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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 92 explicit thread_context_guard(
84 select_scheduler const* ctx) noexcept
85 92 : frame_{ctx, context_stack.get()}
86 {
87 92 context_stack.set(&frame_);
88 92 }
89
90 92 ~thread_context_guard() noexcept
91 {
92 92 context_stack.set(frame_.next);
93 92 }
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 2423 [](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
194 168 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
203 168 post_handler(capy::coro h)
204 168 : h_(h)
205 {
206 168 }
207
208 336 ~post_handler() = default;
209
210 168 void operator()() override
211 {
212 168 auto h = h_;
213
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 168 delete this;
214
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 168 h.resume();
215 168 }
216
217 ✗ void destroy() override
218 {
219 ✗ delete this;
220 ✗ }
221 };
222
223
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 168 auto ph = std::make_unique<post_handler>(h);
224 168 outstanding_work_.fetch_add(1, std::memory_order_relaxed);
225
226
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 168 std::unique_lock lock(mutex_);
227 168 completed_ops_.push(ph.release());
228
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 168 wake_one_thread_and_unlock(lock);
229 168 }
230
231 void
232 162842 select_scheduler::
233 post(scheduler_op* h) const
234 {
235 162842 outstanding_work_.fetch_add(1, std::memory_order_relaxed);
236
237
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 162842 std::unique_lock lock(mutex_);
238 162842 completed_ops_.push(h);
239
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 162842 wake_one_thread_and_unlock(lock);
240 162842 }
241
242 void
243 2420 select_scheduler::
244 on_work_started() noexcept
245 {
246 2420 outstanding_work_.fetch_add(1, std::memory_order_relaxed);
247 2420 }
248
249 void
250 2414 select_scheduler::
251 on_work_finished() noexcept
252 {
253
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 4828 if (outstanding_work_.fetch_sub(1, std::memory_order_acq_rel) == 1)
254 18 stop();
255 2414 }
256
257 bool
258 167632 select_scheduler::
259 running_in_this_thread() const noexcept
260 {
261
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 167632 for (auto* c = context_stack.get(); c != nullptr; c = c->next)
262
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 167477 if (c->key == this)
263 167477 return true;
264 155 return false;
265 }
266
267 void
268 18 select_scheduler::
269 stop()
270 {
271 18 bool expected = false;
272
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 18 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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 18 std::lock_guard lock(mutex_);
278 18 wakeup_event_.notify_all();
279 18 }
280
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 18 interrupt_reactor();
281 }
282 18 }
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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 167501 while (do_one(-1))
315
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 167412 if (n != (std::numeric_limits<std::size_t>::max)())
316 167412 ++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 3 select_scheduler::
339 wait_one(long usec)
340 {
341
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 3 if (stopped_.load(std::memory_order_acquire))
342 ✗ return 0;
343
344
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 6 if (outstanding_work_.load(std::memory_order_acquire) == 0)
345 {
346 ✗ stop();
347 ✗ return 0;
348 }
349
350 3 thread_context_guard ctx(this);
351
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 3 return do_one(usec);
352 3 }
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 4523 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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 4523 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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 4523 std::lock_guard lock(mutex_);
403
404
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 4523 auto& state = registered_fds_[fd];
405
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 4523 if (events & event_read)
406 2343 state.read_op = op;
407
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 4523 if (events & event_write)
408 2180 state.write_op = op;
409
410
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 4523 if (fd > max_fd_)
411 184 max_fd_ = fd;
412 4523 }
413
414 // Wake the reactor so a thread blocked in select() rebuilds its fd_sets
415 // with the newly registered fd.
