1// <stop_token> -*- C++ -*-
2
3// Copyright (C) 2019-2022 Free Software Foundation, Inc.
4//
5// This file is part of the GNU ISO C++ Library.  This library is free
6// software; you can redistribute it and/or modify it under the
7// terms of the GNU General Public License as published by the
8// Free Software Foundation; either version 3, or (at your option)
9// any later version.
10
11// This library is distributed in the hope that it will be useful,
12// but WITHOUT ANY WARRANTY; without even the implied warranty of
13// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
14// GNU General Public License for more details.
15
16// Under Section 7 of GPL version 3, you are granted additional
17// permissions described in the GCC Runtime Library Exception, version
18// 3.1, as published by the Free Software Foundation.
19
20// You should have received a copy of the GNU General Public License and
21// a copy of the GCC Runtime Library Exception along with this program;
22// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
23// <http://www.gnu.org/licenses/>.
24
25/** @file include/stop_token
26 *  This is a Standard C++ Library header.
27 */
28
29#ifndef _GLIBCXX_STOP_TOKEN
30#define _GLIBCXX_STOP_TOKEN
31
32#if __cplusplus > 201703L
33
34#include <atomic>
35#include <bits/std_thread.h>
36
37#include <semaphore>
38
39#define __cpp_lib_jthread 201911L
40
41namespace std _GLIBCXX_VISIBILITY(default)
42{
43_GLIBCXX_BEGIN_NAMESPACE_VERSION
44
45  /// Tag type indicating a stop_source should have no shared-stop-state.
46  struct nostopstate_t { explicit nostopstate_t() = default; };
47  inline constexpr nostopstate_t nostopstate{};
48
49  class stop_source;
50
51  /// Allow testing whether a stop request has been made on a `stop_source`.
52  class stop_token
53  {
54  public:
55    stop_token() noexcept = default;
56
57    stop_token(const stop_token&) noexcept = default;
58    stop_token(stop_token&&) noexcept = default;
59
60    ~stop_token() = default;
61
62    stop_token&
63    operator=(const stop_token&) noexcept = default;
64
65    stop_token&
66    operator=(stop_token&&) noexcept = default;
67
68    [[nodiscard]]
69    bool
70    stop_possible() const noexcept
71    {
72      return static_cast<bool>(_M_state) && _M_state->_M_stop_possible();
73    }
74
75    [[nodiscard]]
76    bool
77    stop_requested() const noexcept
78    {
79      return static_cast<bool>(_M_state) && _M_state->_M_stop_requested();
80    }
81
82    void
83    swap(stop_token& __rhs) noexcept
84    { _M_state.swap(__rhs._M_state); }
85
86    [[nodiscard]]
87    friend bool
88    operator==(const stop_token& __a, const stop_token& __b)
89    { return __a._M_state == __b._M_state; }
90
91    friend void
92    swap(stop_token& __lhs, stop_token& __rhs) noexcept
93    { __lhs.swap(__rhs); }
94
95  private:
96    friend class stop_source;
97    template<typename _Callback>
98      friend class stop_callback;
99
100    static void
101    _S_yield() noexcept
102    {
103#if defined __i386__ || defined __x86_64__
104      __builtin_ia32_pause();
105#endif
106      this_thread::yield();
107    }
108
109#ifndef __cpp_lib_semaphore
110    struct binary_semaphore
111    {
112      explicit binary_semaphore(int __d) : _M_counter(__d > 0) { }
113
114      void release() { _M_counter.fetch_add(1, memory_order::release); }
115
116      void acquire()
117      {
118          int __old = 1;
119          while (!_M_counter.compare_exchange_weak(__old, 0,
120                                                             memory_order::acquire,
121                                                             memory_order::relaxed))
122            {
123              __old = 1;
124              _S_yield();
125            }
126      }
127
128      atomic<int> _M_counter;
129    };
130#endif
131
132    struct _Stop_cb
133    {
134      using __cb_type = void(_Stop_cb*) noexcept;
135      __cb_type* _M_callback;
136      _Stop_cb* _M_prev = nullptr;
137      _Stop_cb* _M_next = nullptr;
