gecko-dev/dom/media/SharedBuffer.h

/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this file,
 * You can obtain one at http://mozilla.org/MPL/2.0/. */

#ifndef MOZILLA_SHAREDBUFFER_H_
#define MOZILLA_SHAREDBUFFER_H_

#include "mozilla/CheckedInt.h"
#include "mozilla/mozalloc.h"
#include "mozilla/MemoryReporting.h"
#include "nsISupportsImpl.h"

namespace mozilla {

class AudioBlockBuffer;
class ThreadSharedFloatArrayBufferList;

/**
 * Base class for objects with a thread-safe refcount and a virtual
 * destructor.
 */
class ThreadSharedObject {
 public:
  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(ThreadSharedObject)

  bool IsShared() { return mRefCnt.get() > 1; }

  virtual AudioBlockBuffer* AsAudioBlockBuffer() { return nullptr; };
  virtual ThreadSharedFloatArrayBufferList*
  AsThreadSharedFloatArrayBufferList() {
    return nullptr;
  };

  virtual size_t SizeOfExcludingThis(MallocSizeOf aMallocSizeOf) const {
    return 0;
  }

  virtual size_t SizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const {
    return aMallocSizeOf(this) + SizeOfExcludingThis(aMallocSizeOf);
  }

 protected:
  // Protected destructor, to discourage deletion outside of Release():
  virtual ~ThreadSharedObject() = default;
};

/**
 * Heap-allocated chunk of arbitrary data with threadsafe refcounting.
 * Typically you would allocate one of these, fill it in, and then treat it as
 * immutable while it's shared.
 * This only guarantees 4-byte alignment of the data. For alignment we simply
 * assume that the memory from malloc is at least 4-byte aligned and the
 * refcount's size is large enough that SharedBuffer's size is divisible by 4.
 */
class SharedBuffer : public ThreadSharedObject {
 public:
  void* Data() { return this + 1; }

  // Ensure that the caller has a CheckedInt.  We have to take one by
  // non-const reference to do that, because if we take one by const
  // reference or value or rvalue reference the implicit constructor on
  // CheckedInt will helpfully synthesize one for us at the callsite
  // even if the caller passes a raw size_t.
  static already_AddRefed<SharedBuffer> Create(CheckedInt<size_t>& aSize,
                                               const fallible_t&) {
    CheckedInt<size_t> allocSize = AllocSize(aSize, fallible);
    if (!allocSize.isValid()) {
      return nullptr;
    }
    void* m = operator new(allocSize.value(), fallible);
    if (!m) {
      return nullptr;
    }
    RefPtr<SharedBuffer> p = new (m) SharedBuffer();
    return p.forget();
  }

  static already_AddRefed<SharedBuffer> Create(CheckedInt<size_t>& aSize) {
    void* m = operator new(AllocSize(aSize));
    RefPtr<SharedBuffer> p = new (m) SharedBuffer();
    return p.forget();
  }

  size_t SizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const override {
    return aMallocSizeOf(this) + SizeOfExcludingThis(aMallocSizeOf);
  }

 private:
  static CheckedInt<size_t> AllocSize(CheckedInt<size_t> aDataSize,
                                      const fallible_t&) {
    CheckedInt<size_t> size = sizeof(SharedBuffer);
    size += aDataSize;
    return size;
  }

  static size_t AllocSize(CheckedInt<size_t> aDataSize) {
    CheckedInt<size_t> size = AllocSize(aDataSize, fallible);
    if (!size.isValid()) {
      MOZ_CRASH();
    }
    return size.value();
  }

  SharedBuffer() {
    NS_ASSERTION(
        (reinterpret_cast<char*>(this + 1) - reinterpret_cast<char*>(this)) %
                4 ==
            0,
        "SharedBuffers should be at least 4-byte aligned");
  }
};

}  // namespace mozilla

#endif /* MOZILLA_SHAREDBUFFER_H_ */