXPConnect, but it should never
         // be the one that initializes gSelf.
         MOZ_ASSERT(NS_IsMainThread());
-        nsCOMPtr<nsIThreadInternal> thread = do_QueryInterface(NS_GetCurrentThread());
-        if (NS_FAILED(thread->AddObserver(gSelf))) {
+        if (NS_FAILED(nsThread::SetMainThreadObserver(gSelf))) {
             NS_RELEASE(gSelf);
             // Fall through to returning null
         }
@@ -186,14 +186,7 @@ nsXPConnect::ReleaseXPConnectSingleton()
 {
     nsXPConnect* xpc = gSelf;
     if (xpc) {
-
-        // The thread subsystem may have been shut down already, so make sure
-        // to check for null here.
-        nsCOMPtr<nsIThreadInternal> thread = do_QueryInterface(NS_GetCurrentThread());
-        if (thread) {
-            MOZ_ASSERT(NS_IsMainThread());
-            thread->RemoveObserver(xpc);
-        }
+        nsThread::SetMainThreadObserver(nullptr);
 
 #ifdef DEBUG
         // force a dump of the JavaScript gc heap if JS is still alive

xpcom/threads/nsThread.cpp

@@ -47,6 +47,8 @@ static PRLogModuleInfo *sLog = PR_NewLogModule("nsThread");
 
 NS_DECL_CI_INTERFACE_GETTER(nsThread)
 
+nsIThreadObserver* nsThread::sMainThreadObserver = nullptr;
+
 namespace mozilla {
 
 // Fun fact: Android's GCC won't convert bool* to int32_t*, so we can't
@@ -580,6 +582,12 @@ nsThread::ProcessNextEvent(bool mayWait, bool *result)
     }
   }
 
+  bool notifyMainThreadObserver =
+    (MAIN_THREAD == mIsMainThread) && sMainThreadObserver;
+  if (notifyMainThreadObserver) 
+   sMainThreadObserver->OnProcessNextEvent(this, mayWait && !ShuttingDown(),
+                                           mRunningEvent);
+
   nsCOMPtr<nsIThreadObserver> obs = mObserver;
   if (obs)
     obs->OnProcessNextEvent(this, mayWait && !ShuttingDown(), mRunningEvent);
@@ -624,6 +632,9 @@ nsThread::ProcessNextEvent(bool mayWait, bool *result)
   if (obs)
     obs->AfterProcessNextEvent(this, mRunningEvent);
 
+  if (notifyMainThreadObserver && sMainThreadObserver)
+    sMainThreadObserver->AfterProcessNextEvent(this, mRunningEvent);
+
   return rv;
 }
 
@@ -732,6 +743,21 @@ nsThread::RemoveObserver(nsIThreadObserver *observer)
   return NS_OK;
 }
 
+nsresult
+nsThread::SetMainThreadObserver(nsIThreadObserver* aObserver)
+{
+  if (aObserver && nsThread::sMainThreadObserver) {
+    return NS_ERROR_NOT_AVAILABLE;
+  }
+
+  if (!NS_IsMainThread()) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
+  nsThread::sMainThreadObserver = aObserver;
+  return NS_OK;
+}
+
 //-----------------------------------------------------------------------------
 
 NS_IMETHODIMP

xpcom/threads/nsThread.h

@@ -50,7 +50,12 @@ public:
   // Clear the observer list.
   void ClearObservers() { mEventObservers.Clear(); }
 
+  static nsresult
+  SetMainThreadObserver(nsIThreadObserver* aObserver);
+
 private:
+  static nsIThreadObserver* sMainThreadObserver;
+
   friend class nsThreadShutdownEvent;
 
   ~nsThread();