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(lock_type& lock, context_type* ctx) override;
131		void interrupt_reactor() const override;
132		long calculate_timeout(long requested_timeout_us) const;
133
134		// Self-pipe for interrupting select()
135		int pipe_fds_[2]; // [0]=read, [1]=write
136
137		// Per-fd tracking for fd_set building
138		mutable std::unordered_map<int, select_descriptor_state*> registered_descs_;
139		mutable int max_fd_ = -1;
140		};
141
142	195x	inline select_scheduler::select_scheduler(capy::execution_context& ctx, int)
143	195x	: pipe_fds_{-1, -1}
144	195x	, max_fd_(-1)
145		{
146	195x	if (::pipe(pipe_fds_) < 0)
147	✗	detail::throw_system_error(make_err(errno), "pipe");
148
149	585x	for (int i = 0; i < 2; ++i)
150		{
151	390x	int flags = ::fcntl(pipe_fds_[i], F_GETFL, 0);
152	390x	if (flags == -1)
153		{
154	✗	int errn = errno;
155	✗	::close(pipe_fds_[0]);
156	✗	::close(pipe_fds_[1]);
157	✗	detail::throw_system_error(make_err(errn), "fcntl F_GETFL");
158		}
159	390x	if (::fcntl(pipe_fds_[i], F_SETFL, flags \| O_NONBLOCK) == -1)
160		{
161	✗	int errn = errno;
162	✗	::close(pipe_fds_[0]);
163	✗	::close(pipe_fds_[1]);
164	✗	detail::throw_system_error(make_err(errn), "fcntl F_SETFL");
165		}
166	390x	if (::fcntl(pipe_fds_[i], F_SETFD, FD_CLOEXEC) == -1)
167		{
168	✗	int errn = errno;
169	✗	::close(pipe_fds_[0]);
170	✗	::close(pipe_fds_[1]);
171	✗	detail::throw_system_error(make_err(errn), "fcntl F_SETFD");
172		}
173		}
174
175	195x	timer_svc_ = &get_timer_service(ctx, *this);
176	195x	timer_svc_->set_on_earliest_changed(
177	3959x	timer_service::callback(this, [](void* p) {
178	3764x	static_cast<select_scheduler*>(p)->interrupt_reactor();
179	3764x	}));
180
181	195x	get_resolver_service(ctx, *this);
182	195x	get_signal_service(ctx, *this);
183	195x	get_stream_file_service(ctx, *this);
184	195x	get_random_access_file_service(ctx, *this);
185
186	195x	completed_ops_.push(&task_op_);
187	195x	}
188
189	390x	inline select_scheduler::~select_scheduler()
190		{
191	195x	if (pipe_fds_[0] >= 0)
192	195x	::close(pipe_fds_[0]);
193	195x	if (pipe_fds_[1] >= 0)
194	195x	::close(pipe_fds_[1]);
195	390x	}
196
197		inline void
198	195x	select_scheduler::shutdown()
199		{
200	195x	shutdown_drain();
201
202	195x	if (pipe_fds_[1] >= 0)
203	195x	interrupt_reactor();
204	195x	}
205
206		inline void
207	7182x	select_scheduler::register_descriptor(
208		int fd, select_descriptor_state* desc) const
209		{
210	7182x	if (fd < 0 \|\| fd >= FD_SETSIZE)
211	✗	detail::throw_system_error(make_err(EINVAL), "select: fd out of range");
212
213	7182x	desc->registered_events = reactor_event_read \| reactor_event_write;
214	7182x	desc->fd = fd;
215	7182x	desc->scheduler_ = this;
216	7182x	desc->mutex.set_enabled(!single_threaded_);
217	7182x	desc->ready_events_.store(0, std::memory_order_relaxed);
218
219		{
