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libs/corosio/src/corosio/src/detail/select/scheduler.cpp

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libs/corosio/src/corosio/src/detail/select/scheduler.cpp
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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 #include <boost/corosio/detail/platform.hpp>
11
12 #if BOOST_COROSIO_HAS_SELECT
13
14 #include "src/detail/select/scheduler.hpp"
15 #include "src/detail/select/op.hpp"
16 #include "src/detail/make_err.hpp"
17 #include "src/detail/posix/resolver_service.hpp"
18 #include "src/detail/posix/signals.hpp"
19
20 #include <boost/corosio/detail/except.hpp>
21 #include <boost/corosio/detail/thread_local_ptr.hpp>
22
23 #include <chrono>
24 #include <limits>
25
26 #include <errno.h>
27 #include <fcntl.h>
28 #include <sys/select.h>
29 #include <sys/socket.h>
30 #include <unistd.h>
31
32 /*
33 select Scheduler - Single Reactor Model
34 =======================================
35
36 This scheduler mirrors the epoll_scheduler design but uses select() instead
37 of epoll for I/O multiplexing. The thread coordination strategy is identical:
38 one thread becomes the "reactor" while others wait on a condition variable.
39
40 Thread Model
41 ------------
42 - ONE thread runs select() at a time (the reactor thread)
43 - OTHER threads wait on wakeup_event_ (condition variable) for handlers
44 - When work is posted, exactly one waiting thread wakes via notify_one()
45
46 Key Differences from epoll
47 --------------------------
48 - Uses self-pipe instead of eventfd for interruption (more portable)
49 - fd_set rebuilding each iteration (O(n) vs O(1) for epoll)
50 - FD_SETSIZE limit (~1024 fds on most systems)
51 - Level-triggered only (no edge-triggered mode)
52
53 Self-Pipe Pattern
54 -----------------
55 To interrupt a blocking select() call (e.g., when work is posted or a timer
56 expires), we write a byte to pipe_fds_[1]. The read end pipe_fds_[0] is
57 always in the read_fds set, so select() returns immediately. We drain the
58 pipe to clear the readable state.
59
60 fd-to-op Mapping
61 ----------------
62 We use an unordered_map<int, fd_state> to track which operations are
63 registered for each fd. This allows O(1) lookup when select() returns
64 ready fds. Each fd can have at most one read op and one write op registered.
65 */
66
67 namespace boost::corosio::detail {
68
69 namespace {
70
71 struct scheduler_context
72 {
73 select_scheduler const* key;
74 scheduler_context* next;
75 };
76
77 corosio::detail::thread_local_ptr<scheduler_context> context_stack;
78
79 struct thread_context_guard
80 {
81 scheduler_context frame_;
82
83 95 explicit thread_context_guard(
84 select_scheduler const* ctx) noexcept
85 95 : frame_{ctx, context_stack.get()}
86 {
87 95 context_stack.set(&frame_);
88 95 }
89
90 95 ~thread_context_guard() noexcept
91 {
92 95 context_stack.set(frame_.next);
93 95 }
94 };
95
96 } // namespace
97
98 120 select_scheduler::
99 select_scheduler(
100 capy::execution_context& ctx,
101 120 int)
102 120 : pipe_fds_{-1, -1}
103 120 , outstanding_work_(0)
104 120 , stopped_(false)
105 120 , shutdown_(false)
106 120 , max_fd_(-1)
107 120 , reactor_running_(false)
108 120 , reactor_interrupted_(false)
109 240 , idle_thread_count_(0)
110 {
111 // Create self-pipe for interrupting select()
112
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 120 if (::pipe(pipe_fds_) < 0)
113 ✗ detail::throw_system_error(make_err(errno), "pipe");
114
115 // Set both ends to non-blocking and close-on-exec
116
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 360 for (int i = 0; i < 2; ++i)
117 {
118
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 240 int flags = ::fcntl(pipe_fds_[i], F_GETFL, 0);
119
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 240 if (flags == -1)
120 {
121 ✗ int errn = errno;
122 ✗ ::close(pipe_fds_[0]);
123 ✗ ::close(pipe_fds_[1]);
124 ✗ detail::throw_system_error(make_err(errn), "fcntl F_GETFL");
125 }
126
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 240 if (::fcntl(pipe_fds_[i], F_SETFL, flags | O_NONBLOCK) == -1)
127 {
128 ✗ int errn = errno;
129 ✗ ::close(pipe_fds_[0]);
