stop_token
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// <stop_token> -*- C++ -*-
// Copyright (C) 2019-2021 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>
#include <bits/std_thread.h>
#include <semaphore>
#define __cpp_lib_jthread 201911L
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();
#endif
this_thread::yield();
}
#ifndef __cpp_lib_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;
std::thread::id _M_requester;
_Stop_state_t() = default;
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));
_M_requester = this_thread::get_id();
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;
// synchronize with destructor of stop_callback that owns *__cb
if (!__gnu_cxx::__is_single_threaded())
__cb->_M_done.release();
}
// 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.
// 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 (!(_M_requester == this_thread::get_id()))
{
// Synchronize with completion of callback.
__cb->_M_done.acquire();
// Safe for ~stop_callback to destroy *__cb now.
return;
}
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