stop_token

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// <stop_token> -*- C++ -*-
 
// Copyright (C) 2019-2020 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.
 
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
 
// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.
 
// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// <http://www.gnu.org/licenses/>.
 
/** @file include/stop_token
 *  This is a Standard C++ Library header.
 */
 
#ifndef _GLIBCXX_STOP_TOKEN
#define _GLIBCXX_STOP_TOKEN
 
#if __cplusplus > 201703L
 
#include <atomic>
 
#ifdef _GLIBCXX_HAS_GTHREADS
# define __cpp_lib_jthread 201911L
# include <bits/gthr.h>
# if __has_include(<semaphore>)
#  include <semaphore>
# endif
#endif
 
namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
 
  /// Tag type indicating a stop_source should have no shared-stop-state.
  struct nostopstate_t { explicit nostopstate_t() = default; };
  inline constexpr nostopstate_t nostopstate{};
 
  class stop_source;
 
  /// Allow testing whether a stop request has been made on a `stop_source`.
  class stop_token
  {
  public:
    stop_token() noexcept = default;
 
    stop_token(const stop_token&) noexcept = default;
    stop_token(stop_token&&) noexcept = default;
 
    ~stop_token() = default;
 
    stop_token&
    operator=(const stop_token&) noexcept = default;
 
    stop_token&
    operator=(stop_token&&) noexcept = default;
 
    [[nodiscard]]
    bool
    stop_possible() const noexcept
    {
      return static_cast<bool>(_M_state) && _M_state->_M_stop_possible();
    }
 
    [[nodiscard]]
    bool
    stop_requested() const noexcept
    {
      return static_cast<bool>(_M_state) && _M_state->_M_stop_requested();
    }
 
    void
    swap(stop_token& __rhs) noexcept
    { _M_state.swap(__rhs._M_state); }
 
    [[nodiscard]]
    friend bool
    operator==(const stop_token& __a, const stop_token& __b)
    { return __a._M_state == __b._M_state; }
 
    friend void
    swap(stop_token& __lhs, stop_token& __rhs) noexcept
    { __lhs.swap(__rhs); }
 
  private:
    friend class stop_source;
    template<typename _Callback>
      friend class stop_callback;
 
    static void
    _S_yield() noexcept
    {
#if defined __i386__ || defined __x86_64__
      __builtin_ia32_pause();
#elif defined _GLIBCXX_HAS_GTHREADS && defined _GLIBCXX_USE_SCHED_YIELD
      __gthread_yield();
#endif
    }
 
#ifndef __cpp_lib_semaphore
    // TODO: replace this with a real implementation of std::binary_semaphore
    struct binary_semaphore
    {
      explicit binary_semaphore(int __d) : _M_counter(__d > 0) { }
 
      void release() { _M_counter.fetch_add(1, memory_order::release); }
 
      void acquire()
      {
        int __old = 1;
        while (!_M_counter.compare_exchange_weak(__old, 0,
                                                 memory_order::acquire,
                                                 memory_order::relaxed))
          {
            __old = 1;
            _S_yield();
          }
      }
 
      atomic<int> _M_counter;
    };
#endif
 
    struct _Stop_cb
    {
      using __cb_type = void(_Stop_cb*) noexcept;
      __cb_type* _M_callback;
      _Stop_cb* _M_prev = nullptr;
      _Stop_cb* _M_next = nullptr;
      bool* _M_destroyed = nullptr;
      binary_semaphore _M_done{0};
 
      [[__gnu__::__nonnull__]]
      explicit
      _Stop_cb(__cb_type* __cb)
      : _M_callback(__cb)
      { }
 
      void _M_run() noexcept { _M_callback(this); }
    };
 
    struct _Stop_state_t
    {
      using value_type = uint32_t;
      static constexpr value_type _S_stop_requested_bit = 1;
      static constexpr value_type _S_locked_bit = 2;
      static constexpr value_type _S_ssrc_counter_inc = 4;
 
      std::atomic<value_type> _M_owners{1};
      std::atomic<value_type> _M_value{_S_ssrc_counter_inc};
      _Stop_cb* _M_head = nullptr;
#ifdef _GLIBCXX_HAS_GTHREADS
      __gthread_t _M_requester;
#endif
 
