include/boost/corosio/native/detail/select/select_scheduler.hpp

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select_scheduler.hpp

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1		//
2		// Copyright (c) 2026 Steve Gerbino
3		//
4		// Distributed under the Boost Software License, Version 1.0. (See accompanying
5		// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
6		//
7		// Official repository: https://github.com/cppalliance/corosio
8		//
9
10		#ifndef BOOST_COROSIO_NATIVE_DETAIL_SELECT_SELECT_SCHEDULER_HPP
11		#define BOOST_COROSIO_NATIVE_DETAIL_SELECT_SELECT_SCHEDULER_HPP
12
13		#include <boost/corosio/detail/platform.hpp>
14
15		#if BOOST_COROSIO_HAS_SELECT
16
17		#include <boost/corosio/detail/config.hpp>
18		#include <boost/capy/ex/execution_context.hpp>
19
20		#include <boost/corosio/native/detail/reactor/reactor_scheduler.hpp>
21
22		#include <boost/corosio/native/detail/select/select_op.hpp>
23		#include <boost/corosio/detail/timer_service.hpp>
24		#include <boost/corosio/native/detail/make_err.hpp>
25		#include <boost/corosio/native/detail/posix/posix_resolver_service.hpp>
26		#include <boost/corosio/native/detail/posix/posix_signal_service.hpp>
27		#include <boost/corosio/native/detail/posix/posix_stream_file_service.hpp>
28		#include <boost/corosio/native/detail/posix/posix_random_access_file_service.hpp>
29
30		#include <boost/corosio/detail/except.hpp>
31
32		#include <sys/select.h>
33		#include <unistd.h>
34		#include <errno.h>
35		#include <fcntl.h>
36
37		#include <atomic>
38		#include <chrono>
39		#include <cstdint>
40		#include <limits>
41		#include <mutex>
42		#include <unordered_map>
43
44		namespace boost::corosio::detail {
45
46		struct select_op;
47		struct select_descriptor_state;
48
49		/** POSIX scheduler using select() for I/O multiplexing.
50
51		This scheduler implements the scheduler interface using the POSIX select()
52		call for I/O event notification. It inherits the shared reactor threading
53		model from reactor_scheduler_base: signal state machine, inline completion
54		budget, work counting, and the do_one event loop.
55
56		The design mirrors epoll_scheduler for behavioral consistency:
57		- Same single-reactor thread coordination model
58		- Same deferred I/O pattern (reactor marks ready; workers do I/O)
59		- Same timer integration pattern
60
61		Known Limitations:
62		- FD_SETSIZE (~1024) limits maximum concurrent connections
63		- O(n) scanning: rebuilds fd_sets each iteration
64		- Level-triggered only (no edge-triggered mode)
65
66		@par Thread Safety
67		All public member functions are thread-safe.
68		*/
69		class BOOST_COROSIO_DECL select_scheduler final : public reactor_scheduler_base
70		{
71		public:
72		/** Construct the scheduler.
73
74		Creates a self-pipe for reactor interruption.
75
76		@param ctx Reference to the owning execution_context.
77		@param concurrency_hint Hint for expected thread count (unused).
78		*/
79		select_scheduler(capy::execution_context& ctx, int concurrency_hint = -1);
80
81		/// Destroy the scheduler.
82		~select_scheduler() override;
83
84		select_scheduler(select_scheduler const&) = delete;
85		select_scheduler& operator=(select_scheduler const&) = delete;
86
87		/// Shut down the scheduler, draining pending operations.
88		void shutdown() override;
89
90		/** Return the maximum file descriptor value supported.
91
92		Returns FD_SETSIZE - 1, the maximum fd value that can be
93		monitored by select(). Operations with fd >= FD_SETSIZE
94		will fail with EINVAL.
95
96		@return The maximum supported file descriptor value.
97		*/
98		static constexpr int max_fd() noexcept
99		{
100		return FD_SETSIZE - 1;
101		}
102
103		/** Register a descriptor for persistent monitoring.
104
105		The fd is added to the registered_descs_ map and will be
106		included in subsequent select() calls. The reactor is
107		interrupted so a blocked select() rebuilds its fd_sets.