130 ✗ ::close(pipe_fds_[1]);
131 ✗ detail::throw_system_error(make_err(errn), "fcntl F_SETFL");
132 }
133
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 240 if (::fcntl(pipe_fds_[i], F_SETFD, FD_CLOEXEC) == -1)
134 {
135 ✗ int errn = errno;
136 ✗ ::close(pipe_fds_[0]);
137 ✗ ::close(pipe_fds_[1]);
138 ✗ detail::throw_system_error(make_err(errn), "fcntl F_SETFD");
139 }
140 }
141
142
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 120 timer_svc_ = &get_timer_service(ctx, *this);
143
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 120 timer_svc_->set_on_earliest_changed(
144 timer_service::callback(
145 this,
146 3120 [](void* p) { static_cast<select_scheduler*>(p)->interrupt_reactor(); }));
147
148 // Initialize resolver service
149
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 120 get_resolver_service(ctx, *this);
150
151 // Initialize signal service
152
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 120 get_signal_service(ctx, *this);
153
154 // Push task sentinel to interleave reactor runs with handler execution
155 120 completed_ops_.push(&task_op_);
156 120 }
157
158 240 select_scheduler::
159 120 ~select_scheduler()
160 {
161
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 120 if (pipe_fds_[0] >= 0)
162 120 ::close(pipe_fds_[0]);
163
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 120 if (pipe_fds_[1] >= 0)
164 120 ::close(pipe_fds_[1]);
165 240 }
166
167 void
168 120 select_scheduler::
169 shutdown()
170 {
171 {
172
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 120 std::unique_lock lock(mutex_);
173 120 shutdown_ = true;
174
175
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 240 while (auto* h = completed_ops_.pop())
176 {
177
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 120 if (h == &task_op_)
178 120 continue;
179 ✗ lock.unlock();
180 ✗ h->destroy();
181 ✗ lock.lock();
182 120 }
183 120 }
184
185 120 outstanding_work_.store(0, std::memory_order_release);
186
187
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 120 if (pipe_fds_[1] >= 0)
188 120 interrupt_reactor();
189
190 120 wakeup_event_.notify_all();
191 120 }
192
193 void
194 3335 select_scheduler::
195 post(capy::coro h) const
196 {
197 struct post_handler final
198 : scheduler_op
199 {
200 capy::coro h_;
201
202 explicit
203 3335 post_handler(capy::coro h)
204 3335 : h_(h)
205 {
206 3335 }
207
208 6670 ~post_handler() = default;
209
210 3335 void operator()() override
211 {
212 3335 auto h = h_;
213
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 3335 delete this;
214
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 3335 h.resume();
215 3335 }
216
217 ✗ void destroy() override
218 {
219 ✗ delete this;
220 ✗ }
221 };
222
223
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 3335 auto ph = std::make_unique<post_handler>(h);
224 3335 outstanding_work_.fetch_add(1, std::memory_order_relaxed);
225
226
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 3335 std::unique_lock lock(mutex_);
227 3335 completed_ops_.push(ph.release());
228
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 3335 wake_one_thread_and_unlock(lock);
229 3335 }
230
231 void
232 229291 select_scheduler::
233 post(scheduler_op* h) const
234 {
235 229291 outstanding_work_.fetch_add(1, std::memory_order_relaxed);
236
237
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 229291 std::unique_lock lock(mutex_);
238 229291 completed_ops_.push(h);
239
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 229291 wake_one_thread_and_unlock(lock);
240 229291 }
241
242 void
243 3532 select_scheduler::
244 on_work_started() noexcept
245 {
246 3532 outstanding_work_.fetch_add(1, std::memory_order_relaxed);
247 3532 }
248
249 void
250 3526 select_scheduler::
251 on_work_finished() noexcept
252 {
253
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 7052 if (outstanding_work_.fetch_sub(1, std::memory_order_acq_rel) == 1)
254 ✗ stop();
255 3526 }
256
257 bool
258 232489 select_scheduler::
259 running_in_this_thread() const noexcept
260 {
261
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 232489 for (auto* c = context_stack.get(); c != nullptr; c = c->next)
262
