nativeruntime/cpp/libutils/SharedBuffer.cpp

/*
 * Copyright (C) 2005 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

// Derived from
// https://cs.android.com/android/platform/superproject/+/android-11.0.0_r1:system/core/libutils/SharedBuffer.cpp

#define LOG_TAG "sharedbuffer"

#include "SharedBuffer.h"

#include <log/log.h>
#include <stdlib.h>
#include <string.h>

// ---------------------------------------------------------------------------

namespace android {

SharedBuffer* SharedBuffer::alloc(size_t size) {
  // Don't overflow if the combined size of the buffer / header is larger than
  // size_max.
  LOG_ALWAYS_FATAL_IF((size >= (SIZE_MAX - sizeof(SharedBuffer))),
                      "Invalid buffer size %zu", size);

  SharedBuffer* sb =
      static_cast<SharedBuffer*>(malloc(sizeof(SharedBuffer) + size));
  if (sb) {
    // Should be std::atomic_init(&sb->mRefs, 1);
    // But that generates a warning with some compilers.
    // The following is OK on Android-supported platforms.
    sb->mRefs.store(1, std::memory_order_relaxed);
    sb->mSize = size;
    sb->mClientMetadata = 0;
  }
  return sb;
}

void SharedBuffer::dealloc(const SharedBuffer* released) {
  free(const_cast<SharedBuffer*>(released));
}

SharedBuffer* SharedBuffer::edit() const {
  if (onlyOwner()) {
    return const_cast<SharedBuffer*>(this);
  }
  SharedBuffer* sb = alloc(mSize);
  if (sb) {
    memcpy(sb->data(), data(), size());
    release();
  }
  return sb;
}

SharedBuffer* SharedBuffer::editResize(size_t newSize) const {
  if (onlyOwner()) {
    SharedBuffer* buf = const_cast<SharedBuffer*>(this);
    if (buf->mSize == newSize) return buf;
    // Don't overflow if the combined size of the new buffer / header is larger
    // than size_max.
    LOG_ALWAYS_FATAL_IF((newSize >= (SIZE_MAX - sizeof(SharedBuffer))),
                        "Invalid buffer size %zu", newSize);

    buf = static_cast<SharedBuffer*>(
        realloc(buf, sizeof(SharedBuffer) + newSize));
    if (buf != nullptr) {
      buf->mSize = newSize;
      return buf;
    }
  }
  SharedBuffer* sb = alloc(newSize);
  if (sb) {
    const size_t mySize = mSize;
    memcpy(sb->data(), data(), newSize < mySize ? newSize : mySize);
    release();
  }
  return sb;
}

SharedBuffer* SharedBuffer::attemptEdit() const {
  if (onlyOwner()) {
    return const_cast<SharedBuffer*>(this);
  }
  return nullptr;
}

SharedBuffer* SharedBuffer::reset(size_t new_size) const {
  // cheap-o-reset.
  SharedBuffer* sb = alloc(new_size);
  if (sb) {
    release();
  }
  return sb;
}

void SharedBuffer::acquire() const {
  mRefs.fetch_add(1, std::memory_order_relaxed);
}

int32_t SharedBuffer::release(uint32_t flags) const {
  const bool useDealloc = ((flags & eKeepStorage) == 0);
  if (onlyOwner()) {
    // Since we're the only owner, our reference count goes to zero.
    mRefs.store(0, std::memory_order_relaxed);
    if (useDealloc) {
      dealloc(this);
    }
    // As the only owner, our previous reference count was 1.
    return 1;
  }
  // There's multiple owners, we need to use an atomic decrement.
  int32_t prevRefCount = mRefs.fetch_sub(1, std::memory_order_release);
  if (prevRefCount == 1) {
    // We're the last reference, we need the acquire fence.
    std::atomic_thread_fence(std::memory_order_acquire);
    if (useDealloc) {
      dealloc(this);
    }
  }
  return prevRefCount;
}

};  // namespace android