Bug 1245789 - Make XPCOMThreadWrapper::GetCurrent() work. r=bholley

XPCOMThreadWrapper::GetCurrent() is failing because it's not keeping
AbstractThread::sCurrentThreadTLS up to date. This causes assertion failures
during startup of the GMP stack when dispatching via InvokeAsync to the GMP
thread, which is an XPCOM thread wrapped by the XPCOMThreadWrapper.

We can trivially initialize AbstractThread::sCurrentThreadTLS to be the
XPCOMThreadWrapper on the target thread, since it's thread-local-storage, and
the target thread won't change.

MozReview-Commit-ID: EIEFZppR2PS

dom/media/gmp/GMPService.cpp

@@ -357,7 +357,7 @@ GeckoMediaPluginService::GetThread(nsIThread** aThread)
       return rv;
     }
 
-    mAbstractGMPThread = CreateXPCOMAbstractThreadWrapper(mGMPThread, false);
+    mAbstractGMPThread = AbstractThread::CreateXPCOMThreadWrapper(mGMPThread, false);
 
     // Tell the thread to initialize plugins
     InitializePlugins();

xpcom/threads/AbstractThread.cpp

@@ -146,10 +146,17 @@ AbstractThread::DispatchDirectTask(already_AddRefed<nsIRunnable> aRunnable)
   GetCurrent()->TailDispatcher().AddDirectTask(Move(aRunnable));
 }
 
+/* static */
 already_AddRefed<AbstractThread>
-CreateXPCOMAbstractThreadWrapper(nsIThread* aThread, bool aRequireTailDispatch)
+AbstractThread::CreateXPCOMThreadWrapper(nsIThread* aThread, bool aRequireTailDispatch)
 {
   RefPtr<XPCOMThreadWrapper> wrapper = new XPCOMThreadWrapper(aThread, aRequireTailDispatch);
+  // Set the thread-local sCurrentThreadTLS to point to the wrapper on the
+  // target thread. This ensures that sCurrentThreadTLS is as expected by
+  // AbstractThread::GetCurrent() on the target thread.
+  nsCOMPtr<nsIRunnable> r =
+    NS_NewRunnableFunction([wrapper]() { sCurrentThreadTLS.set(wrapper); });
+  aThread->Dispatch(r.forget(), NS_DISPATCH_NORMAL);
   return wrapper.forget();
 }
 

xpcom/threads/AbstractThread.h

@@ -42,6 +42,9 @@ public:
 
   AbstractThread(bool aSupportsTailDispatch) : mSupportsTailDispatch(aSupportsTailDispatch) {}
 
+  static already_AddRefed<AbstractThread>
+  CreateXPCOMThreadWrapper(nsIThread* aThread, bool aRequireTailDispatch);
+
   NS_INLINE_DECL_THREADSAFE_REFCOUNTING(AbstractThread);
 
   enum DispatchFailureHandling { AssertDispatchSuccess, DontAssertDispatchSuccess };
@@ -92,9 +95,6 @@ protected:
   const bool mSupportsTailDispatch;
 };
 
-already_AddRefed<AbstractThread> CreateXPCOMAbstractThreadWrapper(nsIThread* aThread,
-                                 bool aRequireTailDispatch);
-
 } // namespace mozilla
 
 #endif