mojo/system/raw_shared_buffer.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 MOJO_SYSTEM_RAW_SHARED_BUFFER_H_
#define MOJO_SYSTEM_RAW_SHARED_BUFFER_H_

#include <stddef.h>

#include "base/macros.h"
#include "base/memory/ref_counted.h"
#include "base/memory/scoped_ptr.h"
#include "mojo/embedder/scoped_platform_handle.h"
#include "mojo/system/system_impl_export.h"

namespace mojo {
namespace system {

class RawSharedBufferMapping;

// |RawSharedBuffer| is a thread-safe, ref-counted wrapper around OS-specific
// shared memory. It has the following features:
//   - A |RawSharedBuffer| simply represents a piece of shared memory that *may*
//     be mapped and *may* be shared to another process.
//   - A single |RawSharedBuffer| may be mapped multiple times. The lifetime of
//     the mapping (owned by |RawSharedBufferMapping|) is separate from the
//     lifetime of the |RawSharedBuffer|.
//   - Sizes/offsets (of the shared memory and mappings) are arbitrary, and not
//     restricted by page size. However, more memory may actually be mapped than
//     requested.
//
// It currently does NOT support the following:
//   - Sharing read-only. (This will probably eventually be supported.)
//
// TODO(vtl): Rectify this with |base::SharedMemory|.
class MOJO_SYSTEM_IMPL_EXPORT RawSharedBuffer
    : public base::RefCountedThreadSafe<RawSharedBuffer> {
 public:
  // Creates a shared buffer of size |num_bytes| bytes (initially zero-filled).
  // |num_bytes| must be nonzero. Returns null on failure.
  static RawSharedBuffer* Create(size_t num_bytes);

  static RawSharedBuffer* CreateFromPlatformHandle(
      size_t num_bytes,
      embedder::ScopedPlatformHandle platform_handle);

  // Maps (some) of the shared buffer into memory; [|offset|, |offset + length|]
  // must be contained in [0, |num_bytes|], and |length| must be at least 1.
  // Returns null on failure.
  scoped_ptr<RawSharedBufferMapping> Map(size_t offset, size_t length);

  // Checks if |offset| and |length| are valid arguments.
  bool IsValidMap(size_t offset, size_t length);

  // Like |Map()|, but doesn't check its arguments (which should have been
  // preflighted using |IsValidMap()|).
  scoped_ptr<RawSharedBufferMapping> MapNoCheck(size_t offset, size_t length);

  // Duplicates the underlying platform handle and passes it to the caller.
  embedder::ScopedPlatformHandle DuplicatePlatformHandle();

  // Passes the underlying platform handle to the caller. This should only be
  // called if there's a unique reference to this object (owned by the caller).
  // After calling this, this object should no longer be used, but should only
  // be disposed of.
  embedder::ScopedPlatformHandle PassPlatformHandle();

  size_t num_bytes() const { return num_bytes_; }

 private:
  friend class base::RefCountedThreadSafe<RawSharedBuffer>;

  explicit RawSharedBuffer(size_t num_bytes);
  ~RawSharedBuffer();

  // Implemented in raw_shared_buffer_{posix,win}.cc:

  // This is called by |Create()| before this object is given to anyone.
  bool Init();

  // This is like |Init()|, but for |CreateFromPlatformHandle()|. (Note: It
  // should verify that |platform_handle| is an appropriate handle for the
  // claimed |num_bytes_|.)
  bool InitFromPlatformHandle(embedder::ScopedPlatformHandle platform_handle);

  // The platform-dependent part of |Map()|; doesn't check arguments.
  scoped_ptr<RawSharedBufferMapping> MapImpl(size_t offset, size_t length);

  const size_t num_bytes_;

  // This is set in |Init()|/|InitFromPlatformHandle()| and never modified
  // (except by |PassPlatformHandle()|; see the comments above its declaration),
  // hence does not need to be protected by a lock.
  embedder::ScopedPlatformHandle handle_;

  DISALLOW_COPY_AND_ASSIGN(RawSharedBuffer);
};

// A mapping of a |RawSharedBuffer| (compararable to a "file view" in Windows);
// see above. Created by |RawSharedBuffer::Map()|. Automatically unmaps memory
// on destruction.
//
// Mappings are NOT thread-safe.
//
// Note: This is an entirely separate class (instead of
// |RawSharedBuffer::Mapping|) so that it can be forward-declared.
class MOJO_SYSTEM_IMPL_EXPORT RawSharedBufferMapping {
 public:
  ~RawSharedBufferMapping() { Unmap(); }

  void* base() const { return base_; }
  size_t length() const { return length_; }

 private:
  friend class RawSharedBuffer;

  RawSharedBufferMapping(void* base,
                         size_t length,
                         void* real_base,
                         size_t real_length)
      : base_(base), length_(length),
        real_base_(real_base), real_length_(real_length) {}
  void Unmap();

  void* const base_;
  const size_t length_;

  void* const real_base_;
  const size_t real_length_;

  DISALLOW_COPY_AND_ASSIGN(RawSharedBufferMapping);
};

}  // namespace system
}  // namespace mojo

#endif  // MOJO_SYSTEM_RAW_SHARED_BUFFER_H_