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 232334 if (c->key == this)
263 232334 return true;
264 155 return false;
265 }
266
267 void
268 1 select_scheduler::
269 stop()
270 {
271 1 bool expected = false;
272
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 1 if (stopped_.compare_exchange_strong(expected, true,
273 std::memory_order_release, std::memory_order_relaxed))
274 {
275 // Wake all threads so they notice stopped_ and exit
276 {
277
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 1 std::lock_guard lock(mutex_);
278 1 wakeup_event_.notify_all();
279 1 }
280
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 1 interrupt_reactor();
281 }
282 1 }
283
284 bool
285 1 select_scheduler::
286 stopped() const noexcept
287 {
288 1 return stopped_.load(std::memory_order_acquire);
289 }
290
291 void
292 34 select_scheduler::
293 restart()
294 {
295 34 stopped_.store(false, std::memory_order_release);
296 34 }
297
298 std::size_t
299 89 select_scheduler::
300 run()
301 {
302
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 89 if (stopped_.load(std::memory_order_acquire))
303 ✗ return 0;
304
305
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 178 if (outstanding_work_.load(std::memory_order_acquire) == 0)
306 {
307 ✗ stop();
308 ✗ return 0;
309 }
310
311 89 thread_context_guard ctx(this);
312
313 89 std::size_t n = 0;
314
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 238646 while (do_one(-1))
315
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 238557 if (n != (std::numeric_limits<std::size_t>::max)())
316 238557 ++n;
317 89 return n;
318 89 }
319
320 std::size_t
321 ✗ select_scheduler::
322 run_one()
323 {
324 ✗ if (stopped_.load(std::memory_order_acquire))
325 ✗ return 0;
326
327 ✗ if (outstanding_work_.load(std::memory_order_acquire) == 0)
328 {
329 ✗ stop();
330 ✗ return 0;
331 }
332
333 ✗ thread_context_guard ctx(this);
334 ✗ return do_one(-1);
335 ✗ }
336
337 std::size_t
338 7 select_scheduler::
339 wait_one(long usec)
340 {
341
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 7 if (stopped_.load(std::memory_order_acquire))
342 ✗ return 0;
343
344
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 14 if (outstanding_work_.load(std::memory_order_acquire) == 0)
345 {
346
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 1 stop();
347 1 return 0;
348 }
349
350 6 thread_context_guard ctx(this);
351
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 6 return do_one(usec);
352 6 }
353
354 std::size_t
355 ✗ select_scheduler::
356 poll()
357 {
358 ✗ if (stopped_.load(std::memory_order_acquire))
359 ✗ return 0;
360
361 ✗ if (outstanding_work_.load(std::memory_order_acquire) == 0)
362 {
363 ✗ stop();
364 ✗ return 0;
365 }
366
367 ✗ thread_context_guard ctx(this);
368
369 ✗ std::size_t n = 0;
370 ✗ while (do_one(0))
371 ✗ if (n != (std::numeric_limits<std::size_t>::max)())
372 ✗ ++n;
373 ✗ return n;
374 ✗ }
375
376 std::size_t
377 ✗ select_scheduler::
378 poll_one()
379 {
380 ✗ if (stopped_.load(std::memory_order_acquire))
381 ✗ return 0;
382
383 ✗ if (outstanding_work_.load(std::memory_order_acquire) == 0)
384 {
385 ✗ stop();
386 ✗ return 0;
387 }
388
389 ✗ thread_context_guard ctx(this);
390 ✗ return do_one(0);
391 ✗ }
392
393 void
394 6057 select_scheduler::
395 register_fd(int fd, select_op* op, int events) const
396 {
397 // Validate fd is within select() limits
398
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 6057 if (fd < 0 || fd >= FD_SETSIZE)
399 ✗ detail::throw_system_error(make_err(EINVAL), "select: fd out of range");
400
401 {
402
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 6057 std::lock_guard lock(mutex_);
403
404
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 6057 auto& state = registered_fds_[fd];
405
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 6057 if (events & event_read)
406 3127 state.read_op = op;
407
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 6057 if (events & event_write)
408 2930 state.write_op = op;
409
410
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 6057 if (fd > max_fd_)
411 185 max_fd_ = fd;
412 6057 }
413
414 // Wake the reactor so a thread blocked in select() rebuilds its fd_sets
415 // with the newly registered fd.
