IPC to pass media data using a lock free circular fifo.

chromecast/media/cma/ipc/media_message_fifo.h

+// Copyright 2014 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 CHROMECAST_MEDIA_CMA_IPC_MEDIA_MESSAGE_FIFO_H_
+#define CHROMECAST_MEDIA_CMA_IPC_MEDIA_MESSAGE_FIFO_H_
+
+#include <list>
+
+#include "base/atomicops.h"
+#include "base/basictypes.h"
+#include "base/callback.h"
+#include "base/logging.h"
+#include "base/macros.h"
+#include "base/memory/ref_counted.h"
+#include "base/memory/scoped_ptr.h"
+#include "base/memory/weak_ptr.h"
+#include "base/threading/thread_checker.h"
+
+namespace chromecast {
+namespace media {
+class MediaMemoryChunk;
+class MediaMessage;
+class MediaMessageFlag;
+
+class MediaMessageFifo {
+ public:
+  // Creates a media message fifo using |mem| as the underlying serialized
+  // structure.
+  // If |init| is true, the underlying fifo structure is initialized.
+  MediaMessageFifo(scoped_ptr<MediaMemoryChunk> mem, bool init);
+  ~MediaMessageFifo();
+
+  // When the consumer and the feeder are living in two different processes,
+  // we might want to convey some messages between these two processes to notify
+  // about some fifo activity.
+  void ObserveReadActivity(const base::Closure& read_event_cb);
+  void ObserveWriteActivity(const base::Closure& write_event_cb);
+
+  // Reserves a writeable block of memory at the back of the fifo,
+  // corresponding to the serialized structure of the message.
+  scoped_ptr<MediaMemoryChunk> ReserveMemory(size_t size);

[xhwang, 2014/09/03 17:40:33]

What if the memory of |size| cannot be reserved? Will NULL be returned?

[damienv1, 2014/09/03 23:59:05]

Done.

+
+  // Pop a message from the queue.
+  // Returns a null pointer if there is no message left.
+  scoped_ptr<MediaMessage> Pop();
+
+  // Flush the fifo.
+  void Flush();
+
+ private:
+  struct Descriptor {
+    size_t size;
+    size_t rd_offset;
+    size_t wr_offset;
+
+    // Ensure the first item has the same alignment as an int64.
+    int64 first_item;
+  };
+
+  // Add some accessors to ensure security on the browser process side.
+  size_t current_rd_offset() const;
+  size_t current_wr_offset() const;
+  size_t internal_rd_offset() const {
+    DCHECK_LT(internal_rd_offset_, size_);
+    return internal_rd_offset_;
+  }
+  size_t internal_wr_offset() const {
+    DCHECK_LT(internal_wr_offset_, size_);
+    return internal_wr_offset_;
+  }
+
+  // Reserve a block of free memory without doing any check on the available
+  // space. Invoke this function only when all the checks have been done.
+  scoped_ptr<MediaMemoryChunk> ReserveMemoryNoCheck(size_t size);
+
+  // Invoked each time there is a memory region in the free space of the fifo
+  // that has possibly been written.
+  void OnWrMemoryReleased();
+
+  // Invoked each time there is a memory region in the allocated space
+  // of the fifo that has possibly been released.
+  void OnRdMemoryReleased();
+
+  void CommitRead(size_t new_rd_offset);
+  void CommitWrite(size_t new_wr_offset);
+  void CommitInternalRead(size_t new_rd_offset);
+  void CommitInternalWrite(size_t new_wr_offset);
+
+  // Having a thread checker does not mean that the feeder and the consumer
+  // cannot live on different threads. If the consumer and the feeder are
+  // living on two different threads, then there should be one MediaMessageFifo
+  // instance for the feeder side and another instance for the consumer side,
+  // both pointing to the same fifo memory structure.
+  base::ThreadChecker thread_checker_;

[xhwang, 2014/09/03 17:40:33]

Given the comments and w/o looking at the impl, I wonder what this checker is
checking?

[damienv1, 2014/09/03 23:59:06]

I updated the comment. Hope it's less confusing.

+
+  // Callbacks invoked to notify either of some read or write activity on the
+  // fifo. This is especially useful when the feeder and consumer are living in
+  // two different processes.
+  base::Closure read_event_cb_;
+  base::Closure write_event_cb_;
+
+  // The serialized structure of the fifo.
+  scoped_ptr<MediaMemoryChunk> mem_;
+
+  // The size in bytes of the fifo is cached locally for security purpose.
+  // (the renderer process cannot modify the size and make the browser process
+  // access out of range addresses).
+  size_t size_;
+
+  // TODO(damienv): This is a work-around since atomicops.h does not define
+  // an atomic size_t type.
+#if SIZE_MAX == UINT32_MAX
+  typedef base::subtle::Atomic32 AtomicSize;
+#elif SIZE_MAX == UINT64_MAX
+  typedef base::subtle::Atomic64 AtomicSize;
+#elif
+#error "Unsupported size_t"
+#endif
+  AtomicSize* rd_offset_;
+  AtomicSize* wr_offset_;
+
+  // Internal read offset: this is where data is actually read from.
+  // The external offset |rd_offset_| is only used to protect data from being
+  // overwritten by the feeder.
+  // At any time, the internal read pointer must be between the external read
+  // offset and the write offset (circular fifo definition of "between").
+  size_t internal_rd_offset_;
+  size_t internal_wr_offset_;
+
+  // Note: all the memory read/write are followed by a memory fence before
+  // updating the rd/wr pointer.
+  void* base_;
+
+  std::list<scoped_refptr<MediaMessageFlag> > rd_flags_;
+  std::list<scoped_refptr<MediaMessageFlag> > wr_flags_;
+
+  base::WeakPtrFactory<MediaMessageFifo> weak_factory_;
+  base::WeakPtr<MediaMessageFifo> weak_this_;
+
+  DISALLOW_COPY_AND_ASSIGN(MediaMessageFifo);
+};
+
+}  // namespace media
+}  // namespace chromecast
+
+#endif  // CHROMECAST_MEDIA_CMA_IPC_MEDIA_MESSAGE_FIFO_H_