mojo/core/channel.cc

// Copyright 2016 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.

#include "mojo/core/channel.h"

#include <stddef.h>
#include <string.h>

#include <algorithm>
#include <limits>
#include <utility>

#include "base/macros.h"
#include "base/memory/aligned_memory.h"
#include "base/numerics/safe_math.h"
#include "base/process/process_handle.h"
#include "build/build_config.h"
#include "mojo/core/configuration.h"
#include "mojo/core/core.h"

#if defined(OS_MACOSX) && !defined(OS_IOS)
#include "base/mac/mach_logging.h"
#elif defined(OS_WIN)
#include "base/win/win_util.h"
#endif

namespace mojo {
namespace core {

namespace {

static_assert(
    IsAlignedForChannelMessage(sizeof(Channel::Message::LegacyHeader)),
    "Invalid LegacyHeader size.");

static_assert(IsAlignedForChannelMessage(sizeof(Channel::Message::Header)),
              "Invalid Header size.");

static_assert(sizeof(Channel::Message::LegacyHeader) == 8,
              "LegacyHeader must be 8 bytes on ChromeOS and Android");

static_assert(offsetof(Channel::Message::LegacyHeader, num_bytes) ==
                  offsetof(Channel::Message::Header, num_bytes),
              "num_bytes should be at the same offset in both Header structs.");
static_assert(offsetof(Channel::Message::LegacyHeader, message_type) ==
                  offsetof(Channel::Message::Header, message_type),
              "message_type should be at the same offset in both Header "
              "structs.");

}  // namespace

const size_t kReadBufferSize = 4096;
const size_t kMaxUnusedReadBufferCapacity = 4096;

// TODO(rockot): Increase this if/when Channel implementations support more.
// Linux: The platform imposes a limit of 253 handles per sendmsg().
// Fuchsia: The zx_channel_write() API supports up to 64 handles.
const size_t kMaxAttachedHandles = 64;

Channel::Message::Message(size_t payload_size, size_t max_handles)
    : Message(payload_size, payload_size, max_handles) {}

Channel::Message::Message(size_t payload_size,
                          size_t max_handles,
                          MessageType message_type)
    : Message(payload_size, payload_size, max_handles, message_type) {}

Channel::Message::Message(size_t capacity,
                          size_t payload_size,
                          size_t max_handles)
#if defined(MOJO_CORE_LEGACY_PROTOCOL)
    : Message(capacity, payload_size, max_handles, MessageType::NORMAL_LEGACY) {
}
#else
    : Message(capacity, payload_size, max_handles, MessageType::NORMAL) {
}
#endif

Channel::Message::Message(size_t capacity,
                          size_t payload_size,
                          size_t max_handles,
                          MessageType message_type)
    : max_handles_(max_handles) {
  DCHECK_GE(capacity, payload_size);
  DCHECK_LE(max_handles_, kMaxAttachedHandles);

  const bool is_legacy_message = (message_type == MessageType::NORMAL_LEGACY);
  size_t extra_header_size = 0;
#if defined(OS_WIN)
  // On Windows we serialize HANDLEs into the extra header space.
  extra_header_size = max_handles_ * sizeof(HandleEntry);
#elif defined(OS_FUCHSIA)
  // On Fuchsia we serialize handle types into the extra header space.
  extra_header_size = max_handles_ * sizeof(HandleInfoEntry);
#elif defined(OS_MACOSX) && !defined(OS_IOS)
  // On OSX, some of the platform handles may be mach ports, which are
  // serialised into the message buffer. Since there could be a mix of fds and
  // mach ports, we store the mach ports as an <index, port> pair (of uint32_t),
  // so that the original ordering of handles can be re-created.
  if (max_handles) {
    extra_header_size =
        sizeof(MachPortsExtraHeader) + (max_handles * sizeof(MachPortsEntry));
  }
#endif
  // Pad extra header data to be aliged to |kChannelMessageAlignment| bytes.
  if (!IsAlignedForChannelMessage(extra_header_size)) {
    extra_header_size += kChannelMessageAlignment -
                         (extra_header_size % kChannelMessageAlignment);
  }
  DCHECK(IsAlignedForChannelMessage(extra_header_size));
  const size_t header_size =
      is_legacy_message ? sizeof(LegacyHeader) : sizeof(Header);
  DCHECK(extra_header_size == 0 || !is_legacy_message);

