libcxx/include/__thread/poll_with_backoff.h

// -*- C++ -*-
//===----------------------------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

#ifndef _LIBCPP___THREAD_POLL_WITH_BACKOFF_H
#define _LIBCPP___THREAD_POLL_WITH_BACKOFF_H

#include <__chrono/duration.h>
#include <__chrono/high_resolution_clock.h>
#include <__config>

#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
#  pragma GCC system_header
#endif

_LIBCPP_BEGIN_NAMESPACE_STD

static _LIBCPP_CONSTEXPR const int __libcpp_polling_count = 64;

enum class __backoff_results : unsigned char {
  __continue_poll   = 1,
  __poll_success    = 2,
  __timeout         = 3,
  __backoff_failure = 4,
};

enum class __poll_with_backoff_results : unsigned char {
  __poll_success    = static_cast<unsigned char>(__backoff_results::__poll_success),
  __timeout         = static_cast<unsigned char>(__backoff_results::__timeout),
  __backoff_failure = static_cast<unsigned char>(__backoff_results::__backoff_failure),
};

// Polls a thread for a condition given by a predicate, and backs off based on a backoff policy
// before polling again.
//
// - __poll is the "test function" that should return true if polling succeeded, and false if it failed.
//
// - __backoff is the "backoff policy", which is called with the duration since we started polling. It should
//   return  __backoff_results::__continue_poll in order to resume polling, and other appropriate  __backoff_results
//   if polling should stop entirely for some reason.
//   In general, backoff policies sleep for some time before returning control to the polling loop.
//
// - __max_elapsed is the maximum duration to try polling for. If the maximum duration is exceeded,
//   the polling loop will return __poll_with_backoff_results::__timeout to report a timeout.

template <class _Poll, class _Backoff>
_LIBCPP_HIDE_FROM_ABI __poll_with_backoff_results __libcpp_thread_poll_with_backoff(
    _Poll&& __poll, _Backoff&& __backoff, chrono::nanoseconds __max_elapsed = chrono::nanoseconds::zero()) {
  auto const __start = chrono::high_resolution_clock::now();
  for (int __count = 0;;) {
    if (__poll())
      return __poll_with_backoff_results::__poll_success;
    if (__count < __libcpp_polling_count) {
      __count += 1;
      continue;
    }
    chrono::nanoseconds const __elapsed = chrono::high_resolution_clock::now() - __start;
    if (__max_elapsed != chrono::nanoseconds::zero() && __max_elapsed < __elapsed)
      return __poll_with_backoff_results::__timeout;
    if (auto __backoff_res = __backoff(__elapsed); __backoff_res == __backoff_results::__continue_poll)
      continue;
    else
      return static_cast<__poll_with_backoff_results>(__backoff_res);
  }
}

// A trivial backoff policy that always immediately returns the control to
// the polling loop.
//
// This is not very well-behaved since it will cause the polling loop to spin,
// so this should most likely only be used on single-threaded systems where there
// are no other threads to compete with.
struct __spinning_backoff_policy {
  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR __backoff_results operator()(chrono::nanoseconds const&) const {
    return __backoff_results::__continue_poll;
  }
};

_LIBCPP_END_NAMESPACE_STD

#endif // _LIBCPP___THREAD_POLL_WITH_BACKOFF_H