C++ OutputBuffer::Get方法代码示例

Result<Ok, nsresult>
WritableSharedMap::Serialize()
{
  // Serializes a new snapshot of the map, initializes a new read-only shared
  // memory region with its contents, and updates all entries to point to that
  // new snapshot.
  //
  // The layout of the snapshot is as follows:
  //
  // - A header containing a uint32 count field containing the number of
  //   entries in the map, followed by that number of serialized entry headers,
  //   as produced by Entry::Code.
  //
  // - A data block containing structured clone data for each of the entries'
  //   values. This data is referenced by absolute byte offsets from the start
  //   of the shared memory region, encoded in each of the entry header values.
  //   Each entry's data is aligned to kStructuredCloneAlign, and therefore may
  //   have alignment padding before it.
  //
  // This serialization format is decoded by the MaybeRebuild() method of
  // read-only SharedMap() instances, and used to populate their mEntries
  // hashtables.
  //
  // Writable instances never read the header blocks, but instead directly
  // update their Entry instances to point to the appropriate offsets in the
  // shared memory region created by this function.

  uint32_t count = mEntries.Count();

  size_t dataSize = 0;
  size_t headerSize = sizeof(count);
  size_t blobCount = 0;

  for (auto& entry : IterHash(mEntries)) {
    headerSize += entry->HeaderSize();
    blobCount += entry->BlobCount();

    dataSize += entry->Size();
    AlignTo(&dataSize, kStructuredCloneAlign);
  }

  size_t offset = headerSize;
  AlignTo(&offset, kStructuredCloneAlign);

  OutputBuffer header;
  header.codeUint32(count);

  MemMapSnapshot mem;
  MOZ_TRY(mem.Init(offset + dataSize));

  auto ptr = mem.Get<char>();

  // We need to build the new array of blobs before we overwrite the existing
  // one, since previously-serialized entries will store their blob references
  // as indexes into our blobs array.
  nsTArray<RefPtr<BlobImpl>> blobImpls(blobCount);

  for (auto& entry : IterHash(mEntries)) {
    AlignTo(&offset, kStructuredCloneAlign);

    size_t blobOffset = blobImpls.Length();
    if (entry->BlobCount()) {
      blobImpls.AppendElements(entry->Blobs());
    }

    entry->ExtractData(&ptr[offset], offset, blobOffset);
    entry->Code(header);

    offset += entry->Size();
  }

  mBlobImpls = std::move(blobImpls);

  // FIXME: We should create a separate OutputBuffer class which can encode to
  // a static memory region rather than dynamically allocating and then
  // copying.
  MOZ_ASSERT(header.cursor() == headerSize);
  memcpy(ptr.get(), header.Get(), header.cursor());

  // We've already updated offsets at this point. We need this to succeed.
  mMap.reset();
  MOZ_RELEASE_ASSERT(mem.Finalize(mMap).isOk());

  return Ok();
}