108
109		@param fd The file descriptor to register.
110		@param desc Pointer to descriptor state for this fd.
111		*/
112		void register_descriptor(int fd, select_descriptor_state* desc) const;
113
114		/** Deregister a persistently registered descriptor.
115
116		@param fd The file descriptor to deregister.
117		*/
118		void deregister_descriptor(int fd) const;
119
120		/** Interrupt the reactor so it rebuilds its fd_sets.
121
122		Called when a write or connect op is registered after
123		the reactor's snapshot was taken. Without this, select()
124		may block not watching for writability on the fd.
125		*/
126		void notify_reactor() const;
127
128		private:
129		void
130		run_task(std::unique_lock<std::mutex>& lock, context_type* ctx,
131		long timeout_us) override;
132		void interrupt_reactor() const override;
133		long calculate_timeout(long requested_timeout_us) const;
134
135		// Self-pipe for interrupting select()
136		int pipe_fds_[2]; // [0]=read, [1]=write
137
138		// Per-fd tracking for fd_set building
139		mutable std::unordered_map<int, select_descriptor_state*> registered_descs_;
140		mutable int max_fd_ = -1;
141		};
142
143	195x	inline select_scheduler::select_scheduler(capy::execution_context& ctx, int)
144	195x	: pipe_fds_{-1, -1}
145	195x	, max_fd_(-1)
146		{
147	195x	if (::pipe(pipe_fds_) < 0)
148	✗	detail::throw_system_error(make_err(errno), "pipe");
149
150	585x	for (int i = 0; i < 2; ++i)
151		{
152	390x	int flags = ::fcntl(pipe_fds_[i], F_GETFL, 0);
153	390x	if (flags == -1)
154		{
155	✗	int errn = errno;
156	✗	::close(pipe_fds_[0]);
157	✗	::close(pipe_fds_[1]);
158	✗	detail::throw_system_error(make_err(errn), "fcntl F_GETFL");
159		}
160	390x	if (::fcntl(pipe_fds_[i], F_SETFL, flags \| O_NONBLOCK) == -1)
161		{
162	✗	int errn = errno;
163	✗	::close(pipe_fds_[0]);
164	✗	::close(pipe_fds_[1]);
165	✗	detail::throw_system_error(make_err(errn), "fcntl F_SETFL");
166		}
167	390x	if (::fcntl(pipe_fds_[i], F_SETFD, FD_CLOEXEC) == -1)
168		{
169	✗	int errn = errno;
170	✗	::close(pipe_fds_[0]);
171	✗	::close(pipe_fds_[1]);
172	✗	detail::throw_system_error(make_err(errn), "fcntl F_SETFD");
173		}
174		}
175
176	195x	timer_svc_ = &get_timer_service(ctx, *this);
177	195x	timer_svc_->set_on_earliest_changed(
178	2637x	timer_service::callback(this, [](void* p) {
179	2442x	static_cast<select_scheduler*>(p)->interrupt_reactor();
180	2442x	}));
181
182	195x	get_resolver_service(ctx, *this);
183	195x	get_signal_service(ctx, *this);
184	195x	get_stream_file_service(ctx, *this);
185	195x	get_random_access_file_service(ctx, *this);
186
187	195x	completed_ops_.push(&task_op_);
188	195x	}
189
190	390x	inline select_scheduler::~select_scheduler()
191		{
192	195x	if (pipe_fds_[0] >= 0)
193	195x	::close(pipe_fds_[0]);
194	195x	if (pipe_fds_[1] >= 0)
195	195x	::close(pipe_fds_[1]);
196	390x	}
197
198		inline void
199	195x	select_scheduler::shutdown()
200		{
201	195x	shutdown_drain();
202
203	195x	if (pipe_fds_[1] >= 0)
204	195x	interrupt_reactor();
205	195x	}
206
207		inline void
208	4591x	select_scheduler::register_descriptor(
209		int fd, select_descriptor_state* desc) const
210		{
211	4591x	if (fd < 0 \|\| fd >= FD_SETSIZE)
