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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