  capacity_ = header_size + extra_header_size + capacity;
  size_ = header_size + extra_header_size + payload_size;
  data_ = static_cast<char*>(
      base::AlignedAlloc(capacity_, kChannelMessageAlignment));
  // Only zero out the header and not the payload. Since the payload is going to
  // be memcpy'd, zeroing the payload is unnecessary work and a significant
  // performance issue when dealing with large messages. Any sanitizer errors
  // complaining about an uninitialized read in the payload area should be
  // treated as an error and fixed.
  memset(data_, 0, header_size + extra_header_size);

  DCHECK(base::IsValueInRangeForNumericType<uint32_t>(size_));
  legacy_header()->num_bytes = static_cast<uint32_t>(size_);

  DCHECK(base::IsValueInRangeForNumericType<uint16_t>(header_size +
                                                      extra_header_size));
  legacy_header()->message_type = message_type;

  if (is_legacy_message) {
    legacy_header()->num_handles = static_cast<uint16_t>(max_handles);
  } else {
    header()->num_header_bytes =
        static_cast<uint16_t>(header_size + extra_header_size);
  }

  if (max_handles_ > 0) {
#if defined(OS_WIN)
    handles_ = reinterpret_cast<HandleEntry*>(mutable_extra_header());
    // Initialize all handles to invalid values.
    for (size_t i = 0; i < max_handles_; ++i)
      handles_[i].handle = base::win::HandleToUint32(INVALID_HANDLE_VALUE);
#elif defined(OS_MACOSX) && !defined(OS_IOS)
    mach_ports_header_ =
        reinterpret_cast<MachPortsExtraHeader*>(mutable_extra_header());
    mach_ports_header_->num_ports = 0;
    // Initialize all handles to invalid values.
    for (size_t i = 0; i < max_handles_; ++i) {
      mach_ports_header_->entries[i] = {0,
                                        static_cast<uint32_t>(MACH_PORT_NULL)};
    }
#endif
  }
}

Channel::Message::~Message() {
  base::AlignedFree(data_);
}

// static
Channel::MessagePtr Channel::Message::Deserialize(
    const void* data,
    size_t data_num_bytes,
    base::ProcessHandle from_process) {
  if (data_num_bytes < sizeof(LegacyHeader))
    return nullptr;

  const LegacyHeader* legacy_header =
      reinterpret_cast<const LegacyHeader*>(data);
  if (legacy_header->num_bytes != data_num_bytes) {
    DLOG(ERROR) << "Decoding invalid message: " << legacy_header->num_bytes
                << " != " << data_num_bytes;
    return nullptr;
  }

  const Header* header = nullptr;
  if (legacy_header->message_type == MessageType::NORMAL)
    header = reinterpret_cast<const Header*>(data);

  uint32_t extra_header_size = 0;
  size_t payload_size = 0;
  const char* payload = nullptr;
  if (!header) {
    payload_size = data_num_bytes - sizeof(LegacyHeader);
    payload = static_cast<const char*>(data) + sizeof(LegacyHeader);
  } else {
    if (header->num_bytes < header->num_header_bytes ||
        header->num_header_bytes < sizeof(Header)) {
      DLOG(ERROR) << "Decoding invalid message: " << header->num_bytes << " < "
                  << header->num_header_bytes;
      return nullptr;
    }
    extra_header_size = header->num_header_bytes - sizeof(Header);
    payload_size = data_num_bytes - header->num_header_bytes;
    payload = static_cast<const char*>(data) + header->num_header_bytes;
  }

