//===-- xray_buffer_queue.h ------------------------------------*- C++ -*-===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file is a part of XRay, a dynamic runtime instrumentation system.
//
// Defines the interface for a buffer queue implementation.
//
//===----------------------------------------------------------------------===//
#ifndef XRAY_BUFFER_QUEUE_H
#define XRAY_BUFFER_QUEUE_H

#include <atomic>
#include <cstdint>
#include <deque>
#include <mutex>
#include <system_error>
#include <unordered_set>

namespace __xray {

/// BufferQueue implements a circular queue of fixed sized buffers (much like a
/// freelist) but is concerned mostly with making it really quick to initialise,
/// finalise, and get/return buffers to the queue. This is one key component of
/// the "flight data recorder" (FDR) mode to support ongoing XRay function call
/// trace collection.
class BufferQueue {
public:
  struct Buffer {
    void *Buffer = nullptr;
    std::size_t Size = 0;
  };

private:
  std::size_t BufferSize;
  std::deque<Buffer> Buffers;
  std::mutex Mutex;
  std::unordered_set<void *> OwnedBuffers;
  std::atomic<bool> Finalizing;

public:
  /// Initialise a queue of size |N| with buffers of size |B|.
  BufferQueue(std::size_t B, std::size_t N);

  /// Updates |Buf| to contain the pointer to an appropriate buffer. Returns an
  /// error in case there are no available buffers to return when we will run
  /// over the upper bound for the total buffers.
  ///
  /// Requirements:
  ///   - BufferQueue is not finalising.
  ///
  /// Returns:
  ///   - std::errc::not_enough_memory on exceeding MaxSize.
  ///   - no error when we find a Buffer.
  ///   - std::errc::state_not_recoverable on finalising BufferQueue.
  std::error_code getBuffer(Buffer &Buf);

  /// Updates |Buf| to point to nullptr, with size 0.
  ///
  /// Returns:
  ///   - ...
  std::error_code releaseBuffer(Buffer &Buf);

  bool finalizing() const { return Finalizing.load(std::memory_order_acquire); }

  // Sets the state of the BufferQueue to finalizing, which ensures that:
  //
  //   - All subsequent attempts to retrieve a Buffer will fail.
  //   - All releaseBuffer operations will not fail.
  //
  // After a call to finalize succeeds, all subsequent calls to finalize will
  // fail with std::errc::state_not_recoverable.
  std::error_code finalize();

  // Cleans up allocated buffers.
  ~BufferQueue();
};

} // namespace __xray

#endif // XRAY_BUFFER_QUEUE_H