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diff --git a/xpcom/base/StaticMutex.h b/xpcom/base/StaticMutex.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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_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 MOZ_ONLY_USED_TO_AVOID_STATIC_CONSTRUCTORS MOZ_CAPABILITY("mutex")
+    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() MOZ_CAPABILITY_ACQUIRE() { Mutex()->Lock(); }
+
+  void Unlock() MOZ_CAPABILITY_RELEASE() { Mutex()->Unlock(); }
+
+  void AssertCurrentThreadOwns() MOZ_ASSERT_CAPABILITY(this) {
+#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*, SequentiallyConsistent> 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& aOther);
+#endif
+
+  // Disallow these operators.
+  StaticMutex& operator=(StaticMutex* aRhs);
+  static void* operator new(size_t) noexcept(true);
+  static void operator delete(void*);
+};
+
+typedef detail::BaseAutoLock<StaticMutex&> StaticMutexAutoLock;
+typedef detail::BaseAutoUnlock<StaticMutex&> StaticMutexAutoUnlock;
+
+}  // namespace mozilla
+
+#endif