diff -r 000000000000 -r 6474c204b198 xpcom/base/StaticMutex.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/xpcom/base/StaticMutex.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,90 @@
+#ifndef mozilla_StaticMutex_h
+#define mozilla_StaticMutex_h
+
+#include "mozilla/Atomics.h"
+#include "mozilla/Mutex.h"
+
+namespace mozilla {
+
+/**
+ * StaticMutex is a Mutex that can (and in fact, must) be used as a
+ * global/static variable.
+ *
+ * The main reason to use StaticMutex as opposed to
+ * StaticAutoPtr<OffTheBooksMutex> is that we instantiate the StaticMutex in a
+ * thread-safe manner the first time it's used.
+ *
+ * The same caveats that apply to StaticAutoPtr apply to StaticMutex.  In
+ * particular, do not use StaticMutex as a stack variable or a class instance
+ * variable, because this class relies on the fact that global variablies are
+ * initialized to 0 in order to initialize mMutex.  It is only safe to use
+ * StaticMutex as a global or static variable.
+ */
+class StaticMutex
+{
+public:
+  // In debug builds, check that mMutex is initialized for us as we expect by
+  // the compiler.  In non-debug builds, don't declare a constructor so that
+  // the compiler can see that the constructor is trivial.
+#ifdef DEBUG
+  StaticMutex()
+  {
+    MOZ_ASSERT(!mMutex);
+  }
+#endif
+
+  void Lock()
+  {
+    Mutex()->Lock();
+  }
+
+  void Unlock()
+  {
+    Mutex()->Unlock();
+  }
+
+  void AssertCurrentThreadOwns()
+  {
+#ifdef DEBUG
+    Mutex()->AssertCurrentThreadOwns();
+#endif
+  }
+
+private:
+  OffTheBooksMutex* Mutex()
+  {
+    if (mMutex) {
+      return mMutex;
+    }
+
+    OffTheBooksMutex* mutex = new OffTheBooksMutex("StaticMutex");
+    if (!mMutex.compareExchange(nullptr, mutex)) {
+      delete mutex;
+    }
+
+    return mMutex;
+  }
+
+  Atomic<OffTheBooksMutex*> mMutex;
+
+
+  // Disallow copy constructor, but only in debug mode.  We only define
+  // a default constructor in debug mode (see above); if we declared
+  // this constructor always, the compiler wouldn't generate a trivial
+  // default constructor for us in non-debug mode.
+#ifdef DEBUG
+  StaticMutex(StaticMutex &other);
+#endif
+
+  // Disallow these operators.
+  StaticMutex& operator=(StaticMutex* rhs);
+  static void* operator new(size_t) CPP_THROW_NEW;
+  static void operator delete(void*);
+};
+
+typedef BaseAutoLock<StaticMutex> StaticMutexAutoLock;
+typedef BaseAutoUnlock<StaticMutex> StaticMutexAutoUnlock;
+
+} // namespace mozilla
+
+#endif