416 6057 interrupt_reactor();
417 6057 }
418
419 void
420 6032 select_scheduler::
421 deregister_fd(int fd, int events) const
422 {
423
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 6032 std::lock_guard lock(mutex_);
424
425
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 6032 auto it = registered_fds_.find(fd);
426
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 6032 if (it == registered_fds_.end())
427 5912 return;
428
429
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 120 if (events & event_read)
430 120 it->second.read_op = nullptr;
431
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 120 if (events & event_write)
432 ✗ it->second.write_op = nullptr;
433
434 // Remove entry if both are null
435
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 120 if (!it->second.read_op && !it->second.write_op)
436 {
437
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 120 registered_fds_.erase(it);
438
439 // Recalculate max_fd_ if needed
440
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 120 if (fd == max_fd_)
441 {
442 119 max_fd_ = pipe_fds_[0]; // At minimum, the pipe read end
443
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 119 for (auto& [registered_fd, state] : registered_fds_)
444 {
445 ✗ if (registered_fd > max_fd_)
446 ✗ max_fd_ = registered_fd;
447 }
448 }
449 }
450 6032 }
451
452 void
453 6057 select_scheduler::
454 work_started() const noexcept
455 {
456 6057 outstanding_work_.fetch_add(1, std::memory_order_relaxed);
457 6057 }
458
459 void
460 238689 select_scheduler::
461 work_finished() const noexcept
462 {
463
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 477378 if (outstanding_work_.fetch_sub(1, std::memory_order_acq_rel) == 1)
464 {
465 // Last work item completed - wake all threads so they can exit.
466 90 std::unique_lock lock(mutex_);
467 90 wakeup_event_.notify_all();
468
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 90 if (reactor_running_ && !reactor_interrupted_)
469 {
470 ✗ reactor_interrupted_ = true;
471 ✗ lock.unlock();
472 ✗ interrupt_reactor();
473 }
474 90 }
475 238689 }
476
477 void
478 12417 select_scheduler::
479 interrupt_reactor() const
480 {
481 12417 char byte = 1;
482
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 12417 [[maybe_unused]] auto r = ::write(pipe_fds_[1], &byte, 1);
483 12417 }
484
485 void
486 232626 select_scheduler::
487 wake_one_thread_and_unlock(std::unique_lock<std::mutex>& lock) const
488 {
489
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 232626 if (idle_thread_count_ > 0)
490 {
491 // Idle worker exists - wake it via condvar
492 ✗ wakeup_event_.notify_one();
493 ✗ lock.unlock();
494 }
495
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 232626 else if (reactor_running_ && !reactor_interrupted_)
496 {
497 // No idle workers but reactor is running - interrupt it
498 3119 reactor_interrupted_ = true;
499 3119 lock.unlock();
500 3119 interrupt_reactor();
501 }
502 else
503 {
504 // No one to wake
505 229507 lock.unlock();
506 }
507 232626 }
508
509 struct work_guard
510 {
511 select_scheduler const* self;
512 238563 ~work_guard() { self->work_finished(); }
513 };
514
515 long
516 8935 select_scheduler::
517 calculate_timeout(long requested_timeout_us) const
518 {
519
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 8935 if (requested_timeout_us == 0)
520 ✗ return 0;
521
522 8935 auto nearest = timer_svc_->nearest_expiry();
523
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 8935 if (nearest == timer_service::time_point::max())
524 35 return requested_timeout_us;
525
526 8900 auto now = std::chrono::steady_clock::now();
527
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 8900 if (nearest <= now)
528 111 return 0;
529
530
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 8789 auto timer_timeout_us = std::chrono::duration_cast<std::chrono::microseconds>(
531
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 17578 nearest - now).count();
532
533
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 8789 if (requested_timeout_us < 0)
534 8789 return static_cast<long>(timer_timeout_us);
535
536 ✗ return static_cast<long>((std::min)(
537 ✗ static_cast<long long>(requested_timeout_us),
