Add helper classes to for managing shared buffers.

mojo/services/media/common/cpp/mapped_shared_buffer.h

Index: mojo/services/media/common/cpp/mapped_shared_buffer.h
diff --git a/mojo/services/media/common/cpp/mapped_shared_buffer.h b/mojo/services/media/common/cpp/mapped_shared_buffer.h
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+++ b/mojo/services/media/common/cpp/mapped_shared_buffer.h
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+// Copyright 2015 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_SERVICES_MEDIA_COMMON_CPP_MAPPED_SHARED_BUFFER_H_
+#define MOJO_SERVICES_MEDIA_COMMON_CPP_MAPPED_SHARED_BUFFER_H_
+
+#include <memory>
+
+#include "mojo/public/cpp/system/buffer.h"
+#include "mojo/services/media/common/cpp/fifo_allocator.h"
+
+namespace mojo {
+namespace media {
+
+// MappedSharedBuffer simplifies the use of shared buffers by taking care of
+// mapping/unmapping and by providing offset/pointer translation. It can be
+// used when the caller wants to allocate its own buffer (InitNew) and when
+// the caller needs to use a buffer supplied by another party (InitFromHandle).
+// It can be used by itself when regions of the buffer are allocated by another
+// party. If the caller needs to allocate regions, SharedMediaBufferAllocator,
+// which is derived from MappedSharedBuffer, provides allocation semantics
+// using FifoAllocator.
+class MappedSharedBuffer {
+ public:
+  MappedSharedBuffer();
+
+  virtual ~MappedSharedBuffer();
+
+  // Initializes by creating a new shared buffer of the indicated size.
+  void InitNew(uint64_t size);
+
+  // Initializes from a handle to an existing shared buffer.
+  void InitFromHandle(ScopedSharedBufferHandle handle, uint64_t size);
+
+  // Indicates whether the buffer is initialized.
+  bool initialized() const;
+
+  // Gets the size of the buffer.
+  uint64_t size() const;
+
+  // Gets a duplicate handle for the shared buffer.
+  ScopedSharedBufferHandle GetDuplicateHandle() const;
+
+  // Translates an offset into a pointer.
+  void* PtrFromOffset(uint64_t offset) const;
+
+  // Translates a pointer into an offset.
+  uint64_t OffsetFromPtr(void *payload_ptr) const;
+
+ protected:
+  void InitInternal(ScopedSharedBufferHandle& handle, uint64_t size);
+
+  // Does nothing. Called when initialization is complete. Subclasses may
+  // override.
+  virtual void OnInit();
+
+ private:
+  struct MappedBufferDeleter {
+    inline void operator()(uint8_t* ptr) const {
+      UnmapBuffer(ptr);
+    }
+  };
+
+  // Size of the shared buffer.
+  uint64_t size_;
+
+  // Shared buffer when initialized with InitNew.
+  std::unique_ptr<SharedBuffer> buffer_;
+
+  // Handle to shared buffer when initialized with InitFromHandle.
+  ScopedSharedBufferHandle handle_;
+
+  // Pointer to the mapped buffer.
+  std::unique_ptr<uint8_t, MappedBufferDeleter> buffer_ptr_;
+};
+
+}  // namespace media
+}  // namespace mojo
+
+#endif // MOJO_SERVICES_MEDIA_COMMON_CPP_MAPPED_SHARED_BUFFER_H_