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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 1902 [](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 2108 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 2108 post_handler(capy::coro h)
204 2108 : h_(h)
205 {
206 2108 }
207
208 4216 ~post_handler() = default;
209
210 2108 void operator()() override
211 {
212 2108 auto h = h_;
213
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 2108 delete this;
214
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 2108 h.resume();
215 2108 }
216
217 ✗ void destroy() override
218 {
219 ✗ delete this;
220 ✗ }
221 };
222
223
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 2108 auto ph = std::make_unique<post_handler>(h);
224 2108 outstanding_work_.fetch_add(1, std::memory_order_relaxed);
225
226
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 2108 std::unique_lock lock(mutex_);
227 2108 completed_ops_.push(ph.release());
228
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 2108 wake_one_thread_and_unlock(lock);
229 2108 }
230
231 void
232 134922 select_scheduler::
233 post(scheduler_op* h) const
234 {
235 134922 outstanding_work_.fetch_add(1, std::memory_order_relaxed);
236
237
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 134922 std::unique_lock lock(mutex_);
238 134922 completed_ops_.push(h);
239
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 134922 wake_one_thread_and_unlock(lock);
240 134922 }
241
242 void
243 2303 select_scheduler::
244 on_work_started() noexcept
245 {
246 2303 outstanding_work_.fetch_add(1, std::memory_order_relaxed);
247 2303 }
248
249 void
250 2297 select_scheduler::
251 on_work_finished() noexcept
252 {
253
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 4594 if (outstanding_work_.fetch_sub(1, std::memory_order_acq_rel) == 1)
254 ✗ stop();
255 2297 }
256
257 bool
258 136899 select_scheduler::
259 running_in_this_thread() const noexcept
260 {
261
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 136899 for (auto* c = context_stack.get(); c != nullptr; c = c->next)
262
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 136744 if (c->key == this)
263 136744 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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 140622 while (do_one(-1))
315
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 140533 if (n != (std::numeric_limits<std::size_t>::max)())
316 140533 ++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 3622 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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 3622 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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 3622 std::lock_guard lock(mutex_);
403
404
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 3622 auto& state = registered_fds_[fd];
405
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 3622 if (events & event_read)
406 1904 state.read_op = op;
407
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 3622 if (events & event_write)
408 1718 state.write_op = op;
409
410
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 3622 if (fd > max_fd_)
411 177 max_fd_ = fd;
412 3622 }
413
414 // Wake the reactor so a thread blocked in select() rebuilds its fd_sets
415 // with the newly registered fd.
416 3622 interrupt_reactor();
417 3622 }
418
419 void
420 3601 select_scheduler::
421 deregister_fd(int fd, int events) const
422 {
423
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 3601 std::lock_guard lock(mutex_);
424
425
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 3601 auto it = registered_fds_.find(fd);
426
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 3601 if (it == registered_fds_.end())
427 3488 return;
428
429
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 113 if (events & event_read)
430 113 it->second.read_op = nullptr;
431
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 113 if (events & event_write)
432 ✗ it->second.write_op = nullptr;
433
434 // Remove entry if both are null
435
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 113 if (!it->second.read_op && !it->second.write_op)
436 {
437
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 113 registered_fds_.erase(it);
438
439 // Recalculate max_fd_ if needed
440
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 113 if (fd == max_fd_)
441 {
442 112 max_fd_ = pipe_fds_[0]; // At minimum, the pipe read end
443
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 112 for (auto& [registered_fd, state] : registered_fds_)
444 {
445 ✗ if (registered_fd > max_fd_)
446 ✗ max_fd_ = registered_fd;
447 }
448 }
449 }
450 3601 }
451
452 void
453 3622 select_scheduler::
454 work_started() const noexcept
455 {
456 3622 outstanding_work_.fetch_add(1, std::memory_order_relaxed);
457 3622 }
458
459 void
460 140658 select_scheduler::
461 work_finished() const noexcept
462 {
463
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 281316 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 140658 }
476
477 void
478 7546 select_scheduler::
479 interrupt_reactor() const
480 {
481 7546 char byte = 1;
482
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 7546 [[maybe_unused]] auto r = ::write(pipe_fds_[1], &byte, 1);
483 7546 }
484
485 void
486 137030 select_scheduler::
487 wake_one_thread_and_unlock(std::unique_lock<std::mutex>& lock) const
488 {
489
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 137030 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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 137030 else if (reactor_running_ && !reactor_interrupted_)
496 {
497 // No idle workers but reactor is running - interrupt it
498 1901 reactor_interrupted_ = true;
499 1901 lock.unlock();
500 1901 interrupt_reactor();
501 }
502 else
503 {
504 // No one to wake
505 135129 lock.unlock();
506 }
507 137030 }
508
509 struct work_guard
510 {
511 select_scheduler const* self;
512 140539 ~work_guard() { self->work_finished(); }
513 };
514
515 long
516 5393 select_scheduler::
517 calculate_timeout(long requested_timeout_us) const
518 {
519
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 5393 if (requested_timeout_us == 0)
520 ✗ return 0;
521
522 5393 auto nearest = timer_svc_->nearest_expiry();
523
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 5393 if (nearest == timer_service::time_point::max())
524 35 return requested_timeout_us;
525
526 5358 auto now = std::chrono::steady_clock::now();
527
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 5358 if (nearest <= now)
528 14 return 0;
529
530
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 5344 auto timer_timeout_us = std::chrono::duration_cast<std::chrono::microseconds>(
531
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 10688 nearest - now).count();
532
533
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 5344 if (requested_timeout_us < 0)
534 5344 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 79179 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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 79179 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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 1346043 FD_ZERO(&read_fds);