220	7182x	conditionally_enabled_mutex::scoped_lock lock(desc->mutex);
221	7182x	desc->impl_ref_.reset();
222	7182x	desc->read_ready = false;
223	7182x	desc->write_ready = false;
224	7182x	}
225
226		{
227	7182x	mutex_type::scoped_lock lock(mutex_);
228	7182x	registered_descs_[fd] = desc;
229	7182x	if (fd > max_fd_)
230	7178x	max_fd_ = fd;
231	7182x	}
232
233	7182x	interrupt_reactor();
234	7182x	}
235
236		inline void
237	7182x	select_scheduler::deregister_descriptor(int fd) const
238		{
239	7182x	mutex_type::scoped_lock lock(mutex_);
240
241	7182x	auto it = registered_descs_.find(fd);
242	7182x	if (it == registered_descs_.end())
243	✗	return;
244
245	7182x	registered_descs_.erase(it);
246
247	7182x	if (fd == max_fd_)
248		{
249	7124x	max_fd_ = pipe_fds_[0];
250	14163x	for (auto& [registered_fd, state] : registered_descs_)
251		{
252	7039x	if (registered_fd > max_fd_)
253	7030x	max_fd_ = registered_fd;
254		}
255		}
256	7182x	}
257
258		inline void
259	26057x	select_scheduler::notify_reactor() const
260		{
261	26057x	interrupt_reactor();
262	26057x	}
263
264		inline void
265	37341x	select_scheduler::interrupt_reactor() const
266		{
267	37341x	char byte = 1;
268	37341x	[[maybe_unused]] auto r = ::write(pipe_fds_[1], &byte, 1);
269	37341x	}
270
271		inline long
272	126526x	select_scheduler::calculate_timeout(long requested_timeout_us) const
273		{
274	126526x	if (requested_timeout_us == 0)
275	✗	return 0;
276
277	126526x	auto nearest = timer_svc_->nearest_expiry();
278	126526x	if (nearest == timer_service::time_point::max())
279	46x	return requested_timeout_us;
280
281	126480x	auto now = std::chrono::steady_clock::now();
282	126480x	if (nearest <= now)
283	363x	return 0;
284
285		auto timer_timeout_us =
286	126117x	std::chrono::duration_cast<std::chrono::microseconds>(nearest - now)
287	126117x	.count();
288
289	126117x	constexpr auto long_max =
290		static_cast<long long>((std::numeric_limits<long>::max)());
291		auto capped_timer_us =
292	126117x	(std::min)((std::max)(static_cast<long long>(timer_timeout_us),
293	126117x	static_cast<long long>(0)),
294	126117x	long_max);
295
296	126117x	if (requested_timeout_us < 0)
297	126117x	return static_cast<long>(capped_timer_us);
298
299		return static_cast<long>(
300	✗	(std::min)(static_cast<long long>(requested_timeout_us),
301	✗	capped_timer_us));
302		}
303
304		inline void
305	151727x	select_scheduler::run_task(
306		lock_type& lock, context_type* ctx)
307		{
308	151727x	long effective_timeout_us = task_interrupted_ ? 0 : calculate_timeout(-1);
309
310		// Snapshot registered descriptors while holding lock.
311		// Record which fds need write monitoring to avoid a hot loop:
312		// select is level-triggered so writable sockets (nearly always
313		// writable) would cause select() to return immediately every
314		// iteration if unconditionally added to write_fds.
