classic_protocol_codec_base.h

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/*
  Copyright (c) 2019, 2025, Oracle and/or its affiliates.
 
  This program is free software; you can redistribute it and/or modify
  it under the terms of the GNU General Public License, version 2.0,
  as published by the Free Software Foundation.
 
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  permission to link the program and your derivative works with the
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  This program is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  GNU General Public License for more details.
 
  You should have received a copy of the GNU General Public License
  along with this program; if not, write to the Free Software
  Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
*/
 
#ifndef MYSQL_ROUTER_CLASSIC_PROTOCOL_CODEC_BASE_H_
#define MYSQL_ROUTER_CLASSIC_PROTOCOL_CODEC_BASE_H_
 
#include <cstddef>  // size_t
#include <cstdint>  // uint8_t
#include <limits>
#include <system_error>  // error_code
#include <type_traits>
#include <utility>  // move
 
#include "mysql/harness/net_ts/buffer.h"
#include "mysql/harness/stdx/bit.h"
#include "mysql/harness/stdx/expected.h"
#include "mysqlrouter/classic_protocol_codec_error.h"
#include "mysqlrouter/classic_protocol_constants.h"
 
namespace classic_protocol {
 
// bytes needed to encode x bits
//
// bits | bytes
// -----+------
//    0 |     0
//    1 |     1
//  ... |   ...
//    8 |     1
//    9 |     2
//  ... |   ...
//
constexpr size_t bytes_per_bits(size_t bits) { return (bits + 7) / 8; }
 
static_assert(bytes_per_bits(0) == 0);
static_assert(bytes_per_bits(1) == 1);
static_assert(bytes_per_bits(8) == 1);
static_assert(bytes_per_bits(9) == 2);
 
/**
 * Codec for a type.
 *
 * requirements for T:
 * - size_t size()
 * - stdx::expected<size_t, std::error_code> encode(net::mutable_buffer);
 * - static stdx::expected<T, std::error_code> decode(buffer_sequence,
 *   capabilities);
 */
template <class T>
class Codec;
 
/**
 * encode a message into a dynamic buffer.
 *
 * @param v message to encode
 * @param caps protocol capabilities
 * @param dyn_buffer dynamic buffer to write into
 * @returns number of bytes written into dynamic buffer or std::error_code on
 * error
 */
template <class T, class DynamicBuffer>
stdx::expected<size_t, std::error_code> encode(const T &v,
                                               capabilities::value_type caps,
                                               DynamicBuffer &&dyn_buffer) {
  //  static_assert(net::is_dynamic_buffer<DynamicBuffer>::value,
  //                "dyn_buffer MUST be a DynamicBuffer");
 
  Codec<T> codec(v, caps);
 
  const auto orig_size = dyn_buffer.size();
  const auto codec_size = codec.size();
 
  // reserve some space to write into
  dyn_buffer.grow(codec_size);
 
  const auto res = codec.encode(dyn_buffer.data(orig_size, codec_size));
  if (!res) {
    dyn_buffer.shrink(codec_size);
    return res;
  }
 
  dyn_buffer.shrink(codec_size - res.value());
 
  return res;
}
 
/**
 * decode a message from a buffer.
 *
 * @param buffer buffer to read from
 * @param caps protocol capabilities
 * @tparam T the message class
 * @returns number of bytes read from 'buffers' and a T on success, or
 * std::error_code on error
 */
template <class T>
stdx::expected<std::pair<size_t, T>, std::error_code> decode(
    const net::const_buffer &buffer, capabilities::value_type caps) {
  return Codec<T>::decode(buffer, caps);
}
 
/**
 * decode a message from a buffer.
 *
 * @param buffer buffer to read from
 * @param caps protocol capabilities
 * @param args arguments that shall be forwarded to T's decode()
 * @tparam T the message class
 * @tparam Args Types of the extra arguments to be forwarded to T's decode()
 * function.
 * @returns number of bytes read from 'buffers' and a T on success, or
 * std::error_code on error
 */
template <class T, class... Args>
stdx::expected<std::pair<size_t, T>, std::error_code> decode(
    const net::const_buffer &buffer, capabilities::value_type caps,
    // clang-format off
    Args &&... args
    // clang-format on
) {
  return Codec<T>::decode(buffer, caps, std::forward<Args>(args)...);
}
 
namespace impl {
 
/**
 * Generator of decoded Types of a buffer.
 *
 * - .step<wire::VarInt>()
 */
class DecodeBufferAccumulator {
 public:
  using buffer_type = net::const_buffer;
  using result_type = stdx::expected<size_t, std::error_code>;
 
  /**
   * construct a DecodeBufferAccumulator.
   *
   * @param buffer a net::const_buffer
   * @param caps classic-protocol capabilities
   * @param consumed bytes to skip from the buffers
   */
  DecodeBufferAccumulator(const net::const_buffer &buffer,
                          capabilities::value_type caps, size_t consumed = 0)
      : buffer_(buffer), caps_(caps), consumed_(consumed) {}
 
  /**
   * decode a Type from the buffer sequence.
   *
   * if it succeeds, moves position in buffer forward and returns
   * decoded Type, otherwise returns error and updates the global error-code in
   * result()
   *
   * 'sz' is unlimited, the whole rest of the current buffer
   * is passed to the decoder.
   *
   * If not, a slice of size 'sz' is taken. If there isn't at least 'sz' bytes
   * in the buffer, it fails.
   *
   * @param sz limits the size of the current buffer.
   */
  template <class T>
  stdx::expected<typename Codec<T>::value_type, std::error_code> step(
      size_t sz = std::numeric_limits<size_t>::max()) {
    if (!res_) return stdx::make_unexpected(res_.error());
 
    auto step_res = step_<T>(sz);
 