538 ✗ static_cast<long long>(timer_timeout_us)));
539 }
540
541 void
542 130724 select_scheduler::
543 run_reactor(std::unique_lock<std::mutex>& lock)
544 {
545 // Calculate timeout considering timers, use 0 if interrupted
546
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 130724 long effective_timeout_us = reactor_interrupted_ ? 0 : calculate_timeout(-1);
547
548 // Build fd_sets from registered_fds_
549 fd_set read_fds, write_fds, except_fds;
550
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 2222308 FD_ZERO(&read_fds);
551
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 2222308 FD_ZERO(&write_fds);
552
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 2222308 FD_ZERO(&except_fds);
553
554 // Always include the interrupt pipe
555 130724 FD_SET(pipe_fds_[0], &read_fds);
556 130724 int nfds = pipe_fds_[0];
557
558 // Add registered fds
559
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 145268 for (auto& [fd, state] : registered_fds_)
560 {
561
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 14544 if (state.read_op)
562 11614 FD_SET(fd, &read_fds);
563
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 14544 if (state.write_op)
564 {
565 2930 FD_SET(fd, &write_fds);
566 // Also monitor for errors on connect operations
567 2930 FD_SET(fd, &except_fds);
568 }
569
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 14544 if (fd > nfds)
570 11617 nfds = fd;
571 }
572
573 // Convert timeout to timeval
574 struct timeval tv;
575 130724 struct timeval* tv_ptr = nullptr;
576
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 130724 if (effective_timeout_us >= 0)
577 {
578 130689 tv.tv_sec = effective_timeout_us / 1000000;
579 130689 tv.tv_usec = effective_timeout_us % 1000000;
580 130689 tv_ptr = &tv;
581 }
582
583
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 130724 lock.unlock();
584
585
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 130724 int ready = ::select(nfds + 1, &read_fds, &write_fds, &except_fds, tv_ptr);
586 130724 int saved_errno = errno;
587
588 // Process timers outside the lock
589
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 130724 timer_svc_->process_expired();
590
591
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 130724 if (ready < 0 && saved_errno != EINTR)
592 ✗ detail::throw_system_error(make_err(saved_errno), "select");
593
594 // Re-acquire lock before modifying completed_ops_
595
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 130724 lock.lock();
596
597 // Drain the interrupt pipe if readable
598
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 130724 if (ready > 0 && FD_ISSET(pipe_fds_[0], &read_fds))
599 {
600 char buf[256];
601
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 18444 while (::read(pipe_fds_[0], buf, sizeof(buf)) > 0) {}
602 }
603
604 // Process I/O completions
605 130724 int completions_queued = 0;
606
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 130724 if (ready > 0)
607 {
608 // Iterate over registered fds (copy keys to avoid iterator invalidation)
609 9222 std::vector<int> fds_to_check;
610
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 9222 fds_to_check.reserve(registered_fds_.size());
611
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 20874 for (auto& [fd, state] : registered_fds_)
612
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 11652 fds_to_check.push_back(fd);
613
614
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 20874 for (int fd : fds_to_check)
615 {
616
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 11652 auto it = registered_fds_.find(fd);
617
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 11652 if (it == registered_fds_.end())
618 ✗ continue;
619
620 11652 auto& state = it->second;
621
622 // Check for errors (especially for connect operations)
623 11652 bool has_error = FD_ISSET(fd, &except_fds);
624
625 // Process read readiness
626
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 11652 if (state.read_op && (FD_ISSET(fd, &read_fds) || has_error))
627 {
628 3007 auto* op = state.read_op;
629 // Claim the op by exchanging to unregistered. Both registering and
630 // registered states mean the op is ours to complete.