212	✗	detail::throw_system_error(make_err(EINVAL), "select: fd out of range");
213
214	4591x	desc->registered_events = reactor_event_read \| reactor_event_write;
215	4591x	desc->fd = fd;
216	4591x	desc->scheduler_ = this;
217	4591x	desc->ready_events_.store(0, std::memory_order_relaxed);
218
219		{
220	4591x	std::lock_guard lock(desc->mutex);
221	4591x	desc->impl_ref_.reset();
222	4591x	desc->read_ready = false;
223	4591x	desc->write_ready = false;
224	4591x	}
225
226		{
227	4591x	std::lock_guard lock(mutex_);
228	4591x	registered_descs_[fd] = desc;
229	4591x	if (fd > max_fd_)
230	4587x	max_fd_ = fd;
231	4591x	}
232
233	4591x	interrupt_reactor();
234	4591x	}
235
236		inline void
237	4591x	select_scheduler::deregister_descriptor(int fd) const
238		{
239	4591x	std::lock_guard lock(mutex_);
240
241	4591x	auto it = registered_descs_.find(fd);
242	4591x	if (it == registered_descs_.end())
243	✗	return;
244
245	4591x	registered_descs_.erase(it);
246
247	4591x	if (fd == max_fd_)
248		{
249	4534x	max_fd_ = pipe_fds_[0];
250	8983x	for (auto& [registered_fd, state] : registered_descs_)
251		{
252	4449x	if (registered_fd > max_fd_)
253	4440x	max_fd_ = registered_fd;
254		}
255		}
256	4591x	}
257
258		inline void
259	17952x	select_scheduler::notify_reactor() const
260		{
261	17952x	interrupt_reactor();
262	17952x	}
263
264		inline void
265	25323x	select_scheduler::interrupt_reactor() const
266		{
267	25323x	char byte = 1;
268	25323x	[[maybe_unused]] auto r = ::write(pipe_fds_[1], &byte, 1);
269	25323x	}
270
271		inline long
272	79141x	select_scheduler::calculate_timeout(long requested_timeout_us) const
273		{
274	79141x	if (requested_timeout_us == 0)
275	✗	return 0;
276
277	79141x	auto nearest = timer_svc_->nearest_expiry();
278	79141x	if (nearest == timer_service::time_point::max())
279	46x	return requested_timeout_us;
280
281	79095x	auto now = std::chrono::steady_clock::now();
282	79095x	if (nearest <= now)
283	315x	return 0;
284
285		auto timer_timeout_us =
286	78780x	std::chrono::duration_cast<std::chrono::microseconds>(nearest - now)
287	78780x	.count();
288
289	78780x	constexpr auto long_max =
290		static_cast<long long>((std::numeric_limits<long>::max)());
291		auto capped_timer_us =
292	78780x	(std::min)((std::max)(static_cast<long long>(timer_timeout_us),
293	78780x	static_cast<long long>(0)),
294	78780x	long_max);
295
296	78780x	if (requested_timeout_us < 0)
297	78774x	return static_cast<long>(capped_timer_us);
298
299		return static_cast<long>(
300	6x	(std::min)(static_cast<long long>(requested_timeout_us),
301	6x	capped_timer_us));
302		}
303
304		inline void
305	91216x	select_scheduler::run_task(
306		std::unique_lock<std::mutex>& lock, context_type* ctx, long timeout_us)
307		{
308		long effective_timeout_us =
309	91216x	task_interrupted_ ? 0 : calculate_timeout(timeout_us);
310
311		// Snapshot registered descriptors while holding lock.
312		// Record which fds need write monitoring to avoid a hot loop:
313		// select is level-triggered so writable sockets (nearly always
314		// writable) would cause select() to return immediately every
315		// iteration if unconditionally added to write_fds.