    // capture the first failure
    if (!step_res) res_ = stdx::make_unexpected(step_res.error());
 
    return step_res;
  }
 
  /**
   * try decoding a Type from the buffer sequence.
   *
   * if it succeeds, moves position in buffer-sequence forward and returns
   * decoded Type, otherwise returns error and does NOT update the global
   * error-code in result()
   */
  template <class T>
  stdx::expected<typename Codec<T>::value_type, std::error_code> try_step(
      size_t sz = std::numeric_limits<size_t>::max()) {
    if (!res_) return stdx::make_unexpected(res_.error());
 
    return step_<T>(sz);
  }
 
  /**
   * get result of the step().
   *
   * if a step() failed, result is the error-code of the first failed step()
   *
   * @returns consumed bytes by all steps(), or error of first failed step()
   */
  result_type result() const {
    if (!res_) return res_;
 
    return consumed_;
  }
 
 private:
  template <class T>
  stdx::expected<typename Codec<T>::value_type, std::error_code> step_(
      size_t sz) {
    auto buf = buffer_ + consumed_;
 
    if (sz != std::numeric_limits<size_t>::max()) {
      // not enough data.
      if (buf.size() < sz) {
        return stdx::make_unexpected(
            make_error_code(codec_errc::not_enough_input));
      }
    }
 
    auto decode_res = Codec<T>::decode(net::buffer(buf, sz), caps_);
    if (!decode_res) return stdx::make_unexpected(decode_res.error());
 
    consumed_ += decode_res->first;
    return decode_res->second;
  }
 
  net::const_buffer buffer_;
  const capabilities::value_type caps_;
  size_t consumed_;
 
  result_type res_;
};
 
/**
 * accumulator of encoded buffers.
 *
 * writes the .step()ed encoded types into buffer.
 *
 * EncodeBufferAccumulator(buffer, caps)
 *   .step(wire::VarInt(42))
 *   .step(wire::VarInt(512))
 *   .result()
 *
 * The class should be used together with EncodeSizeAccumulator which shares
 * the same interface.
 */
class EncodeBufferAccumulator {
 public:
  using result_type = stdx::expected<size_t, std::error_code>;
 
  /**
   * construct a encode-buffer-accumulator.
   *
   * @param buffer mutable-buffer to encode into
   * @param caps protocol capabilities
   * @param consumed bytes already used in the in buffer
   */
  EncodeBufferAccumulator(net::mutable_buffer buffer,
                          capabilities::value_type caps, size_t consumed = 0)
      : buffer_{buffer}, caps_{caps}, consumed_{consumed} {}
 
  /**
   * encode a T into the buffer and move position forward.
   *
   * no-op of a previous step failed.
   */
  template <class T>
  EncodeBufferAccumulator &step(const T &v) {
    if (!res_) return *this;
 
    auto res = Codec<T>(v, caps_).encode(buffer_ + consumed_);
    if (!res) {  // it failed.
      res_ = res;
    } else {
      consumed_ += *res;
    }
 
    return *this;
  }
 
  /**
   * get result the steps().
   *
   * @returns last used position in buffer, or first error in case of a step()
   * failed.
   */
  result_type result() const {
    if (!res_) return res_;
 
    return {consumed_};
  }
 
 private:
  const net::mutable_buffer buffer_;
  const capabilities::value_type caps_;
  size_t consumed_{};
 
  result_type res_;
};
 
/**
 * accumulates the sizes of encoded T's.
 *
 * e.g. the size of tw
 *
 * EncodeSizeAccumulator(caps)
 *   .step(wire::VarInt(42))    // 1
 *   .step(wire::VarInt(512))   // 2
 *   .result()                  // = 3
 *
 * The class should be used together with EncodeBufferAccumulator which shares
 * the same interface.
 */
class EncodeSizeAccumulator {
 public:
  using result_type = size_t;
 
  /**
   * construct a EncodeSizeAccumulator.
   */
  constexpr explicit EncodeSizeAccumulator(capabilities::value_type caps)
      : caps_{caps} {}
 
  /**
   * accumulate the size() of encoded T.
   *
   * calls Codec<T>(v, caps).size()
   */
  template <class T>
  constexpr EncodeSizeAccumulator &step(const T &v) noexcept {
    consumed_ += Codec<T>(v, caps_).size();
 
    return *this;
  }
 
  /**
   * @returns size of all steps().
   */
  constexpr result_type result() const { return consumed_; }
 
 private:
  size_t consumed_{};
  const capabilities::value_type caps_;
};
 
/**
 * CRTP base for the Codec's encode part.
 *
 * derived classes must provide a 'accumulate_fields()' which
 * maps each field by the Mapper and returns the result
 *
 * used by .size() and .encode() as both have to process the same
 * fields in the same order, just with different mappers
 */
template <class T>
class EncodeBase {
 public:
  constexpr explicit EncodeBase(capabilities::value_type caps) : caps_{caps} {}
 
  constexpr size_t size() const noexcept {
    return static_cast<const T *>(this)->accumulate_fields(
        EncodeSizeAccumulator(caps_));
  }
 
  stdx::expected<size_t, std::error_code> encode(
      net::mutable_buffer buffer) const {
    return static_cast<const T *>(this)->accumulate_fields(
        EncodeBufferAccumulator(buffer, caps_));
  }
 
  constexpr capabilities::value_type caps() const noexcept { return caps_; }
 
 private:
  const capabilities::value_type caps_;
};
 
}  // namespace impl
}  // namespace classic_protocol
 
#endif