631 3007 auto prev = op->registered.exchange(
632 select_registration_state::unregistered, std::memory_order_acq_rel);
633
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 3007 if (prev != select_registration_state::unregistered)
634 {
635 3007 state.read_op = nullptr;
636
637
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 3007 if (has_error)
638 {
639 ✗ int errn = 0;
640 ✗ socklen_t len = sizeof(errn);
641 ✗ if (::getsockopt(fd, SOL_SOCKET, SO_ERROR, &errn, &len) < 0)
642 ✗ errn = errno;
643 ✗ if (errn == 0)
644 ✗ errn = EIO;
645 ✗ op->complete(errn, 0);
646 }
647 else
648 {
649 3007 op->perform_io();
650 }
651
652 3007 completed_ops_.push(op);
653 3007 ++completions_queued;
654 }
655 }
656
657 // Process write readiness
658
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 11652 if (state.write_op && (FD_ISSET(fd, &write_fds) || has_error))
659 {
660 2930 auto* op = state.write_op;
661 // Claim the op by exchanging to unregistered. Both registering and
662 // registered states mean the op is ours to complete.
663 2930 auto prev = op->registered.exchange(
664 select_registration_state::unregistered, std::memory_order_acq_rel);
665
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 2930 if (prev != select_registration_state::unregistered)
666 {
667 2930 state.write_op = nullptr;
668
669
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 2930 if (has_error)
670 {
671 ✗ int errn = 0;
672 ✗ socklen_t len = sizeof(errn);
673 ✗ if (::getsockopt(fd, SOL_SOCKET, SO_ERROR, &errn, &len) < 0)
674 ✗ errn = errno;
675 ✗ if (errn == 0)
676 ✗ errn = EIO;
677 ✗ op->complete(errn, 0);
678 }
679 else
680 {
681 2930 op->perform_io();
682 }
683
684 2930 completed_ops_.push(op);
685 2930 ++completions_queued;
686 }
687 }
688
689 // Clean up empty entries
690
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 11652 if (!state.read_op && !state.write_op)
691
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 5937 registered_fds_.erase(it);
692 }
693 9222 }
694
695
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 130724 if (completions_queued > 0)
696 {
697
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 3010 if (completions_queued == 1)
698 83 wakeup_event_.notify_one();
699 else
700 2927 wakeup_event_.notify_all();
701 }
702 130724 }
703
704 std::size_t
705 238652 select_scheduler::
706 do_one(long timeout_us)
707 {
708
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 238652 std::unique_lock lock(mutex_);
709
710 for (;;)
711 {
712
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 369376 if (stopped_.load(std::memory_order_acquire))
713 ✗ return 0;
714
715 369376 scheduler_op* op = completed_ops_.pop();
716
717
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 369376 if (op == &task_op_)
718 {
719 130813 bool more_handlers = !completed_ops_.empty();
720
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 130813 if (!more_handlers)
722 {
723
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 18048 if (outstanding_work_.load(std::memory_order_acquire) == 0)
724 {
725 89 completed_ops_.push(&task_op_);
726 89 return 0;
727 }
728
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 8935 if (timeout_us == 0)
729 {
730 ✗ completed_ops_.push(&task_op_);
731 ✗ return 0;
732 }
733 }
734
735
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 130724 reactor_interrupted_ = more_handlers || timeout_us == 0;
736 130724 reactor_running_ = true;
737
738
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 130724 if (more_handlers && idle_thread_count_ > 0)
739 ✗ wakeup_event_.notify_one();
740
741
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 130724 run_reactor(lock);
742
743 130724 reactor_running_ = false;
744 130724 completed_ops_.push(&task_op_);
745 130724 continue;
746 130724 }
747
748
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 238563 if (op != nullptr)
749 {
750
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 238563 lock.unlock();
751 238563 work_guard g{this};
752
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 238563 (*op)();
753 238563 return 1;
754 238563 }
755
756 ✗ if (outstanding_work_.load(std::memory_order_acquire) == 0)
757 ✗ return 0;
758
759 ✗ if (timeout_us == 0)
760 ✗ return 0;
761
762 ✗ ++idle_thread_count_;
763 ✗ if (timeout_us < 0)
764 ✗ wakeup_event_.wait(lock);
765 else
766 ✗ wakeup_event_.wait_for(lock, std::chrono::microseconds(timeout_us));
767 ✗ --idle_thread_count_;
768 130724 }
769 238652 }
770
771 } // namespace boost::corosio::detail
772
773 #endif // BOOST_COROSIO_HAS_SELECT
774