416 4523 interrupt_reactor();
417 4523 }
418
419 void
420 4530 select_scheduler::
421 deregister_fd(int fd, int events) const
422 {
423
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 4530 std::lock_guard lock(mutex_);
424
425
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 4530 auto it = registered_fds_.find(fd);
426
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 4530 if (it == registered_fds_.end())
427 4411 return;
428
429
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 119 if (events & event_read)
430 119 it->second.read_op = nullptr;
431
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 119 if (events & event_write)
432 ✗ it->second.write_op = nullptr;
433
434 // Remove entry if both are null
435
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 119 if (!it->second.read_op && !it->second.write_op)
436 {
437
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 119 registered_fds_.erase(it);
438
439 // Recalculate max_fd_ if needed
440
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 119 if (fd == max_fd_)
441 {
442 118 max_fd_ = pipe_fds_[0]; // At minimum, the pipe read end
443
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 118 for (auto& [registered_fd, state] : registered_fds_)
444 {
445 ✗ if (registered_fd > max_fd_)
446 ✗ max_fd_ = registered_fd;
447 }
448 }
449 }
450 4530 }
451
452 void
453 4523 select_scheduler::
454 work_started() const noexcept
455 {
456 4523 outstanding_work_.fetch_add(1, std::memory_order_relaxed);
457 4523 }
458
459 void
460 167539 select_scheduler::
461 work_finished() const noexcept
462 {
463
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 335078 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 72 std::unique_lock lock(mutex_);
467 72 wakeup_event_.notify_all();
468
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 72 if (reactor_running_ && !reactor_interrupted_)
469 {
470 ✗ reactor_interrupted_ = true;
471 ✗ lock.unlock();
472 ✗ interrupt_reactor();
473 }
474 72 }
475 167539 }
476
477 void
478 7292 select_scheduler::
479 interrupt_reactor() const
480 {
481 7292 char byte = 1;
482
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 7292 [[maybe_unused]] auto r = ::write(pipe_fds_[1], &byte, 1);
483 7292 }
484
485 void
486 163010 select_scheduler::
487 wake_one_thread_and_unlock(std::unique_lock<std::mutex>& lock) const
488 {
489
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 163010 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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 163010 else if (reactor_running_ && !reactor_interrupted_)
496 {
497 // No idle workers but reactor is running - interrupt it
498 208 reactor_interrupted_ = true;
499 208 lock.unlock();
500 208 interrupt_reactor();
501 }
502 else
503 {
504 // No one to wake
505 162802 lock.unlock();
506 }
507 163010 }
508
509 struct work_guard
510 {
511 select_scheduler const* self;
512 341853 ~work_guard() { self->work_finished(); }
513 };
514
515 long
516 6822 select_scheduler::
517 calculate_timeout(long requested_timeout_us) const
518 {
519
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 6822 if (requested_timeout_us == 0)
520 ✗ return 0;
521
522 6822 auto nearest = timer_svc_->nearest_expiry();
523
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 6822 if (nearest == timer_service::time_point::max())
524 36 return requested_timeout_us;
525
526 6786 auto now = std::chrono::steady_clock::now();
527
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 6786 if (nearest <= now)
528 38 return 0;
529
530
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 6748 auto timer_timeout_us = std::chrono::duration_cast<std::chrono::microseconds>(
531
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 13496 nearest - now).count();
532
533
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 6748 if (requested_timeout_us < 0)
534 6748 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 82753 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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 82753 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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 1406801 FD_ZERO(&read_fds);
551
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 1406801 FD_ZERO(&write_fds);
552
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 1406801 FD_ZERO(&except_fds);
553
554 // Always include the interrupt pipe
555 82753 FD_SET(pipe_fds_[0], &read_fds);
556 82753 int nfds = pipe_fds_[0];
557
558 // Add registered fds
559
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 91479 for (auto& [fd, state] : registered_fds_)
560 {
561
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 8726 if (state.read_op)
562 6546 FD_SET(fd, &read_fds);
563
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 8726 if (state.write_op)
564 {
565 2180 FD_SET(fd, &write_fds);
566 // Also monitor for errors on connect operations
567 2180 FD_SET(fd, &except_fds);
568 }
569
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 8726 if (fd > nfds)
570 6550 nfds = fd;
571 }
572
573 // Convert timeout to timeval
574 struct timeval tv;
575 82753 struct timeval* tv_ptr = nullptr;
576
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 82753 if (effective_timeout_us >= 0)
577 {
578 82717 tv.tv_sec = effective_timeout_us / 1000000;
579 82717 tv.tv_usec = effective_timeout_us % 1000000;
580 82717 tv_ptr = &tv;
581 }
582
583
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 82753 lock.unlock();
584
585
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 82753 int ready = ::select(nfds + 1, &read_fds, &write_fds, &except_fds, tv_ptr);
586 82753 int saved_errno = errno;