138      bool* _M_destroyed = nullptr;
139      binary_semaphore _M_done{0};
140
141      [[__gnu__::__nonnull__]]
142      explicit
143      _Stop_cb(__cb_type* __cb)
144      : _M_callback(__cb)
145      { }
146
147      void _M_run() noexcept { _M_callback(this); }
148    };
149
150    struct _Stop_state_t
151    {
152      using value_type = uint32_t;
153      static constexpr value_type _S_stop_requested_bit = 1;
154      static constexpr value_type _S_locked_bit = 2;
155      static constexpr value_type _S_ssrc_counter_inc = 4;
156
157      std::atomic<value_type> _M_owners{1};
158      std::atomic<value_type> _M_value{_S_ssrc_counter_inc};
159      _Stop_cb* _M_head = nullptr;
160      std::thread::id _M_requester;
161
162      _Stop_state_t() = default;
163
164      bool
165      _M_stop_possible() noexcept
166      {
167          // true if a stop request has already been made or there are still
168          // stop_source objects that would allow one to be made.
169          return _M_value.load(memory_order::acquire) & ~_S_locked_bit;
170      }
171
172      bool
173      _M_stop_requested() noexcept
174      {
175          return _M_value.load(memory_order::acquire) & _S_stop_requested_bit;
176      }
177
178      void
179      _M_add_owner() noexcept
180      {
181          _M_owners.fetch_add(1, memory_order::relaxed);
182      }
183
184      void
185      _M_release_ownership() noexcept
186      {
187          if (_M_owners.fetch_sub(1, memory_order::acq_rel) == 1)
188            delete this;
189      }
190
191      void
192      _M_add_ssrc() noexcept
193      {
194          _M_value.fetch_add(_S_ssrc_counter_inc, memory_order::relaxed);
195      }
196
197      void
198      _M_sub_ssrc() noexcept
199      {
200          _M_value.fetch_sub(_S_ssrc_counter_inc, memory_order::release);
201      }
202
203      // Obtain lock.
204      void
205      _M_lock() noexcept
206      {
207          // Can use relaxed loads to get the current value.
208          // The successful call to _M_try_lock is an acquire operation.
209          auto __old = _M_value.load(memory_order::relaxed);
210          while (!_M_try_lock(__old, memory_order::relaxed))
211            { }
212      }
213
214      // Precondition: calling thread holds the lock.
215      void
216      _M_unlock() noexcept
217      {
218          _M_value.fetch_sub(_S_locked_bit, memory_order::release);
219      }
220
221      bool
222      _M_request_stop() noexcept
223      {
224          // obtain lock and set stop_requested bit
225          auto __old = _M_value.load(memory_order::acquire);
226          do
227            {
228              if (__old & _S_stop_requested_bit) // stop request already made
229                return false;
230            }
231          while (!_M_try_lock_and_stop(__old));
232
233          _M_requester = this_thread::get_id();
234
235          while (_M_head)
236            {
237              bool __last_cb;
238              _Stop_cb* __cb = _M_head;
239              _M_head = _M_head->_M_next;
240              if (_M_head)
241                {
242                    _M_head->_M_prev = nullptr;
243                    __last_cb = false;
244                }
245              else
246                __last_cb = true;
247
248              // Allow other callbacks to be unregistered while __cb runs.
249              _M_unlock();
250
251              bool __destroyed = false;
252              __cb->_M_destroyed = &__destroyed;
253
254              // run callback
255              __cb->_M_run();
256
257              if (!__destroyed)
258                {
259                    __cb->_M_destroyed = nullptr;
260
261                    // synchronize with destructor of stop_callback that owns *__cb
262                    if (!__gnu_cxx::__is_single_threaded())
263                      __cb->_M_done.release();
264                }
265
266              // Avoid relocking if we already know there are no more callbacks.
