Create a reader-writer lock implementaiton in base.

base/synchronization/read_write_lock.h

Index: base/synchronization/read_write_lock.h
diff --git a/base/synchronization/read_write_lock.h b/base/synchronization/read_write_lock.h
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+++ b/base/synchronization/read_write_lock.h
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+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef BASE_SYNCHRONIZATION_READ_WRITE_LOCK_H_
+#define BASE_SYNCHRONIZATION_READ_WRITE_LOCK_H_
+
+#include "base/base_export.h"
+#include "base/macros.h"
+#include "build/build_config.h"
+
+#if defined(OS_NACL)
+#include "base/synchronization/lock.h"
+#endif
+
+#if defined(OS_WIN)
+#include <windows.h>
+#elif defined(OS_POSIX)
+#include <pthread.h>
+#else
+#  error No reader-writer lock defined for this platform.
+#endif
+
+namespace base {
+namespace subtle {
+
+// An OS-independent wrapper around reader-writer locks. There's no magic here.
+//
+// You are strongly encouraged to use base::Lock instead of this, unless you
+// can demonstrate contention and show that this would lead to an improvement.
+// This lock does not make any guarantees of fairness, which can lead to writer
+// starvation under certain access patterns. You should carefully consider your
+// writer access patterns before using this lock.
+class BASE_EXPORT ReadWriteLock {
+ public:
+  ReadWriteLock();
+  ~ReadWriteLock();
+
+  // Reader lock functions.
+  void ReadAcquire();
+  void ReadRelease();
+
+  // Writer lock functions.
+  void WriteAcquire();
+  void WriteRelease();
+
+ private:
+#if defined(OS_WIN)
+  using NativeHandle = SRWLOCK;
+#elif defined(OS_NACL)
+  using NativeHandle = Lock;
+#elif defined(OS_POSIX)
+  using NativeHandle = pthread_rwlock_t;
+#endif
+
+  NativeHandle native_handle_;
+
+#if defined(OS_NACL)
+  // Even though NaCl has a pthread_rwlock implementation, the build rules don't
+  // make it universally available. So instead, implement a slower and trivial
+  // reader-writer lock using a regular mutex.
+  // TODO(amistry): Remove this and use the posix implementation when it's
+  // available in all build configurations.
+  uint32_t readers_ = 0;
+  // base::Lock does checking to ensure the lock is acquired and released on the
+  // same thread. This is not the case for this lock, so use pthread mutexes
+  // directly here.
+  pthread_mutex_t writer_lock_ = PTHREAD_MUTEX_INITIALIZER;
+#endif
+
+  DISALLOW_COPY_AND_ASSIGN(ReadWriteLock);
+};
+
+class AutoReadLock {
+ public:
+  explicit AutoReadLock(ReadWriteLock& lock) : lock_(lock) {
+    lock_.ReadAcquire();
+  }
+  ~AutoReadLock() {
+    lock_.ReadRelease();
+  }
+
+ private:
+  ReadWriteLock& lock_;
+  DISALLOW_COPY_AND_ASSIGN(AutoReadLock);
+};
+
+class AutoWriteLock {
+ public:
+  explicit AutoWriteLock(ReadWriteLock& lock) : lock_(lock) {
+    lock_.WriteAcquire();
+  }
+  ~AutoWriteLock() {
+    lock_.WriteRelease();
+  }
+
+ private:
+  ReadWriteLock& lock_;
+  DISALLOW_COPY_AND_ASSIGN(AutoWriteLock);
+};
+
+}  // namespace subtle
+}  // namespace base
+
+#endif  // BASE_SYNCHRONIZATION_READ_WRITE_LOCK_H_