#if defined(OS_WIN)
  uint32_t max_handles = extra_header_size / sizeof(HandleEntry);
#elif defined(OS_FUCHSIA)
  uint32_t max_handles = extra_header_size / sizeof(HandleInfoEntry);
#elif defined(OS_MACOSX) && !defined(OS_IOS)
  if (extra_header_size > 0 &&
      extra_header_size < sizeof(MachPortsExtraHeader)) {
    DLOG(ERROR) << "Decoding invalid message: " << extra_header_size << " < "
                << sizeof(MachPortsExtraHeader);
    return nullptr;
  }
  uint32_t max_handles =
      extra_header_size == 0
          ? 0
          : (extra_header_size - sizeof(MachPortsExtraHeader)) /
                sizeof(MachPortsEntry);
#else
  const uint32_t max_handles = 0;
#endif  // defined(OS_WIN)

  const uint16_t num_handles =
      header ? header->num_handles : legacy_header->num_handles;
  if (num_handles > max_handles || max_handles > kMaxAttachedHandles) {
    DLOG(ERROR) << "Decoding invalid message: " << num_handles << " > "
                << max_handles;
    return nullptr;
  }

  MessagePtr message(
      new Message(payload_size, max_handles, legacy_header->message_type));
  DCHECK_EQ(message->data_num_bytes(), data_num_bytes);

  // Copy all payload bytes.
  if (payload_size)
    memcpy(message->mutable_payload(), payload, payload_size);

  if (header) {
    DCHECK_EQ(message->extra_header_size(), extra_header_size);
    DCHECK_EQ(message->header()->num_header_bytes, header->num_header_bytes);

    if (message->extra_header_size()) {
      // Copy extra header bytes.
      memcpy(message->mutable_extra_header(),
             static_cast<const char*>(data) + sizeof(Header),
             message->extra_header_size());
    }
    message->header()->num_handles = header->num_handles;
  } else {
    message->legacy_header()->num_handles = legacy_header->num_handles;
  }

#if defined(OS_WIN)
  std::vector<PlatformHandleInTransit> handles(num_handles);
  for (size_t i = 0; i < num_handles; i++) {
    HANDLE handle = base::win::Uint32ToHandle(message->handles_[i].handle);
    if (from_process == base::kNullProcessHandle) {
      handles[i] = PlatformHandleInTransit(
          PlatformHandle(base::win::ScopedHandle(handle)));
    } else {
      handles[i] = PlatformHandleInTransit(
          PlatformHandleInTransit::TakeIncomingRemoteHandle(handle,
                                                            from_process));
    }
  }
  message->SetHandles(std::move(handles));
#endif

  return message;
}

size_t Channel::Message::capacity() const {
  if (is_legacy_message())
    return capacity_ - sizeof(LegacyHeader);
  return capacity_ - header()->num_header_bytes;
}

void Channel::Message::ExtendPayload(size_t new_payload_size) {
  size_t capacity_without_header = capacity();
  size_t header_size = capacity_ - capacity_without_header;
  if (new_payload_size > capacity_without_header) {
    size_t new_capacity =
        std::max(capacity_without_header * 2, new_payload_size) + header_size;
    void* new_data = base::AlignedAlloc(new_capacity, kChannelMessageAlignment);
    memcpy(new_data, data_, capacity_);
    base::AlignedFree(data_);
    data_ = static_cast<char*>(new_data);
    capacity_ = new_capacity;

    if (max_handles_ > 0) {
// We also need to update the cached extra header addresses in case the
// payload buffer has been relocated.
#if defined(OS_WIN)
      handles_ = reinterpret_cast<HandleEntry*>(mutable_extra_header());
#elif defined(OS_MACOSX) && !defined(OS_IOS)
      mach_ports_header_ =
          reinterpret_cast<MachPortsExtraHeader*>(mutable_extra_header());
#endif
    }
  }
  size_ = header_size + new_payload_size;
  DCHECK(base::IsValueInRangeForNumericType<uint32_t>(size_));
  legacy_header()->num_bytes = static_cast<uint32_t>(size_);
}

