platform/impl/tls_write_buffer.h

// Copyright 2019 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.

#ifndef PLATFORM_IMPL_TLS_WRITE_BUFFER_H_
#define PLATFORM_IMPL_TLS_WRITE_BUFFER_H_

#include <atomic>
#include <memory>

#include "absl/types/span.h"
#include "platform/base/macros.h"

namespace openscreen {

// This class is responsible for buffering TLS Write data. The approach taken by
// this class is to allow for a single thread to act as a publisher of data and
// for a separate thread to act as the consumer of that data. The data in
// question is written to a lockless FIFO queue.
class TlsWriteBuffer {
 public:
  TlsWriteBuffer();
  ~TlsWriteBuffer();

  // Pushes the provided data into the buffer, returning true if successful.
  // Returns false if there was insufficient space left. Either all or none of
  // the data is pushed into the buffer.
  bool Push(const void* data, size_t len);

  // Returns a subset of the readable region of data. At time of reading, more
  // data may be available for reading than what is represented in this Span.
  absl::Span<const uint8_t> GetReadableRegion();

  // Marks the provided number of bytes as consumed by the consumer thread.
  void Consume(size_t byte_count);

  // The amount of space to allocate in the buffer.
  static constexpr size_t kBufferSizeBytes = 1 << 19;  // 0.5 MB.

 private:
  // Buffer where data to be written over the TLS connection is stored.
  uint8_t buffer_[kBufferSizeBytes];

  // Total number of bytes read or written so far. Atomics are used both to
  // ensure that read and write operations are atomic for uint64s on all systems
  // and to ensure that different values for these values aren't loaded from
  // each CPU's physical cache.
  std::atomic_size_t bytes_read_so_far_{0};
  std::atomic_size_t bytes_written_so_far_{0};

  OSP_DISALLOW_COPY_AND_ASSIGN(TlsWriteBuffer);
};

}  // namespace openscreen

#endif  // PLATFORM_IMPL_TLS_WRITE_BUFFER_H_