315		struct fd_entry
316		{
317		int fd;
318		select_descriptor_state* desc;
319		bool needs_write;
320		};
321		fd_entry snapshot[FD_SETSIZE];
322	151727x	int snapshot_count = 0;
323
324	449307x	for (auto& [fd, desc] : registered_descs_)
325		{
326	297580x	if (snapshot_count < FD_SETSIZE)
327		{
328	297580x	conditionally_enabled_mutex::scoped_lock desc_lock(desc->mutex);
329	297580x	snapshot[snapshot_count].fd = fd;
330	297580x	snapshot[snapshot_count].desc = desc;
331	297580x	snapshot[snapshot_count].needs_write =
332	297580x	(desc->write_op \|\| desc->connect_op);
333	297580x	++snapshot_count;
334	297580x	}
335		}
336
337	151727x	if (lock.owns_lock())
338	126526x	lock.unlock();
339
340	151727x	task_cleanup on_exit{this, &lock, ctx};
341
342		fd_set read_fds, write_fds, except_fds;
343	2579359x	FD_ZERO(&read_fds);
344	2579359x	FD_ZERO(&write_fds);
345	2579359x	FD_ZERO(&except_fds);
346
347	151727x	FD_SET(pipe_fds_[0], &read_fds);
348	151727x	int nfds = pipe_fds_[0];
349
350	449307x	for (int i = 0; i < snapshot_count; ++i)
351		{
352	297580x	int fd = snapshot[i].fd;
353	297580x	FD_SET(fd, &read_fds);
354	297580x	if (snapshot[i].needs_write)
355	3538x	FD_SET(fd, &write_fds);
356	297580x	FD_SET(fd, &except_fds);
357	297580x	if (fd > nfds)
358	151462x	nfds = fd;
359		}
360
361		struct timeval tv;
362	151727x	struct timeval* tv_ptr = nullptr;
363	151727x	if (effective_timeout_us >= 0)
364		{
365	151681x	tv.tv_sec = effective_timeout_us / 1000000;
366	151681x	tv.tv_usec = effective_timeout_us % 1000000;
367	151681x	tv_ptr = &tv;
368		}
369
370	151727x	int ready = ::select(nfds + 1, &read_fds, &write_fds, &except_fds, tv_ptr);
371
372		// EINTR: signal interrupted select(), just retry.
373		// EBADF: an fd was closed between snapshot and select(); retry
374		// with a fresh snapshot from registered_descs_.
375	151727x	if (ready < 0)
376		{
377	✗	if (errno == EINTR \|\| errno == EBADF)
378	✗	return;
379	✗	detail::throw_system_error(make_err(errno), "select");
380		}
381
382		// Process timers outside the lock
383	151727x	timer_svc_->process_expired();
384
385	151727x	op_queue local_ops;
386
387	151727x	if (ready > 0)
388		{
389	135723x	if (FD_ISSET(pipe_fds_[0], &read_fds))
390		{
391		char buf[256];
392	39200x	while (::read(pipe_fds_[0], buf, sizeof(buf)) > 0)
393		{
394		}
395		}
396
397	408437x	for (int i = 0; i < snapshot_count; ++i)
398		{
399	272714x	int fd = snapshot[i].fd;
400	272714x	select_descriptor_state* desc = snapshot[i].desc;
401
402	272714x	std::uint32_t flags = 0;
403	272714x	if (FD_ISSET(fd, &read_fds))
404	128627x	flags \|= reactor_event_read;
405	272714x	if (FD_ISSET(fd, &write_fds))
406	3538x	flags \|= reactor_event_write;
407	272714x	if (FD_ISSET(fd, &except_fds))
408	✗	flags \|= reactor_event_error;
409
410	272714x	if (flags == 0)
411	140552x	continue;
412
413	132162x	desc->add_ready_events(flags);
414
415	132162x	bool expected = false;
416	132162x	if (desc->is_enqueued_.compare_exchange_strong(
417		expected, true, std::memory_order_release,
418		std::memory_order_relaxed))
419		{
420	132162x	local_ops.push(desc);
421		}
422		}
423		}
424
425	151727x	lock.lock();
426
427	151727x	if (!local_ops.empty())
428	128627x	completed_ops_.splice(local_ops);
429	151727x	}
430
431		} // namespace boost::corosio::detail
432
433		#endif // BOOST_COROSIO_HAS_SELECT
434
435		#endif // BOOST_COROSIO_NATIVE_DETAIL_SELECT_SELECT_SCHEDULER_HPP
436