      _Stop_state_t() noexcept { }
 
      bool
      _M_stop_possible() noexcept
      {
        // true if a stop request has already been made or there are still
        // stop_source objects that would allow one to be made.
        return _M_value.load(memory_order::acquire) & ~_S_locked_bit;
      }
 
      bool
      _M_stop_requested() noexcept
      {
        return _M_value.load(memory_order::acquire) & _S_stop_requested_bit;
      }
 
      void
      _M_add_owner() noexcept
      {
        _M_owners.fetch_add(1, memory_order::relaxed);
      }
 
      void
      _M_release_ownership() noexcept
      {
        if (_M_owners.fetch_sub(1, memory_order::acq_rel) == 1)
          delete this;
      }
 
      void
      _M_add_ssrc() noexcept
      {
        _M_value.fetch_add(_S_ssrc_counter_inc, memory_order::relaxed);
      }
 
      void
      _M_sub_ssrc() noexcept
      {
        _M_value.fetch_sub(_S_ssrc_counter_inc, memory_order::release);
      }
 
      // Obtain lock.
      void
      _M_lock() noexcept
      {
        // Can use relaxed loads to get the current value.
        // The successful call to _M_try_lock is an acquire operation.
        auto __old = _M_value.load(memory_order::relaxed);
        while (!_M_try_lock(__old, memory_order::relaxed))
          { }
      }
 
      // Precondition: calling thread holds the lock.
      void
      _M_unlock() noexcept
      {
        _M_value.fetch_sub(_S_locked_bit, memory_order::release);
      }
 
      bool
      _M_request_stop() noexcept
      {
        // obtain lock and set stop_requested bit
        auto __old = _M_value.load(memory_order::acquire);
        do
          {
            if (__old & _S_stop_requested_bit) // stop request already made
              return false;
          }
        while (!_M_try_lock_and_stop(__old));
 
#ifdef _GLIBCXX_HAS_GTHREADS
#ifdef _GLIBCXX_NATIVE_THREAD_ID
        _M_requester = _GLIBCXX_NATIVE_THREAD_ID;
#else
        _M_requester = __gthread_self();
#endif
#endif
 
        while (_M_head)
          {
            bool __last_cb;
            _Stop_cb* __cb = _M_head;
            _M_head = _M_head->_M_next;
            if (_M_head)
              {
                _M_head->_M_prev = nullptr;
                __last_cb = false;
              }
            else
              __last_cb = true;
 
            // Allow other callbacks to be unregistered while __cb runs.
            _M_unlock();
 
            bool __destroyed = false;
            __cb->_M_destroyed = &__destroyed;
 
            // run callback
            __cb->_M_run();
 
            if (!__destroyed)
              {
                __cb->_M_destroyed = nullptr;
#ifdef _GLIBCXX_HAS_GTHREADS
                // synchronize with destructor of stop_callback that owns *__cb
                __cb->_M_done.release();
#endif
              }
 
            // Avoid relocking if we already know there are no more callbacks.
            if (__last_cb)
              return true;
 
            _M_lock();
          }
 
        _M_unlock();
        return true;
      }
 
      [[__gnu__::__nonnull__]]
      bool
      _M_register_callback(_Stop_cb* __cb) noexcept
      {
        auto __old = _M_value.load(memory_order::acquire);
        do
          {
            if (__old & _S_stop_requested_bit) // stop request already made
              {
                __cb->_M_run(); // run synchronously
                return false;
              }
 
            if (__old < _S_ssrc_counter_inc) // no stop_source owns *this
              // No need to register callback if no stop request can be made.
              // Returning false also means the stop_callback does not share
              // ownership of this state, but that's not observable.
              return false;
          }
        while (!_M_try_lock(__old));
 
        __cb->_M_next = _M_head;
        if (_M_head)
          {
            _M_head->_M_prev = __cb;
          }
        _M_head = __cb;
        _M_unlock();
        return true;
      }
 
      // Called by ~stop_callback just before destroying *__cb.
      [[__gnu__::__nonnull__]]
      void
      _M_remove_callback(_Stop_cb* __cb)
      {
        _M_lock();
 
        if (__cb == _M_head)
          {
            _M_head = _M_head->_M_next;
            if (_M_head)
              _M_head->_M_prev = nullptr;
            _M_unlock();
            return;
          }
        else if (__cb->_M_prev)
          {
            __cb->_M_prev->_M_next = __cb->_M_next;
            if (__cb->_M_next)
              __cb->_M_next->_M_prev = __cb->_M_prev;
            _M_unlock();
            return;
          }
 
        _M_unlock();
 
        // Callback is not in the list, so must have been removed by a call to
        // _M_request_stop.
 