316		struct fd_entry
317		{
318		int fd;
319		select_descriptor_state* desc;
320		bool needs_write;
321		};
322		fd_entry snapshot[FD_SETSIZE];
323	91216x	int snapshot_count = 0;
324
325	281071x	for (auto& [fd, desc] : registered_descs_)
326		{
327	189855x	if (snapshot_count < FD_SETSIZE)
328		{
329	189855x	std::lock_guard desc_lock(desc->mutex);
330	189855x	snapshot[snapshot_count].fd = fd;
331	189855x	snapshot[snapshot_count].desc = desc;
332	189855x	snapshot[snapshot_count].needs_write =
333	189855x	(desc->write_op \|\| desc->connect_op);
334	189855x	++snapshot_count;
335	189855x	}
336		}
337
338	91216x	if (lock.owns_lock())
339	79141x	lock.unlock();
340
341	91216x	task_cleanup on_exit{this, &lock, ctx};
342
343		fd_set read_fds, write_fds, except_fds;
344	1550672x	FD_ZERO(&read_fds);
345	1550672x	FD_ZERO(&write_fds);
346	1550672x	FD_ZERO(&except_fds);
347
348	91216x	FD_SET(pipe_fds_[0], &read_fds);
349	91216x	int nfds = pipe_fds_[0];
350
351	281071x	for (int i = 0; i < snapshot_count; ++i)
352		{
353	189855x	int fd = snapshot[i].fd;
354	189855x	FD_SET(fd, &read_fds);
355	189855x	if (snapshot[i].needs_write)
356	2242x	FD_SET(fd, &write_fds);
357	189855x	FD_SET(fd, &except_fds);
358	189855x	if (fd > nfds)
359	90953x	nfds = fd;
360		}
361
362		struct timeval tv;
363	91216x	struct timeval* tv_ptr = nullptr;
364	91216x	if (effective_timeout_us >= 0)
365		{
366	91170x	tv.tv_sec = effective_timeout_us / 1000000;
367	91170x	tv.tv_usec = effective_timeout_us % 1000000;
368	91170x	tv_ptr = &tv;
369		}
370
371	91216x	int ready = ::select(nfds + 1, &read_fds, &write_fds, &except_fds, tv_ptr);
372
373		// EINTR: signal interrupted select(), just retry.
374		// EBADF: an fd was closed between snapshot and select(); retry
375		// with a fresh snapshot from registered_descs_.
376	91216x	if (ready < 0)
377		{
378	✗	if (errno == EINTR \|\| errno == EBADF)
379	✗	return;
380	✗	detail::throw_system_error(make_err(errno), "select");
381		}
382
383		// Process timers outside the lock
384	91216x	timer_svc_->process_expired();
385
386	91216x	op_queue local_ops;
387
388	91216x	if (ready > 0)
389		{
390	83608x	if (FD_ISSET(pipe_fds_[0], &read_fds))
391		{
392		char buf[256];
393	21936x	while (::read(pipe_fds_[0], buf, sizeof(buf)) > 0)
394		{
395		}
396		}
397
398	262784x	for (int i = 0; i < snapshot_count; ++i)
399		{
400	179176x	int fd = snapshot[i].fd;
401	179176x	select_descriptor_state* desc = snapshot[i].desc;
402
403	179176x	std::uint32_t flags = 0;
404	179176x	if (FD_ISSET(fd, &read_fds))
405	78082x	flags \|= reactor_event_read;
406	179176x	if (FD_ISSET(fd, &write_fds))
407	2242x	flags \|= reactor_event_write;
408	179176x	if (FD_ISSET(fd, &except_fds))
409	✗	flags \|= reactor_event_error;
410
411	179176x	if (flags == 0)
412	98854x	continue;
413
414	80322x	desc->add_ready_events(flags);
415
416	80322x	bool expected = false;
417	80322x	if (desc->is_enqueued_.compare_exchange_strong(
418		expected, true, std::memory_order_release,
419		std::memory_order_relaxed))
420		{
421	80322x	local_ops.push(desc);
422		}
423		}
424		}
425
426	91216x	lock.lock();
427
428	91216x	if (!local_ops.empty())
429	78082x	completed_ops_.splice(local_ops);
430	91216x	}
431
432		} // namespace boost::corosio::detail
433
434		#endif // BOOST_COROSIO_HAS_SELECT
435
436		#endif // BOOST_COROSIO_NATIVE_DETAIL_SELECT_SELECT_SCHEDULER_HPP
437