267              if (__last_cb)
268                return true;
269
270              _M_lock();
271            }
272
273          _M_unlock();
274          return true;
275      }
276
277      [[__gnu__::__nonnull__]]
278      bool
279      _M_register_callback(_Stop_cb* __cb) noexcept
280      {
281          auto __old = _M_value.load(memory_order::acquire);
282          do
283            {
284              if (__old & _S_stop_requested_bit) // stop request already made
285                {
286                    __cb->_M_run(); // run synchronously
287                    return false;
288                }
289
290              if (__old < _S_ssrc_counter_inc) // no stop_source owns *this
291                // No need to register callback if no stop request can be made.
292                // Returning false also means the stop_callback does not share
293                // ownership of this state, but that's not observable.
294                return false;
295            }
296          while (!_M_try_lock(__old));
297
298        __cb->_M_next = _M_head;
299        if (_M_head)
300          {
301            _M_head->_M_prev = __cb;
302          }
303        _M_head = __cb;
304          _M_unlock();
305        return true;
306      }
307
308      // Called by ~stop_callback just before destroying *__cb.
309      [[__gnu__::__nonnull__]]
310      void
311      _M_remove_callback(_Stop_cb* __cb)
312      {
313          _M_lock();
314
315        if (__cb == _M_head)
316          {
317            _M_head = _M_head->_M_next;
318            if (_M_head)
319                _M_head->_M_prev = nullptr;
320              _M_unlock();
321              return;
322          }
323          else if (__cb->_M_prev)
324          {
325            __cb->_M_prev->_M_next = __cb->_M_next;
326            if (__cb->_M_next)
327                __cb->_M_next->_M_prev = __cb->_M_prev;
328              _M_unlock();
329              return;
330          }
331
332          _M_unlock();
333
334          // Callback is not in the list, so must have been removed by a call to
335          // _M_request_stop.
336
337          // Despite appearances there is no data race on _M_requester. The only
338          // write to it happens before the callback is removed from the list,
339          // and removing it from the list happens before this read.
340          if (!(_M_requester == this_thread::get_id()))
341            {
342              // Synchronize with completion of callback.
343              __cb->_M_done.acquire();
344              // Safe for ~stop_callback to destroy *__cb now.
345              return;
346            }
347
348          if (__cb->_M_destroyed)
349            *__cb->_M_destroyed = true;
350      }
351
352      // Try to obtain the lock.
353      // Returns true if the lock is acquired (with memory order acquire).
354      // Otherwise, sets __curval = _M_value.load(__failure) and returns false.
355      // Might fail spuriously, so must be called in a loop.
356      bool
357      _M_try_lock(value_type& __curval,
358                      memory_order __failure = memory_order::acquire) noexcept
359      {
360          return _M_do_try_lock(__curval, 0, memory_order::acquire, __failure);
361      }
362
363      // Try to obtain the lock to make a stop request.
364      // Returns true if the lock is acquired and the _S_stop_requested_bit is
365      // set (with memory order acq_rel so that other threads see the request).
366      // Otherwise, sets __curval = _M_value.load(memory_order::acquire) and
367      // returns false.
368      // Might fail spuriously, so must be called in a loop.