const void* Channel::Message::extra_header() const {
  DCHECK(!is_legacy_message());
  return data_ + sizeof(Header);
}

void* Channel::Message::mutable_extra_header() {
  DCHECK(!is_legacy_message());
  return data_ + sizeof(Header);
}

size_t Channel::Message::extra_header_size() const {
  return header()->num_header_bytes - sizeof(Header);
}

void* Channel::Message::mutable_payload() {
  if (is_legacy_message())
    return static_cast<void*>(legacy_header() + 1);
  return data_ + header()->num_header_bytes;
}

const void* Channel::Message::payload() const {
  if (is_legacy_message())
    return static_cast<const void*>(legacy_header() + 1);
  return data_ + header()->num_header_bytes;
}

size_t Channel::Message::payload_size() const {
  if (is_legacy_message())
    return legacy_header()->num_bytes - sizeof(LegacyHeader);
  return size_ - header()->num_header_bytes;
}

size_t Channel::Message::num_handles() const {
  return is_legacy_message() ? legacy_header()->num_handles
                             : header()->num_handles;
}

bool Channel::Message::has_handles() const {
  return (is_legacy_message() ? legacy_header()->num_handles
                              : header()->num_handles) > 0;
}

#if defined(OS_MACOSX) && !defined(OS_IOS)
bool Channel::Message::has_mach_ports() const {
  if (!has_handles())
    return false;

  for (const auto& handle : handle_vector_) {
    if (handle.is_mach_port_name() || handle.handle().is_mach_port())
      return true;
  }
  return false;
}
#endif

bool Channel::Message::is_legacy_message() const {
  return legacy_header()->message_type == MessageType::NORMAL_LEGACY;
}

Channel::Message::LegacyHeader* Channel::Message::legacy_header() const {
  return reinterpret_cast<LegacyHeader*>(data_);
}

Channel::Message::Header* Channel::Message::header() const {
  DCHECK(!is_legacy_message());
  return reinterpret_cast<Header*>(data_);
}

void Channel::Message::SetHandles(std::vector<PlatformHandle> new_handles) {
  std::vector<PlatformHandleInTransit> handles;
  handles.reserve(new_handles.size());
  for (auto& h : new_handles) {
    handles.emplace_back(PlatformHandleInTransit(std::move(h)));
  }
  SetHandles(std::move(handles));
}

void Channel::Message::SetHandles(
    std::vector<PlatformHandleInTransit> new_handles) {
  if (is_legacy_message()) {
    // Old semantics for ChromeOS and Android
    if (legacy_header()->num_handles == 0) {
      CHECK(new_handles.empty());
      return;
    }
    CHECK_EQ(new_handles.size(), legacy_header()->num_handles);
    std::swap(handle_vector_, new_handles);
    return;
  }

  if (max_handles_ == 0) {
    CHECK(new_handles.empty());
    return;
  }

  CHECK_LE(new_handles.size(), max_handles_);
  header()->num_handles = static_cast<uint16_t>(new_handles.size());
  std::swap(handle_vector_, new_handles);
#if defined(OS_WIN)
  memset(handles_, 0, extra_header_size());
  for (size_t i = 0; i < handle_vector_.size(); i++) {
    HANDLE handle = handle_vector_[i].remote_handle();
    if (handle == INVALID_HANDLE_VALUE)
      handle = handle_vector_[i].handle().GetHandle().Get();
    handles_[i].handle = base::win::HandleToUint32(handle);
  }
#endif  // defined(OS_WIN)

