Create a reader-writer lock implementaiton in base.

base/synchronization/rw_lock.h

Index: base/synchronization/rw_lock.h
diff --git a/base/synchronization/rw_lock.h b/base/synchronization/rw_lock.h
new file mode 100644
index 0000000000000000000000000000000000000000..04d3f18f4f5d0de67e8b1e4a46e89190df1d9cce
--- /dev/null
+++ b/base/synchronization/rw_lock.h
@@ -0,0 +1,92 @@
+// 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_RW_LOCK_H_
+#define BASE_SYNCHRONIZATION_RW_LOCK_H_
+
+#include "base/base_export.h"
+#include "base/macros.h"
+#include "build/build_config.h"
+
+#if defined(OS_WIN)
+#include <windows.h>
+#elif defined(OS_NACL)
+#include "base/synchronization/lock.h"
+#elif defined(OS_POSIX)
+#include <pthread.h>
+#else
+#  error No reader-writer lock defined for this platform.
+#endif
+
+namespace base {
+
+// An OS-independent wrapper around reader-writer locks. There's no magic here.
+class BASE_EXPORT RWLock {

[danakj, 2016/05/19 23:01:36]

ReadWriteLock or ReaderWriterLock. Then make the file name match.

[Anand Mistry (off Chromium), 2016/05/20 04:06:38]

Done.

+ public:
+  RWLock();
+  ~RWLock();
+
+  // 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;
+  Lock writer_lock_;
+#endif
+
+  DISALLOW_COPY_AND_ASSIGN(RWLock);
+};
+
+class AutoReadLock {
+ public:
+  explicit AutoReadLock(RWLock &lock) : lock_(lock) {

[danakj, 2016/05/19 23:01:36]

the & should go on the type not the variable name

This is actually a style guide violation and should be taking a pointer. I know
you're being consistent with AutoLock which violates this also.. so.. I guess..

[Anand Mistry (off Chromium), 2016/05/20 04:06:38]

Done.

+    lock_.ReadAcquire();
+  }
+  ~AutoReadLock() {
+    lock_.ReadRelease();
+  }
+
+ private:
+  RWLock& lock_;
+  DISALLOW_COPY_AND_ASSIGN(AutoReadLock);
+};
+
+class AutoWriteLock {
+ public:
+  explicit AutoWriteLock(RWLock &lock) : lock_(lock) {

[danakj, 2016/05/19 23:01:36]

ditto

[Anand Mistry (off Chromium), 2016/05/20 04:06:38]

Done.

+    lock_.WriteAcquire();
+  }
+  ~AutoWriteLock() {
+    lock_.WriteRelease();
+  }
+
+ private:
+  RWLock& lock_;
+  DISALLOW_COPY_AND_ASSIGN(AutoWriteLock);
+};
+
+}  // namespace base
+
+#endif  // BASE_SYNCHRONIZATION_RW_LOCK_H_