Add a SharedMemSynchronizer class.

media/audio/cross_process_notification.h

Index: media/audio/cross_process_notification.h
diff --git a/media/audio/cross_process_notification.h b/media/audio/cross_process_notification.h
new file mode 100644
index 0000000000000000000000000000000000000000..6765969c1a2e44187bf7b88b9acfe78e2f6ef99f
--- /dev/null
+++ b/media/audio/cross_process_notification.h
@@ -0,0 +1,174 @@
+// Copyright (c) 2012 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 MEDIA_AUDIO_CROSS_PROCESS_NOTIFICATION_H_
+#define MEDIA_AUDIO_CROSS_PROCESS_NOTIFICATION_H_
+
+#include <vector>
+
+#include "base/basictypes.h"
+#include "base/process.h"
+#include "base/threading/non_thread_safe.h"
+#include "media/base/media_export.h"
+
+#if defined(OS_WIN)
+#include "base/win/scoped_handle.h"
+#else
+#include "base/file_descriptor_posix.h"
+#include "base/sync_socket.h"
+#endif
+
+// A mechanism to synchronize access to a shared resource between two parties
+// when the usage pattern resembles that of two players playing a game of chess.
+// Each end has an instance of CrossProcessNotification and calls Signal() when
+// it has finished reading from/writing to the shared memory section.

[Ami GONE FROM CHROMIUM, 2012/03/14 16:40:51]

s/shared memory section/shared resource/

[tommi (sloooow) - chröme, 2012/03/14 21:20:38]

Done.

+// Before accessing the resource, it must call Wait() in order to know when the
+// other end has called Signal().
+//
+// Here's some pseudo code for how this class can be used:
+//
+// This method is used by both processes as it's a general way to access the
+// shared memory section, write to it and then grant the privilege to the
+// other process:
+//
+// void WriteToSharedMemory(CrossProcessNotification* notification,
+//                          SharedMemory* mem,
+//                          const char my_char) {
+//   notification->Wait();  // Wait for the other process to yield access.
+//   reinterpret_cast<char*>(mem->memory())[0] = my_char;
+//   notification->Signal();  // Grant the other process access.
+// }
+//
+// Process A:
+//
+// class A {
+//  public:
+//   void Initialize(base::ProcessHandle process_b) {
+//     mem_.CreateNamed("foo", false, 1024);
+//
+//     CrossProcessNotification other;
+//     CHECK(CrossProcessNotification::InitializePair(&notification_, &other));
+//     CrossProcessNotification::IPCHandle handle_1, handle_2;
+//     CHECK(other.ShareToProcess(process_b, &handle_1, &handle_2));
+//     // This could be implemented by using some IPC mechanism
+//     // such as MessageLoop.
+//     SendToProcessB(mem_, handle_1, handle_2);
+//     // Allow process B the first chance to write to the memory:
+//     notification_.Signal();
+//     // Once B is done, we'll write 'A' to the shared memory.
+//     WriteToSharedMemory(&notification_, &mem_, 'A');
+//   }
+//
+//   CrossProcessNotification notification_;
+//   SharedMemory mem_;
+// };
+//
+// Process B:
+//
+// class B {
+//  public:
+//   // Called when we receive the IPC message from A.
+//   void Initialize(SharedMemoryHandle mem,
+//                   CrossProcessNotification::IPCHandle handle_1,
+//                   CrossProcessNotification::IPCHandle handle_2) {
+//     mem_.reset(new SharedMemory(mem, false));
+//     notification_.reset(new CrossProcessNotification(handle_1, handle_2));
+//     WriteToSharedMemory(&notification_, &mem_, 'B');
+//   }
+//
+//   CrossProcessNotification notification_;
+//   scoped_ptr<SharedMemory> mem_;
+// };
+//
+class MEDIA_EXPORT CrossProcessNotification {
+ public:
+#if defined(OS_WIN)
+  typedef HANDLE IPCHandle;
+#else
+  typedef base::FileDescriptor IPCHandle;
+#endif
+
+  typedef std::vector<CrossProcessNotification*> Notifications;
+
+  // Default ctor.  Initializes a NULL notification.  User must call
+  // InitializePair() to initialize the instance along with a connected one.
+  CrossProcessNotification();
+
+  // Ctor for the user that does not call InitializePair but instead receives
+  // handles from the one that did.  These handles come from a call to
+  // ShareToProcess.
+  CrossProcessNotification(IPCHandle handle_1, IPCHandle handle_2);
+  ~CrossProcessNotification();
+
+  // Raises a signal that the shared resource now can be accessed by the other
+  // party.
+  // NOTE: Calling Signal() more than once without calling Wait() in between
+  // is not a supported scenario and will result in undefined behavior (and
+  // different depending on platform).
+  void Signal();
+
+  // Waits for the other party to finish using the shared memory.

[Ami GONE FROM CHROMIUM, 2012/03/14 16:40:51]

s/memory/resource/

[tommi (sloooow) - chröme, 2012/03/14 21:20:38]

Done.

+  // NOTE: As with Signal(), you must not call Wait() more than once without
+  // calling Signal() in between.
+  void Wait();
+
+  bool IsValid() const;
+
+  // Copies the internal handles to the output parameters, |handle_1| and
+  // |handle_2|.  The operation can fail, so the caller must be prepared to
+  // handle that case.
+  bool ShareToProcess(base::ProcessHandle process, IPCHandle* handle_1,
+                      IPCHandle* handle_2);
+
+  // Initializes a pair of CrossProcessNotification instances.  Note that this
+  // can fail (e.g. due to EMFILE on Linux).
+  static bool InitializePair(CrossProcessNotification* a,
+                             CrossProcessNotification* b);
+
+  // Use an instance of this class when you have to repeatedly wait for multiple
+  // notifications on the same thread.  The class will store information about
+  // which notification was last signaled and try to distribute the signals so
+  // that all notifications get a chance to be processed in times of high load
+  // and a busy one won't starve the others.
+  // TODO(tommi): Support a way to abort the wait.
+  class MEDIA_EXPORT WaitForMultiple :
+      public NON_EXPORTED_BASE(base::NonThreadSafe) {
+   public:
+    // Caller must make sure that the lifetime of the array is greater than
+    // that of the WaitForMultiple instance.
+    explicit WaitForMultiple(const Notifications* notifications);
+
+    // Waits for any of the notifications to be signaled.  Returns the 0 based
+    // index of a signaled synchronizer.

[Ami GONE FROM CHROMIUM, 2012/03/14 16:40:51]

s/synchronizer/notification/

[tommi (sloooow) - chröme, 2012/03/14 21:20:38]

Done.

+    int Wait();
+
+    // Call when the array changes.  This should be called on the same thread
+    // as Wait() is called on and the array must never change while a Wait()
+    // is in progress.
+    void Reset(const Notifications* notifications);
+
+   private:
+    const Notifications* notifications_;
+    size_t wait_offset_;
+  };
+
+ private:
+  // Only called by the WaitForMultiple class.  See documentation
+  // for WaitForMultiple and comments inside WaitMultiple for details.
+  static int WaitMultiple(const Notifications& notifications,
+                          size_t wait_offset);
+
+#if defined(OS_WIN)
+  base::win::ScopedHandle mine_;
+  base::win::ScopedHandle other_;
+#else
+  typedef base::CancelableSyncSocket SocketClass;
+  SocketClass socket_;
+#endif
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(CrossProcessNotification);
+};
+
+#endif  // MEDIA_AUDIO_CROSS_PROCESS_NOTIFICATION_H_