pw_transfer/context.cc

// Copyright 2021 The Pigweed Authors
//
// 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
//
//     https://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.

#include "pw_transfer/internal/context.h"

#include "pw_assert/check.h"
#include "pw_status/try.h"
#include "pw_varint/varint.h"

namespace pw::transfer::internal {

uint32_t Context::MaxWriteChunkSize(uint32_t max_chunk_size_bytes,
                                    uint32_t channel_id) const {
  // Start with the user-provided maximum chunk size, which should be the usable
  // payload length on the RPC ingress path after any transport overhead.
  ssize_t max_size = max_chunk_size_bytes;

  // Subtract the RPC overhead (pw_rpc/internal/packet.proto).
  //
  //   type:       1 byte key, 1 byte value (CLIENT_STREAM)
  //   channel_id: 1 byte key, varint value (calculate from stream)
  //   service_id: 1 byte key, 4 byte value
  //   method_id:  1 byte key, 4 byte value
  //   payload:    1 byte key, varint length (remaining space)
  //   status:     0 bytes (not set in stream packets)
  //
  //   TOTAL: 14 bytes + encoded channel_id size + encoded payload length
  //
  max_size -= 14;
  max_size -= varint::EncodedSize(channel_id);
  max_size -= varint::EncodedSize(max_size);

  // Subtract the transfer service overhead for a client write chunk
  // (pw_transfer/transfer.proto).
  //
  //   transfer_id: 1 byte key, varint value (calculate)
  //   offset:      1 byte key, varint value (calculate)
  //   data:        1 byte key, varint length (remaining space)
  //
  //   TOTAL: 3 + encoded transfer_id + encoded offset + encoded data length
  //
  size_t max_offset_in_window = offset() + pending_bytes();
  max_size -= 3;
  max_size -= varint::EncodedSize(transfer_id());
  max_size -= varint::EncodedSize(max_offset_in_window);
  max_size -= varint::EncodedSize(max_size);

  // A resulting value of zero (or less) renders write transfers unusable, as
  // there is no space to send any payload. This should be considered a
  // programmer error in the transfer service setup.
  PW_CHECK_INT_GT(
      max_size,
      0,
      "Transfer service maximum chunk size is too small to fit a payload. "
      "Increase max_chunk_size_bytes to support write transfers.");

  return max_size;
}

}  // namespace pw::transfer::internal