diff -r 000000000000 -r 6474c204b198 xpcom/threads/SyncRunnable.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/xpcom/threads/SyncRunnable.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,93 @@
+/* -*- Mode: C++; tab-width: 12; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_SyncRunnable_h
+#define mozilla_SyncRunnable_h
+
+#include "nsThreadUtils.h"
+#include "mozilla/Monitor.h"
+
+namespace mozilla {
+
+/**
+ * This class will wrap a nsIRunnable and dispatch it to the main thread
+ * synchronously. This is different from nsIEventTarget.DISPATCH_SYNC:
+ * this class does not spin the event loop waiting for the event to be
+ * dispatched. This means that you don't risk reentrance from pending
+ * messages, but you must be sure that the target thread does not ever block
+ * on this thread, or else you will deadlock.
+ *
+ * Typical usage:
+ * nsRefPtr<SyncRunnable> sr = new SyncRunnable(new myrunnable...());
+ * sr->DispatchToThread(t);
+ *
+ * We also provide a convenience wrapper:
+ * SyncRunnable::DispatchToThread(new myrunnable...());
+ *
+ */
+class SyncRunnable : public nsRunnable
+{
+public:
+  SyncRunnable(nsIRunnable* r)
+    : mRunnable(r)
+    , mMonitor("SyncRunnable")
+    , mDone(false)
+  { }
+
+  void DispatchToThread(nsIEventTarget* thread,
+                        bool forceDispatch = false)
+  {
+    nsresult rv;
+    bool on;
+
+    if (!forceDispatch) {
+      rv = thread->IsOnCurrentThread(&on);
+      MOZ_ASSERT(NS_SUCCEEDED(rv));
+      if (NS_SUCCEEDED(rv) && on) {
+        mRunnable->Run();
+        return;
+      }
+    }
+
+    rv = thread->Dispatch(this, NS_DISPATCH_NORMAL);
+    if (NS_SUCCEEDED(rv)) {
+      mozilla::MonitorAutoLock lock(mMonitor);
+      while (!mDone) {
+        lock.Wait();
+      }
+    }
+  }
+
+  static void DispatchToThread(nsIEventTarget* thread,
+                               nsIRunnable* r,
+                               bool forceDispatch = false)
+  {
+    nsRefPtr<SyncRunnable> s(new SyncRunnable(r));
+    s->DispatchToThread(thread, forceDispatch);
+  }
+
+protected:
+  NS_IMETHODIMP Run()
+  {
+    mRunnable->Run();
+
+    mozilla::MonitorAutoLock lock(mMonitor);
+    MOZ_ASSERT(!mDone);
+
+    mDone = true;
+    mMonitor.Notify();
+
+    return NS_OK;
+  }
+
+private:
+  nsCOMPtr<nsIRunnable> mRunnable;
+  mozilla::Monitor mMonitor;
+  bool mDone;
+};
+
+} // namespace mozilla
+
+#endif // mozilla_SyncRunnable_h