text: // Copyright (c) 2012 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 "net/spdy/spdy_frame_builder.h"
  
  #include <limits>
  
  #include "base/logging.h"
  #include "net/spdy/spdy_bug_tracker.h"
  #include "net/spdy/spdy_framer.h"
  #include "net/spdy/spdy_protocol.h"
  
  namespace net {
  
  namespace {
  
  // A special structure for the 8 bit flags and 24 bit length fields.
  union FlagsAndLength {
    uint8_t flags[4];  // 8 bits
    uint32_t length;   // 24 bits
  };
  
  // Creates a FlagsAndLength.
  FlagsAndLength CreateFlagsAndLength(uint8_t flags, size_t length) {
    DCHECK_EQ(0u, length & ~static_cast<size_t>(kLengthMask));
    FlagsAndLength flags_length;
    flags_length.length = base::HostToNet32(static_cast<uint32_t>(length));
    DCHECK_EQ(0, flags & ~kControlFlagsMask);
    flags_length.flags[0] = flags;
    return flags_length;
  }
  
  }  // namespace
  
  SpdyFrameBuilder::SpdyFrameBuilder(size_t size, SpdyMajorVersion version)
      : buffer_(new char[size]),
        capacity_(size),
        length_(0),
        offset_(0),
        version_(version) {
  }
  
  SpdyFrameBuilder::~SpdyFrameBuilder() {
  }
  
  char* SpdyFrameBuilder::GetWritableBuffer(size_t length) {
    if (!CanWrite(length)) {
      return NULL;
    }
    return buffer_.get() + offset_ + length_;
  }
  
  bool SpdyFrameBuilder::Seek(size_t length) {
    if (!CanWrite(length)) {
      return false;
    }
  
    length_ += length;
    return true;
  }
  
  bool SpdyFrameBuilder::WriteControlFrameHeader(const SpdyFramer& framer,
                                                 SpdyFrameType type,
                                                 uint8_t flags) {
    DCHECK_EQ(SPDY3, version_);
    DCHECK(SpdyConstants::IsValidFrameType(
        version_, SpdyConstants::SerializeFrameType(version_, type)));
    bool success = true;
    FlagsAndLength flags_length = CreateFlagsAndLength(
        flags, capacity_ - framer.GetControlFrameHeaderSize());
    success &= WriteUInt16(kControlFlagMask | kSpdy3Version);
    success &= WriteUInt16(
        SpdyConstants::SerializeFrameType(framer.protocol_version(), type));
    success &= WriteBytes(&flags_length, sizeof(flags_length));
    DCHECK_EQ(framer.GetControlFrameHeaderSize(), length());
    return success;
  }
  
  bool SpdyFrameBuilder::WriteDataFrameHeader(const SpdyFramer& framer,
                                              SpdyStreamId stream_id,
                                              uint8_t flags) {
    if (version_ == HTTP2) {
      return BeginNewFrame(framer, DATA, flags, stream_id);
    }
    DCHECK_EQ(0u, stream_id & ~kStreamIdMask);
    bool success = true;
    success &= WriteUInt32(stream_id);
    size_t length_field = capacity_ - framer.GetDataFrameMinimumSize();
    DCHECK_EQ(0u, length_field & ~static_cast<size_t>(kLengthMask));
    FlagsAndLength flags_length;
    flags_length.length = base::HostToNet32(static_cast<uint32_t>(length_field));
    DCHECK_EQ(0, flags & ~kDataFlagsMask);
    flags_length.flags[0] = flags;
    success &= WriteBytes(&flags_length, sizeof(flags_length));
    DCHECK_EQ(framer.GetDataFrameMinimumSize(), length());
    return success;
  }
  
  bool SpdyFrameBuilder::BeginNewFrame(const SpdyFramer& framer,
                                       SpdyFrameType type,
                                       uint8_t flags,
                                       SpdyStreamId stream_id) {
    DCHECK(SpdyConstants::IsValidFrameType(
        version_, SpdyConstants::SerializeFrameType(version_, type)));
    DCHECK_EQ(0u, stream_id & ~kStreamIdMask);
    DCHECK_EQ(HTTP2, framer.protocol_version());
    bool success = true;
    if (length_ > 0) {
      // Update length field for previous frame.
      OverwriteLength(framer, length_ - framer.GetPrefixLength(type));
      SPDY_BUG_IF(framer.GetFrameMaximumSize() < length_)
          << "Frame length  " << length_
          << " is longer than the maximum allowed length.";
    }
  
    offset_ += length_;
    length_ = 0;
  
