1// -*- C++ -*-
2//===----------------------------------------------------------------------===//
3//
4// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
5// See https://llvm.org/LICENSE.txt for license information.
6// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
7//
8//===----------------------------------------------------------------------===//
9
10#ifndef _LIBCPP___THREAD_POLL_WITH_BACKOFF_H
11#define _LIBCPP___THREAD_POLL_WITH_BACKOFF_H
12
13#include <__availability>
14#include <__chrono/duration.h>
15#include <__chrono/high_resolution_clock.h>
16#include <__config>
17
18#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
19#  pragma GCC system_header
20#endif
21
22_LIBCPP_BEGIN_NAMESPACE_STD
23
24static _LIBCPP_CONSTEXPR const int __libcpp_polling_count = 64;
25
26// Polls a thread for a condition given by a predicate, and backs off based on a backoff policy
27// before polling again.
28//
29// - __f is the "test function" that should return true if polling succeeded, and false if it failed.
30//
31// - __bf is the "backoff policy", which is called with the duration since we started polling. It should
32//   return false in order to resume polling, and true if polling should stop entirely for some reason.
33//   In general, backoff policies sleep for some time before returning control to the polling loop.
34//
35// - __max_elapsed is the maximum duration to try polling for. If the maximum duration is exceeded,
36//   the polling loop will return false to report a timeout.
37template<class _Fn, class _BFn>
38_LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI
39bool __libcpp_thread_poll_with_backoff(_Fn&& __f, _BFn&& __bf, chrono::nanoseconds __max_elapsed = chrono::nanoseconds::zero()) {
40    auto const __start = chrono::high_resolution_clock::now();
41    for (int __count = 0;;) {
42      if (__f())
43        return true; // _Fn completion means success
44      if (__count < __libcpp_polling_count) {
45        __count += 1;
46        continue;
47      }
48      chrono::nanoseconds const __elapsed = chrono::high_resolution_clock::now() - __start;
49      if (__max_elapsed != chrono::nanoseconds::zero() && __max_elapsed < __elapsed)
50          return false; // timeout failure
51      if (__bf(__elapsed))
52        return false; // _BFn completion means failure
53    }
54}
55
56// A trivial backoff policy that always immediately returns the control to
57// the polling loop.
58//
59// This is not very well-behaved since it will cause the polling loop to spin,
60// so this should most likely only be used on single-threaded systems where there
61// are no other threads to compete with.
62struct __spinning_backoff_policy {
63  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR
64  bool operator()(chrono::nanoseconds const&) const {
65      return false;
66  }
67};
68
69_LIBCPP_END_NAMESPACE_STD
70
71#endif // _LIBCPP___THREAD_POLL_WITH_BACKOFF_H
72