587
588 // Process timers outside the lock
589
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 82753 timer_svc_->process_expired();
590
591
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 82753 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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 82753 lock.lock();
596
597 // Drain the interrupt pipe if readable
598
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 82753 if (ready > 0 && FD_ISSET(pipe_fds_[0], &read_fds))
599 {
600 char buf[256];
601
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 9126 while (::read(pipe_fds_[0], buf, sizeof(buf)) > 0) {}
602 }
603
604 // Process I/O completions
605 82753 int completions_queued = 0;
606
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 82753 if (ready > 0)
607 {
608 // Iterate over registered fds (copy keys to avoid iterator invalidation)
609 4565 std::vector<int> fds_to_check;
610
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 4565 fds_to_check.reserve(registered_fds_.size());
611
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 11117 for (auto& [fd, state] : registered_fds_)
612
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 6552 fds_to_check.push_back(fd);
613
614
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 11117 for (int fd : fds_to_check)
615 {
616
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 6552 auto it = registered_fds_.find(fd);
617
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 6552 if (it == registered_fds_.end())
618 ✗ continue;
619
620 6552 auto& state = it->second;
621
622 // Check for errors (especially for connect operations)
623 6552 bool has_error = FD_ISSET(fd, &except_fds);
624
625 // Process read readiness
626
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 6552 if (state.read_op && (FD_ISSET(fd, &read_fds) || has_error))
627 {
628 2224 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 2224 auto prev = op->registered.exchange(
632 select_registration_state::unregistered, std::memory_order_acq_rel);
633
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 2224 if (prev != select_registration_state::unregistered)
634 {
635 2224 state.read_op = nullptr;
636
637
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 2224 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 2224 op->perform_io();
650 }
651
652 2224 completed_ops_.push(op);
653 2224 ++completions_queued;
654 }
655 }
656
657 // Process write readiness
658
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 6552 if (state.write_op && (FD_ISSET(fd, &write_fds) || has_error))
659 {
660 2180 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 2180 auto prev = op->registered.exchange(
664 select_registration_state::unregistered, std::memory_order_acq_rel);
665
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 2180 if (prev != select_registration_state::unregistered)
666 {
667 2180 state.write_op = nullptr;
668
669
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 2180 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 2180 op->perform_io();
682 }
683
684 2180 completed_ops_.push(op);
685 2180 ++completions_queued;
686 }
687 }
688
689 // Clean up empty entries
690
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 6552 if (!state.read_op && !state.write_op)
691
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 4404 registered_fds_.erase(it);
692 }
693 4565 }
694
695
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 82753 if (completions_queued > 0)
696 {
697
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 2228 if (completions_queued == 1)
698 52 wakeup_event_.notify_one();
699 else
700 2176 wakeup_event_.notify_all();
701 }
702 82753 }
703
704 std::size_t
705 167504 select_scheduler::
706 do_one(long timeout_us)
707 {
708
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 167504 std::unique_lock lock(mutex_);
709
710 for (;;)
711 {
712
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 250257 if (stopped_.load(std::memory_order_acquire))
713 18 return 0;
714
715 250239 scheduler_op* op = completed_ops_.pop();
716
717
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 250239 if (op == &task_op_)
718 {
719 82825 bool more_handlers = !completed_ops_.empty();
720
721
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 82825 if (!more_handlers)
722 {
723
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 13788 if (outstanding_work_.load(std::memory_order_acquire) == 0)
724 {
725 72 completed_ops_.push(&task_op_);
726 72 return 0;
727 }
728
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 6822 if (timeout_us == 0)
729 {
730 ✗ completed_ops_.push(&task_op_);
731 ✗ return 0;
732 }
733 }
734
735
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 82753 reactor_interrupted_ = more_handlers || timeout_us == 0;
736 82753 reactor_running_ = true;
737
738
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 82753 if (more_handlers && idle_thread_count_ > 0)
739 ✗ wakeup_event_.notify_one();
740
741
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 82753 run_reactor(lock);
742
743 82753 reactor_running_ = false;
744 82753 completed_ops_.push(&task_op_);
745 82753 continue;
746 82753 }
747
748
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 167414 if (op != nullptr)
749 {
750
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 167414 lock.unlock();
751 167414 work_guard g{this};
752
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 167414 (*op)();
753 167414 return 1;
754 167414 }
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 82753 }
769 167504 }
770
771 } // namespace boost::corosio::detail
772
773 #endif // BOOST_COROSIO_HAS_SELECT
774