    // Assume all remaining capacity will be used for this frame. If not,
    // the length will get overwritten when we begin the next frame.
    // Don't check for length limits here because this may be larger than the
    // actual frame length.
    success &= WriteUInt24(capacity_ - offset_ - framer.GetPrefixLength(type));
    success &= WriteUInt8(
        SpdyConstants::SerializeFrameType(version_, type));
    success &= WriteUInt8(flags);
    success &= WriteUInt32(stream_id);
    DCHECK_EQ(framer.GetDataFrameMinimumSize(), length_);
    return success;
  }
  
  bool SpdyFrameBuilder::WriteStringPiece16(const base::StringPiece& value) {
    if (value.size() > 0xffff) {
      DCHECK(false) << "Tried to write string with length > 16bit.";
      return false;
    }
  
    if (!WriteUInt16(static_cast<uint16_t>(value.size()))) {
      return false;
    }
  
    return WriteBytes(value.data(), static_cast<uint16_t>(value.size()));
  }
  
  bool SpdyFrameBuilder::WriteStringPiece32(const base::StringPiece& value) {
    if (!WriteUInt32(value.size())) {
      return false;
    }
  
    return WriteBytes(value.data(), value.size());
  }
  
  bool SpdyFrameBuilder::WriteBytes(const void* data, uint32_t data_len) {
    if (!CanWrite(data_len)) {
      return false;
    }
  
    char* dest = GetWritableBuffer(data_len);
    memcpy(dest, data, data_len);
    Seek(data_len);
    return true;
  }
  
  bool SpdyFrameBuilder::RewriteLength(const SpdyFramer& framer) {
    return OverwriteLength(framer,
                           length_ - framer.GetControlFrameHeaderSize());
  }
  
  bool SpdyFrameBuilder::OverwriteLength(const SpdyFramer& framer,
                                         size_t length) {
    if (version_ == SPDY3) {
      DCHECK_GE(framer.GetFrameMaximumSize() - framer.GetFrameMinimumSize(),
                length);
    } else {
      DCHECK_GE(framer.GetFrameMaximumSize(), length);
    }
    bool success = false;
    const size_t old_length = length_;
  
    if (version_ == SPDY3) {
      FlagsAndLength flags_length = CreateFlagsAndLength(
          0,  // We're not writing over the flags value anyway.
          length);
  
      // Write into the correct location by temporarily faking the offset.
      length_ = 5;  // Offset at which the length field occurs.
      success = WriteBytes(reinterpret_cast<char*>(&flags_length) + 1,
                           sizeof(flags_length) - 1);
    } else {
      length_ = 0;
      success = WriteUInt24(length);
    }
  
    length_ = old_length;
    return success;
  }
  
  bool SpdyFrameBuilder::OverwriteFlags(const SpdyFramer& framer, uint8_t flags) {
    DCHECK_EQ(HTTP2, framer.protocol_version());
    bool success = false;
    const size_t old_length = length_;
    // Flags are the fifth octet in the frame prefix.
    length_ = 4;
    success = WriteUInt8(flags);
    length_ = old_length;
    return success;
  }
  
  bool SpdyFrameBuilder::CanWrite(size_t length) const {
    if (length > kLengthMask) {
      DCHECK(false);
      return false;
    }
  
    if (offset_ + length_ + length > capacity_) {
      DCHECK(false);
      return false;
    }
  
    return true;
  }
  
  }  // namespace net
  