#if defined(OS_MACOSX) && !defined(OS_IOS)
  size_t mach_port_index = 0;
  if (mach_ports_header_) {
    for (size_t i = 0; i < max_handles_; ++i) {
      mach_ports_header_->entries[i] = {0,
                                        static_cast<uint32_t>(MACH_PORT_NULL)};
    }
    for (size_t i = 0; i < handle_vector_.size(); i++) {
      if (!handle_vector_[i].is_mach_port_name() &&
          !handle_vector_[i].handle().is_mach_port()) {
        DCHECK(handle_vector_[i].handle().is_valid_fd());
        continue;
      }

      mach_port_t port = handle_vector_[i].is_mach_port_name()
                             ? handle_vector_[i].mach_port_name()
                             : handle_vector_[i].handle().GetMachPort().get();
      mach_ports_header_->entries[mach_port_index].index = i;
      mach_ports_header_->entries[mach_port_index].mach_port = port;
      mach_port_index++;
    }
    mach_ports_header_->num_ports = static_cast<uint16_t>(mach_port_index);
  }
#endif
}

std::vector<PlatformHandleInTransit> Channel::Message::TakeHandles() {
#if defined(OS_MACOSX) && !defined(OS_IOS)
  if (mach_ports_header_) {
    for (size_t i = 0; i < max_handles_; ++i) {
      mach_ports_header_->entries[i] = {0,
                                        static_cast<uint32_t>(MACH_PORT_NULL)};
    }
    mach_ports_header_->num_ports = 0;
  }
#endif
  if (is_legacy_message())
    legacy_header()->num_handles = 0;
  else
    header()->num_handles = 0;
  return std::move(handle_vector_);
}

std::vector<PlatformHandleInTransit>
Channel::Message::TakeHandlesForTransport() {
#if defined(OS_WIN)
  // Not necessary on Windows.
  NOTREACHED();
  return std::vector<PlatformHandleInTransit>();
#elif defined(OS_MACOSX) && !defined(OS_IOS)
  std::vector<PlatformHandleInTransit> non_mach_handles;
  for (auto& handle : handle_vector_) {
    if (handle.is_mach_port_name() || handle.handle().is_mach_port()) {
      // Ownership is effectively transferred to the receiving process
      // out-of-band via MachPortRelay.
      handle.CompleteTransit();
    } else {
      non_mach_handles.emplace_back(std::move(handle));
    }
  }
  handle_vector_.clear();
  return non_mach_handles;
#else
  return std::move(handle_vector_);
#endif
}

// Helper class for managing a Channel's read buffer allocations. This maintains
// a single contiguous buffer with the layout:
//
//   [discarded bytes][occupied bytes][unoccupied bytes]
//
// The Reserve() method ensures that a certain capacity of unoccupied bytes are
// available. It does not claim that capacity and only allocates new capacity
// when strictly necessary.
//
// Claim() marks unoccupied bytes as occupied.
//
// Discard() marks occupied bytes as discarded, signifying that their contents
// can be forgotten or overwritten.
//
// Realign() moves occupied bytes to the front of the buffer so that those
// occupied bytes are properly aligned.
//
// The most common Channel behavior in practice should result in very few
// allocations and copies, as memory is claimed and discarded shortly after
// being reserved, and future reservations will immediately reuse discarded
// memory.
class Channel::ReadBuffer {
 public:
  ReadBuffer() {
    size_ = kReadBufferSize;
    data_ =
        static_cast<char*>(base::AlignedAlloc(size_, kChannelMessageAlignment));
  }

  ~ReadBuffer() {
    DCHECK(data_);
    base::AlignedFree(data_);
  }

  const char* occupied_bytes() const { return data_ + num_discarded_bytes_; }

  size_t num_occupied_bytes() const {
    return num_occupied_bytes_ - num_discarded_bytes_;
  }

  // Ensures the ReadBuffer has enough contiguous space allocated to hold
  // |num_bytes| more bytes; returns the address of the first available byte.
  char* Reserve(size_t num_bytes) {
    if (num_occupied_bytes_ + num_bytes > size_) {
      size_ = std::max(size_ * 2, num_occupied_bytes_ + num_bytes);
      void* new_data = base::AlignedAlloc(size_, kChannelMessageAlignment);
      memcpy(new_data, data_, num_occupied_bytes_);
      base::AlignedFree(data_);
      data_ = static_cast<char*>(new_data);
    }

    return data_ + num_occupied_bytes_;
  }

  // Marks the first |num_bytes| unoccupied bytes as occupied.
  void Claim(size_t num_bytes) {
    DCHECK_LE(num_occupied_bytes_ + num_bytes, size_);
    num_occupied_bytes_ += num_bytes;
  }