369      bool
370      _M_try_lock_and_stop(value_type& __curval) noexcept
371      {
372          return _M_do_try_lock(__curval, _S_stop_requested_bit,
373                                    memory_order::acq_rel, memory_order::acquire);
374      }
375
376      bool
377      _M_do_try_lock(value_type& __curval, value_type __newbits,
378                         memory_order __success, memory_order __failure) noexcept
379      {
380          if (__curval & _S_locked_bit)
381            {
382              _S_yield();
383              __curval = _M_value.load(__failure);
384              return false;
385            }
386          __newbits |= _S_locked_bit;
387          return _M_value.compare_exchange_weak(__curval, __curval | __newbits,
388                                                        __success, __failure);
389      }
390    };
391
392    struct _Stop_state_ref
393    {
394      _Stop_state_ref() = default;
395
396      [[__gnu__::__access__(__none__, 2)]]
397      explicit
398      _Stop_state_ref(const stop_source&)
399      : _M_ptr(new _Stop_state_t())
400      { }
401
402      _Stop_state_ref(const _Stop_state_ref& __other) noexcept
403      : _M_ptr(__other._M_ptr)
404      {
405          if (_M_ptr)
406            _M_ptr->_M_add_owner();
407      }
408
409      _Stop_state_ref(_Stop_state_ref&& __other) noexcept
410      : _M_ptr(__other._M_ptr)
411      {
412          __other._M_ptr = nullptr;
413      }
414
415      _Stop_state_ref&
416      operator=(const _Stop_state_ref& __other) noexcept
417      {
418          if (auto __ptr = __other._M_ptr; __ptr != _M_ptr)
419            {
420              if (__ptr)
421                __ptr->_M_add_owner();
422              if (_M_ptr)
423                _M_ptr->_M_release_ownership();
424              _M_ptr = __ptr;
425            }
426          return *this;
427      }
428
429      _Stop_state_ref&
430      operator=(_Stop_state_ref&& __other) noexcept
431      {
432          _Stop_state_ref(std::move(__other)).swap(*this);
433          return *this;
434      }
435
436      ~_Stop_state_ref()
437      {
438          if (_M_ptr)
439            _M_ptr->_M_release_ownership();
440      }
441
442      void
443      swap(_Stop_state_ref& __other) noexcept
444      { std::swap(_M_ptr, __other._M_ptr); }
445
446      explicit operator bool() const noexcept { return _M_ptr != nullptr; }
447
448      _Stop_state_t* operator->() const noexcept { return _M_ptr; }
449
450#if __cpp_impl_three_way_comparison >= 201907L
451      friend bool
452      operator==(const _Stop_state_ref&, const _Stop_state_ref&) = default;
453#else
454      friend bool
455      operator==(const _Stop_state_ref& __lhs, const _Stop_state_ref& __rhs)
456      noexcept
457      { return __lhs._M_ptr == __rhs._M_ptr; }
458
459      friend bool
460      operator!=(const _Stop_state_ref& __lhs, const _Stop_state_ref& __rhs)
461      noexcept
462      { return __lhs._M_ptr != __rhs._M_ptr; }
463#endif
464
465    private:
466      _Stop_state_t* _M_ptr = nullptr;
467    };
468
469    _Stop_state_ref _M_state;
470
471    explicit
472    stop_token(const _Stop_state_ref& __state) noexcept
473    : _M_state{__state}
474    { }
475  };
476
477  /// A type that allows a stop request to be made.