#ifdef _GLIBCXX_HAS_GTHREADS
#ifdef _GLIBCXX_NATIVE_THREAD_ID
        auto __tid = _GLIBCXX_NATIVE_THREAD_ID;
#else
        auto __tid = __gthread_self();
#endif
        // Despite appearances there is no data race on _M_requester. The only
        // write to it happens before the callback is removed from the list,
        // and removing it from the list happens before this read.
        if (!__gthread_equal(_M_requester, __tid))
          {
            // Synchronize with completion of callback.
            __cb->_M_done.acquire();
            // Safe for ~stop_callback to destroy *__cb now.
            return;
          }
#endif
        if (__cb->_M_destroyed)
          *__cb->_M_destroyed = true;
      }
 
      // Try to obtain the lock.
      // Returns true if the lock is acquired (with memory order acquire).
      // Otherwise, sets __curval = _M_value.load(__failure) and returns false.
      // Might fail spuriously, so must be called in a loop.
      bool
      _M_try_lock(value_type& __curval,
                  memory_order __failure = memory_order::acquire) noexcept
      {
        return _M_do_try_lock(__curval, 0, memory_order::acquire, __failure);
      }
 
      // Try to obtain the lock to make a stop request.
      // Returns true if the lock is acquired and the _S_stop_requested_bit is
      // set (with memory order acq_rel so that other threads see the request).
      // Otherwise, sets __curval = _M_value.load(memory_order::acquire) and
      // returns false.
      // Might fail spuriously, so must be called in a loop.
      bool
      _M_try_lock_and_stop(value_type& __curval) noexcept
      {
        return _M_do_try_lock(__curval, _S_stop_requested_bit,
                              memory_order::acq_rel, memory_order::acquire);
      }
 
      bool
      _M_do_try_lock(value_type& __curval, value_type __newbits,
                     memory_order __success, memory_order __failure) noexcept
      {
        if (__curval & _S_locked_bit)
          {
            _S_yield();
            __curval = _M_value.load(__failure);
            return false;
          }
        __newbits |= _S_locked_bit;
        return _M_value.compare_exchange_weak(__curval, __curval | __newbits,
                                              __success, __failure);
      }
    };
 
    struct _Stop_state_ref
    {
      _Stop_state_ref() = default;
 
      explicit
      _Stop_state_ref(const stop_source&)
      : _M_ptr(new _Stop_state_t())
      { }
 
      _Stop_state_ref(const _Stop_state_ref& __other) noexcept
      : _M_ptr(__other._M_ptr)
      {
        if (_M_ptr)
          _M_ptr->_M_add_owner();
      }
 
      _Stop_state_ref(_Stop_state_ref&& __other) noexcept
      : _M_ptr(__other._M_ptr)
      {
        __other._M_ptr = nullptr;
      }
 
      _Stop_state_ref&
      operator=(const _Stop_state_ref& __other) noexcept
      {
        if (auto __ptr = __other._M_ptr; __ptr != _M_ptr)
          {
            if (__ptr)
              __ptr->_M_add_owner();
            if (_M_ptr)
              _M_ptr->_M_release_ownership();
            _M_ptr = __ptr;
          }
        return *this;
      }
 
      _Stop_state_ref&
      operator=(_Stop_state_ref&& __other) noexcept
      {
        _Stop_state_ref(std::move(__other)).swap(*this);
        return *this;
      }
 
      ~_Stop_state_ref()
      {
        if (_M_ptr)
          _M_ptr->_M_release_ownership();
      }
 
      void
      swap(_Stop_state_ref& __other) noexcept
      { std::swap(_M_ptr, __other._M_ptr); }
 
      explicit operator bool() const noexcept { return _M_ptr != nullptr; }
 
      _Stop_state_t* operator->() const noexcept { return _M_ptr; }
 
#if __cpp_impl_three_way_comparison >= 201907L
      friend bool
      operator==(const _Stop_state_ref&, const _Stop_state_ref&) = default;
#else
      friend bool
      operator==(const _Stop_state_ref& __lhs, const _Stop_state_ref& __rhs)
      noexcept
      { return __lhs._M_ptr == __rhs._M_ptr; }
 
      friend bool
      operator!=(const _Stop_state_ref& __lhs, const _Stop_state_ref& __rhs)
      noexcept
      { return __lhs._M_ptr != __rhs._M_ptr; }
#endif
 
    private:
      _Stop_state_t* _M_ptr = nullptr;
    };
 
    _Stop_state_ref _M_state;
 
    explicit
    stop_token(const _Stop_state_ref& __state) noexcept
    : _M_state{__state}
    { }
  };
 