  // Marks the first |num_bytes| occupied bytes as discarded. This may result in
  // shrinkage of the internal buffer, and it is not safe to assume the result
  // of a previous Reserve() call is still valid after this.
  void Discard(size_t num_bytes) {
    DCHECK_LE(num_discarded_bytes_ + num_bytes, num_occupied_bytes_);
    num_discarded_bytes_ += num_bytes;

    if (num_discarded_bytes_ == num_occupied_bytes_) {
      // We can just reuse the buffer from the beginning in this common case.
      num_discarded_bytes_ = 0;
      num_occupied_bytes_ = 0;
    }

    if (num_discarded_bytes_ > kMaxUnusedReadBufferCapacity) {
      // In the uncommon case that we have a lot of discarded data at the
      // front of the buffer, simply move remaining data to a smaller buffer.
      size_t num_preserved_bytes = num_occupied_bytes_ - num_discarded_bytes_;
      size_ = std::max(num_preserved_bytes, kReadBufferSize);
      char* new_data = static_cast<char*>(
          base::AlignedAlloc(size_, kChannelMessageAlignment));
      memcpy(new_data, data_ + num_discarded_bytes_, num_preserved_bytes);
      base::AlignedFree(data_);
      data_ = new_data;
      num_discarded_bytes_ = 0;
      num_occupied_bytes_ = num_preserved_bytes;
    }

    if (num_occupied_bytes_ == 0 && size_ > kMaxUnusedReadBufferCapacity) {
      // Opportunistically shrink the read buffer back down to a small size if
      // it's grown very large. We only do this if there are no remaining
      // unconsumed bytes in the buffer to avoid copies in most the common
      // cases.
      size_ = kMaxUnusedReadBufferCapacity;
      base::AlignedFree(data_);
      data_ = static_cast<char*>(
          base::AlignedAlloc(size_, kChannelMessageAlignment));
    }
  }

  void Realign() {
    size_t num_bytes = num_occupied_bytes();
    memmove(data_, occupied_bytes(), num_bytes);
    num_discarded_bytes_ = 0;
    num_occupied_bytes_ = num_bytes;
  }

 private:
  char* data_ = nullptr;

  // The total size of the allocated buffer.
  size_t size_ = 0;

  // The number of discarded bytes at the beginning of the allocated buffer.
  size_t num_discarded_bytes_ = 0;

  // The total number of occupied bytes, including discarded bytes.
  size_t num_occupied_bytes_ = 0;

  DISALLOW_COPY_AND_ASSIGN(ReadBuffer);
};

Channel::Channel(Delegate* delegate)
    : delegate_(delegate), read_buffer_(new ReadBuffer) {}

Channel::~Channel() {}

void Channel::ShutDown() {
  ShutDownImpl();
  delegate_ = nullptr;
}

char* Channel::GetReadBuffer(size_t* buffer_capacity) {
  DCHECK(read_buffer_);
  size_t required_capacity = *buffer_capacity;
  if (!required_capacity)
    required_capacity = kReadBufferSize;

  *buffer_capacity = required_capacity;
  return read_buffer_->Reserve(required_capacity);
}

bool Channel::OnReadComplete(size_t bytes_read, size_t* next_read_size_hint) {
  bool did_consume_message = false;
  read_buffer_->Claim(bytes_read);
  while (read_buffer_->num_occupied_bytes() >= sizeof(Message::LegacyHeader)) {
    // Ensure the occupied data is properly aligned. If it isn't, a SIGBUS could
    // happen on architectures that don't allow misaligned words access (i.e.
    // anything other than x86). Only re-align when necessary to avoid copies.
    if (!IsAlignedForChannelMessage(
            reinterpret_cast<uintptr_t>(read_buffer_->occupied_bytes()))) {
      read_buffer_->Realign();
    }

