diff -r 000000000000 -r 6474c204b198 widget/gtk/WidgetTraceEvent.cpp
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/widget/gtk/WidgetTraceEvent.cpp	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,78 @@
+/* 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/. */
+
+#include "mozilla/WidgetTraceEvent.h"
+
+#include <glib.h>
+#include <mozilla/CondVar.h>
+#include <mozilla/Mutex.h>
+#include <stdio.h>
+
+using mozilla::CondVar;
+using mozilla::Mutex;
+using mozilla::MutexAutoLock;
+
+namespace {
+
+Mutex* sMutex = nullptr;
+CondVar* sCondVar = nullptr;
+bool sTracerProcessed = false;
+
+// This function is called from the main (UI) thread.
+gboolean TracerCallback(gpointer data)
+{
+  mozilla::SignalTracerThread();
+  return FALSE;
+}
+
+} // namespace
+
+namespace mozilla {
+
+bool InitWidgetTracing()
+{
+  sMutex = new Mutex("Event tracer thread mutex");
+  sCondVar = new CondVar(*sMutex, "Event tracer thread condvar");
+  return sMutex && sCondVar;
+}
+
+void CleanUpWidgetTracing()
+{
+  delete sMutex;
+  delete sCondVar;
+  sMutex = nullptr;
+  sCondVar = nullptr;
+}
+
+// This function is called from the background tracer thread.
+bool FireAndWaitForTracerEvent()
+{
+  NS_ABORT_IF_FALSE(sMutex && sCondVar, "Tracing not initialized!");
+
+  // Send a default-priority idle event through the
+  // event loop, and wait for it to finish.
+  MutexAutoLock lock(*sMutex);
+  NS_ABORT_IF_FALSE(!sTracerProcessed, "Tracer synchronization state is wrong");
+  g_idle_add_full(G_PRIORITY_DEFAULT,
+                  TracerCallback,
+                  nullptr,
+                  nullptr);
+  while (!sTracerProcessed)
+    sCondVar->Wait();
+  sTracerProcessed = false;
+  return true;
+}
+
+void SignalTracerThread()
+{
+  if (!sMutex || !sCondVar)
+    return;
+  MutexAutoLock lock(*sMutex);
+  if (!sTracerProcessed) {
+    sTracerProcessed = true;
+    sCondVar->Notify();
+  }
+}
+
+}  // namespace mozilla