  /// A type that allows a stop request to be made.
  class stop_source
  {
  public:
    stop_source() : _M_state(*this)
    { }
 
    explicit stop_source(std::nostopstate_t) noexcept
    { }
 
    stop_source(const stop_source& __other) noexcept
    : _M_state(__other._M_state)
    {
      if (_M_state)
        _M_state->_M_add_ssrc();
    }
 
    stop_source(stop_source&&) noexcept = default;
 
    stop_source&
    operator=(const stop_source& __other) noexcept
    {
      if (_M_state != __other._M_state)
        {
          stop_source __sink(std::move(*this));
          _M_state = __other._M_state;
          if (_M_state)
            _M_state->_M_add_ssrc();
        }
      return *this;
    }
 
    stop_source&
    operator=(stop_source&&) noexcept = default;
 
    ~stop_source()
    {
      if (_M_state)
        _M_state->_M_sub_ssrc();
    }
 
    [[nodiscard]]
    bool
    stop_possible() const noexcept
    {
      return static_cast<bool>(_M_state);
    }
 
    [[nodiscard]]
    bool
    stop_requested() const noexcept
    {
      return static_cast<bool>(_M_state) && _M_state->_M_stop_requested();
    }
 
    bool
    request_stop() const noexcept
    {
      if (stop_possible())
        return _M_state->_M_request_stop();
      return false;
    }
 
    [[nodiscard]]
    stop_token
    get_token() const noexcept
    {
      return stop_token{_M_state};
    }
 
    void
    swap(stop_source& __other) noexcept
    {
      _M_state.swap(__other._M_state);
    }
 
    [[nodiscard]]
    friend bool
    operator==(const stop_source& __a, const stop_source& __b) noexcept
    {
      return __a._M_state == __b._M_state;
    }
 
    friend void
    swap(stop_source& __lhs, stop_source& __rhs) noexcept
    {
      __lhs.swap(__rhs);
    }
 
  private:
    stop_token::_Stop_state_ref _M_state;
  };
 
  /// A wrapper for callbacks to be run when a stop request is made.
  template<typename _Callback>
    class [[nodiscard]] stop_callback
    {
      static_assert(is_nothrow_destructible_v<_Callback>);
      static_assert(is_invocable_v<_Callback>);
 
    public:
      using callback_type = _Callback;
 
      template<typename _Cb,
               enable_if_t<is_constructible_v<_Callback, _Cb>, int> = 0>
        explicit
        stop_callback(const stop_token& __token, _Cb&& __cb)
        noexcept(is_nothrow_constructible_v<_Callback, _Cb>)
        : _M_cb(std::forward<_Cb>(__cb))
        {
          if (auto __state = __token._M_state)
            {
              if (__state->_M_register_callback(&_M_cb))
                _M_state.swap(__state);
            }
        }
 
      template<typename _Cb,
               enable_if_t<is_constructible_v<_Callback, _Cb>, int> = 0>
        explicit
        stop_callback(stop_token&& __token, _Cb&& __cb)
        noexcept(is_nothrow_constructible_v<_Callback, _Cb>)
        : _M_cb(std::forward<_Cb>(__cb))
        {
          if (auto& __state = __token._M_state)
            {
              if (__state->_M_register_callback(&_M_cb))
                _M_state.swap(__state);
            }
        }
 
      ~stop_callback()
      {
        if (_M_state)
          {
            _M_state->_M_remove_callback(&_M_cb);
          }
      }
 
      stop_callback(const stop_callback&) = delete;
      stop_callback& operator=(const stop_callback&) = delete;
      stop_callback(stop_callback&&) = delete;
      stop_callback& operator=(stop_callback&&) = delete;
 
    private:
      struct _Cb_impl : stop_token::_Stop_cb
      {
        template<typename _Cb>
          explicit
          _Cb_impl(_Cb&& __cb)
          : _Stop_cb(&_S_execute),
            _M_cb(std::forward<_Cb>(__cb))
          { }
 
        _Callback _M_cb;
 
        [[__gnu__::__nonnull__]]
        static void
        _S_execute(_Stop_cb* __that) noexcept
        {
          _Callback& __cb = static_cast<_Cb_impl*>(__that)->_M_cb;
          std::forward<_Callback>(__cb)();
        }
      };
 
      _Cb_impl _M_cb;
      stop_token::_Stop_state_ref _M_state;
    };
 
  template<typename _Callback>
    stop_callback(stop_token, _Callback) -> stop_callback<_Callback>;
 
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace
#endif // __cplusplus > 201703L
#endif // _GLIBCXX_STOP_TOKEN