    // We have at least enough data available for a LegacyHeader.
    const Message::LegacyHeader* legacy_header =
        reinterpret_cast<const Message::LegacyHeader*>(
            read_buffer_->occupied_bytes());

    const size_t kMaxMessageSize = GetConfiguration().max_message_num_bytes;
    if (legacy_header->num_bytes < sizeof(Message::LegacyHeader) ||
        legacy_header->num_bytes > kMaxMessageSize) {
      LOG(ERROR) << "Invalid message size: " << legacy_header->num_bytes;
      return false;
    }

    if (read_buffer_->num_occupied_bytes() < legacy_header->num_bytes) {
      // Not enough data available to read the full message. Hint to the
      // implementation that it should try reading the full size of the message.
      *next_read_size_hint =
          legacy_header->num_bytes - read_buffer_->num_occupied_bytes();
      return true;
    }

    const Message::Header* header = nullptr;
    if (legacy_header->message_type != Message::MessageType::NORMAL_LEGACY) {
      header = reinterpret_cast<const Message::Header*>(legacy_header);
    }

    size_t extra_header_size = 0;
    const void* extra_header = nullptr;
    size_t payload_size = 0;
    void* payload = nullptr;
    if (header) {
      if (header->num_header_bytes < sizeof(Message::Header) ||
          header->num_header_bytes > header->num_bytes) {
        LOG(ERROR) << "Invalid message header size: "
                   << header->num_header_bytes;
        return false;
      }
      extra_header_size = header->num_header_bytes - sizeof(Message::Header);
      extra_header = extra_header_size ? header + 1 : nullptr;
      payload_size = header->num_bytes - header->num_header_bytes;
      payload = payload_size
                    ? reinterpret_cast<Message::Header*>(
                          const_cast<char*>(read_buffer_->occupied_bytes()) +
                          header->num_header_bytes)
                    : nullptr;
    } else {
      payload_size = legacy_header->num_bytes - sizeof(Message::LegacyHeader);
      payload = payload_size
                    ? const_cast<Message::LegacyHeader*>(&legacy_header[1])
                    : nullptr;
    }

    const uint16_t num_handles =
        header ? header->num_handles : legacy_header->num_handles;
    std::vector<PlatformHandle> handles;
    bool deferred = false;
    if (num_handles > 0) {
      if (!GetReadPlatformHandles(payload, payload_size, num_handles,
                                  extra_header, extra_header_size, &handles,
                                  &deferred)) {
        return false;
      }

      if (handles.empty()) {
        // Not enough handles available for this message.
        break;
      }
    }

    // We've got a complete message! Dispatch it and try another.
    if (legacy_header->message_type != Message::MessageType::NORMAL_LEGACY &&
        legacy_header->message_type != Message::MessageType::NORMAL) {
      DCHECK(!deferred);
      if (!OnControlMessage(legacy_header->message_type, payload, payload_size,
                            std::move(handles))) {
        return false;
      }
      did_consume_message = true;
    } else if (deferred) {
      did_consume_message = true;
    } else if (delegate_) {
      delegate_->OnChannelMessage(payload, payload_size, std::move(handles));
      did_consume_message = true;
    }

    read_buffer_->Discard(legacy_header->num_bytes);
  }

  *next_read_size_hint = did_consume_message ? 0 : kReadBufferSize;
  return true;
}

void Channel::OnError(Error error) {
  if (delegate_)
    delegate_->OnChannelError(error);
}

bool Channel::OnControlMessage(Message::MessageType message_type,
                               const void* payload,
                               size_t payload_size,
                               std::vector<PlatformHandle> handles) {
  return false;
}

}  // namespace core
}  // namespace mojo