551
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 1346043 FD_ZERO(&write_fds);
552
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 1346043 FD_ZERO(&except_fds);
553
554 // Always include the interrupt pipe
555 79179 FD_SET(pipe_fds_[0], &read_fds);
556 79179 int nfds = pipe_fds_[0];
557
558 // Add registered fds
559
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 87763 for (auto& [fd, state] : registered_fds_)
560 {
561
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 8584 if (state.read_op)
562 6866 FD_SET(fd, &read_fds);
563
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 8584 if (state.write_op)
564 {
565 1718 FD_SET(fd, &write_fds);
566 // Also monitor for errors on connect operations
567 1718 FD_SET(fd, &except_fds);
568 }
569
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 8584 if (fd > nfds)
570 6869 nfds = fd;
571 }
572
573 // Convert timeout to timeval
574 struct timeval tv;
575 79179 struct timeval* tv_ptr = nullptr;
576
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 79179 if (effective_timeout_us >= 0)
577 {
578 79144 tv.tv_sec = effective_timeout_us / 1000000;
579 79144 tv.tv_usec = effective_timeout_us % 1000000;
580 79144 tv_ptr = &tv;
581 }
582
583
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 79179 lock.unlock();
584
585
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 79179 int ready = ::select(nfds + 1, &read_fds, &write_fds, &except_fds, tv_ptr);
586 79179 int saved_errno = errno;
587
588 // Process timers outside the lock
589
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 79179 timer_svc_->process_expired();
590
591
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 79179 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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 79179 lock.lock();
596
597 // Drain the interrupt pipe if readable
598
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 79179 if (ready > 0 && FD_ISSET(pipe_fds_[0], &read_fds))
599 {
600 char buf[256];
601
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 11320 while (::read(pipe_fds_[0], buf, sizeof(buf)) > 0) {}
602 }
603
604 // Process I/O completions
605 79179 int completions_queued = 0;
606
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 79179 if (ready > 0)
607 {
608 // Iterate over registered fds (copy keys to avoid iterator invalidation)
609 5660 std::vector<int> fds_to_check;
610
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 5660 fds_to_check.reserve(registered_fds_.size());
611
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 12559 for (auto& [fd, state] : registered_fds_)
612
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 6899 fds_to_check.push_back(fd);
613
614
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 12559 for (int fd : fds_to_check)
615 {
616
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 6899 auto it = registered_fds_.find(fd);
617
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 6899 if (it == registered_fds_.end())
618 ✗ continue;
619
620 6899 auto& state = it->second;
621
622 // Check for errors (especially for connect operations)
623 6899 bool has_error = FD_ISSET(fd, &except_fds);
624
625 // Process read readiness
626
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 6899 if (state.read_op && (FD_ISSET(fd, &read_fds) || has_error))
627 {
628 1791 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 1791 auto prev = op->registered.exchange(
632 select_registration_state::unregistered, std::memory_order_acq_rel);
633
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 1791 if (prev != select_registration_state::unregistered)
634 {
635 1791 state.read_op = nullptr;
636
637
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 1791 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 1791 op->perform_io();
650 }
651
652 1791 completed_ops_.push(op);
653 1791 ++completions_queued;
654 }
655 }
656
657 // Process write readiness
658
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 6899 if (state.write_op && (FD_ISSET(fd, &write_fds) || has_error))
659 {
660 1718 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 1718 auto prev = op->registered.exchange(
664 select_registration_state::unregistered, std::memory_order_acq_rel);
665
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 1718 if (prev != select_registration_state::unregistered)
666 {
667 1718 state.write_op = nullptr;
668
669
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 1718 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 1718 op->perform_io();
682 }
683
684 1718 completed_ops_.push(op);
685 1718 ++completions_queued;
686 }
687 }
688
689 // Clean up empty entries
690
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 6899 if (!state.read_op && !state.write_op)
691
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 3509 registered_fds_.erase(it);
692 }
693 5660 }
694
695
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 79179 if (completions_queued > 0)
696 {
697
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 1794 if (completions_queued == 1)
698 79 wakeup_event_.notify_one();
699 else
700 1715 wakeup_event_.notify_all();
701 }
702 79179 }
703
704 std::size_t
705 140628 select_scheduler::
706 do_one(long timeout_us)
707 {
708
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 140628 std::unique_lock lock(mutex_);
709
710 for (;;)
711 {
712
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 219807 if (stopped_.load(std::memory_order_acquire))
713 ✗ return 0;
714
715 219807 scheduler_op* op = completed_ops_.pop();
716
717
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 219807 if (op == &task_op_)
718 {
719 79268 bool more_handlers = !completed_ops_.empty();
720
721
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 79268 if (!more_handlers)
722 {
723
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 10964 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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 5393 if (timeout_us == 0)
729 {
730 ✗ completed_ops_.push(&task_op_);
731 ✗ return 0;
732 }
733 }
734
735
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 79179 reactor_interrupted_ = more_handlers || timeout_us == 0;
736 79179 reactor_running_ = true;
737
738
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 79179 if (more_handlers && idle_thread_count_ > 0)
739 ✗ wakeup_event_.notify_one();
740
741
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 79179 run_reactor(lock);
742
743 79179 reactor_running_ = false;
744 79179 completed_ops_.push(&task_op_);
745 79179 continue;
746 79179 }
747
748
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 140539 if (op != nullptr)
749 {
750
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 140539 lock.unlock();
751 140539 work_guard g{this};
752
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 140539 (*op)();
753 140539 return 1;
754 140539 }
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 79179 }
769 140628 }
770
771 } // namespace boost::corosio::detail
772
773 #endif // BOOST_COROSIO_HAS_SELECT
774