478  class stop_source
479  {
480  public:
481    stop_source() : _M_state(*this)
482    { }
483
484    explicit stop_source(std::nostopstate_t) noexcept
485    { }
486
487    stop_source(const stop_source& __other) noexcept
488    : _M_state(__other._M_state)
489    {
490      if (_M_state)
491          _M_state->_M_add_ssrc();
492    }
493
494    stop_source(stop_source&&) noexcept = default;
495
496    stop_source&
497    operator=(const stop_source& __other) noexcept
498    {
499      if (_M_state != __other._M_state)
500          {
501            stop_source __sink(std::move(*this));
502            _M_state = __other._M_state;
503            if (_M_state)
504              _M_state->_M_add_ssrc();
505          }
506      return *this;
507    }
508
509    stop_source&
510    operator=(stop_source&&) noexcept = default;
511
512    ~stop_source()
513    {
514      if (_M_state)
515          _M_state->_M_sub_ssrc();
516    }
517
518    [[nodiscard]]
519    bool
520    stop_possible() const noexcept
521    {
522      return static_cast<bool>(_M_state);
523    }
524
525    [[nodiscard]]
526    bool
527    stop_requested() const noexcept
528    {
529      return static_cast<bool>(_M_state) && _M_state->_M_stop_requested();
530    }
531
532    bool
533    request_stop() const noexcept
534    {
535      if (stop_possible())
536        return _M_state->_M_request_stop();
537      return false;
538    }
539
540    [[nodiscard]]
541    stop_token
542    get_token() const noexcept
543    {
544      return stop_token{_M_state};
545    }
546
547    void
548    swap(stop_source& __other) noexcept
549    {
550      _M_state.swap(__other._M_state);
551    }
552
553    [[nodiscard]]
554    friend bool
555    operator==(const stop_source& __a, const stop_source& __b) noexcept
556    {
557      return __a._M_state == __b._M_state;
558    }
559
560    friend void
561    swap(stop_source& __lhs, stop_source& __rhs) noexcept
562    {
563      __lhs.swap(__rhs);
564    }
565
566  private:
567    stop_token::_Stop_state_ref _M_state;
568  };
569
570  /// A wrapper for callbacks to be run when a stop request is made.
571  template<typename _Callback>
572    class [[nodiscard]] stop_callback
573    {
574      static_assert(is_nothrow_destructible_v<_Callback>);
575      static_assert(is_invocable_v<_Callback>);
576
577    public:
578      using callback_type = _Callback;
579
580      template<typename _Cb,
581               enable_if_t<is_constructible_v<_Callback, _Cb>, int> = 0>
582        explicit
583          stop_callback(const stop_token& __token, _Cb&& __cb)
584        noexcept(is_nothrow_constructible_v<_Callback, _Cb>)
585          : _M_cb(std::forward<_Cb>(__cb))
586        {
587            if (auto __state = __token._M_state)
588              {
589                if (__state->_M_register_callback(&_M_cb))
590                    _M_state.swap(__state);
591              }
592        }
593
594      template<typename _Cb,
595               enable_if_t<is_constructible_v<_Callback, _Cb>, int> = 0>
596        explicit
597          stop_callback(stop_token&& __token, _Cb&& __cb)
598        noexcept(is_nothrow_constructible_v<_Callback, _Cb>)
599          : _M_cb(std::forward<_Cb>(__cb))
600          {
601            if (auto& __state = __token._M_state)
602              {
603                if (__state->_M_register_callback(&_M_cb))
604                    _M_state.swap(__state);
605              }
606          }
607
608      ~stop_callback()
609      {
610          if (_M_state)
611            {
612              _M_state->_M_remove_callback(&_M_cb);
613            }
614      }
615
616      stop_callback(const stop_callback&) = delete;
617      stop_callback& operator=(const stop_callback&) = delete;
618      stop_callback(stop_callback&&) = delete;
619      stop_callback& operator=(stop_callback&&) = delete;
620
621    private:
622      struct _Cb_impl : stop_token::_Stop_cb
623      {
624          template<typename _Cb>
625            explicit
626            _Cb_impl(_Cb&& __cb)
627            : _Stop_cb(&_S_execute),
628              _M_cb(std::forward<_Cb>(__cb))
629            { }
630
631          _Callback _M_cb;
632
633          [[__gnu__::__nonnull__]]
634          static void
635          _S_execute(_Stop_cb* __that) noexcept
636          {
637            _Callback& __cb = static_cast<_Cb_impl*>(__that)->_M_cb;
638            std::forward<_Callback>(__cb)();
639          }
640      };
641
642      _Cb_impl _M_cb;
643      stop_token::_Stop_state_ref _M_state;
644    };
645
646  template<typename _Callback>
647    stop_callback(stop_token, _Callback) -> stop_callback<_Callback>;
648
649_GLIBCXX_END_NAMESPACE_VERSION
650} // namespace
651#endif // __cplusplus > 201703L
652#endif // _GLIBCXX_STOP_TOKEN
653