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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 3467 [](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 3628 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 3628 post_handler(capy::coro h)
204 3628 : h_(h)
205 {
206 3628 }
207
208 7256 ~post_handler() = default;
209
210 3628 void operator()() override
211 {
212 3628 auto h = h_;
213
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 3628 delete this;
214
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 3628 h.resume();
215 3628 }
216
217 ✗ void destroy() override
218 {
219 ✗ delete this;
220 ✗ }
221 };
222
223
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 3628 auto ph = std::make_unique<post_handler>(h);
224 3628 outstanding_work_.fetch_add(1, std::memory_order_relaxed);
225
226
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 3628 std::unique_lock lock(mutex_);
227 3628 completed_ops_.push(ph.release());
228
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 3628 wake_one_thread_and_unlock(lock);
229 3628 }
230
231 void
232 160922 select_scheduler::
233 post(scheduler_op* h) const
234 {
235 160922 outstanding_work_.fetch_add(1, std::memory_order_relaxed);
236
237
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 160922 std::unique_lock lock(mutex_);
238 160922 completed_ops_.push(h);
239
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 160922 wake_one_thread_and_unlock(lock);
240 160922 }
241
242 void
243 3864 select_scheduler::
244 on_work_started() noexcept
245 {
246 3864 outstanding_work_.fetch_add(1, std::memory_order_relaxed);
247 3864 }
248
249 void
250 3858 select_scheduler::
251 on_work_finished() noexcept
252 {
253
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 7716 if (outstanding_work_.fetch_sub(1, std::memory_order_acq_rel) == 1)
254 ✗ stop();
255 3858 }
256
257 bool
258 164378 select_scheduler::
259 running_in_this_thread() const noexcept
260 {
261
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 164378 for (auto* c = context_stack.get(); c != nullptr; c = c->next)
262
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 164223 if (c->key == this)
263 164223 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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 171158 while (do_one(-1))
315
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 171069 if (n != (std::numeric_limits<std::size_t>::max)())
316 171069 ++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 6645 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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 6645 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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 6645 std::lock_guard lock(mutex_);
403
404
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 6645 auto& state = registered_fds_[fd];
405
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 6645 if (events & event_read)
406 3421 state.read_op = op;
407
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 6645 if (events & event_write)
408 3224 state.write_op = op;
409
410
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 6645 if (fd > max_fd_)
411 184 max_fd_ = fd;
412 6645 }
413
414 // Wake the reactor so a thread blocked in select() rebuilds its fd_sets
415 // with the newly registered fd.
416 6645 interrupt_reactor();
417 6645 }
418
419 void
420 6620 select_scheduler::
421 deregister_fd(int fd, int events) const
422 {
423
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 6620 std::lock_guard lock(mutex_);
424
425
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 6620 auto it = registered_fds_.find(fd);
426
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 6620 if (it == registered_fds_.end())
427 6500 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 6620 }
451
452 void
453 6645 select_scheduler::
454 work_started() const noexcept
455 {
456 6645 outstanding_work_.fetch_add(1, std::memory_order_relaxed);
457 6645 }
458
459 void
460 171201 select_scheduler::
461 work_finished() const noexcept
462 {
463
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 342402 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 171201 }
476
477 void
478 13683 select_scheduler::
479 interrupt_reactor() const
480 {
481 13683 char byte = 1;
482
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 13683 [[maybe_unused]] auto r = ::write(pipe_fds_[1], &byte, 1);
483 13683 }
484
485 void
486 164550 select_scheduler::
487 wake_one_thread_and_unlock(std::unique_lock<std::mutex>& lock) const
488 {
489
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 164550 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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 164550 else if (reactor_running_ && !reactor_interrupted_)
496 {
497 // No idle workers but reactor is running - interrupt it
498 3450 reactor_interrupted_ = true;
499 3450 lock.unlock();
500 3450 interrupt_reactor();
501 }
502 else
503 {
504 // No one to wake
505 161100 lock.unlock();
506 }
507 164550 }
508
509 struct work_guard
510 {
511 select_scheduler const* self;
512 171075 ~work_guard() { self->work_finished(); }
513 };
514
515 long
516 9943 select_scheduler::
517 calculate_timeout(long requested_timeout_us) const
518 {
519
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 9943 if (requested_timeout_us == 0)
520 ✗ return 0;
521
522 9943 auto nearest = timer_svc_->nearest_expiry();
523
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 9943 if (nearest == timer_service::time_point::max())
524 35 return requested_timeout_us;
525
526 9908 auto now = std::chrono::steady_clock::now();
527
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 9908 if (nearest <= now)
528 47 return 0;
529
530
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 9861 auto timer_timeout_us = std::chrono::duration_cast<std::chrono::microseconds>(
531
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 19722 nearest - now).count();
532
533
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 9861 if (requested_timeout_us < 0)
534 9861 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 95496 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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 95496 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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 1623432 FD_ZERO(&read_fds);
551
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 1623432 FD_ZERO(&write_fds);
552
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 1623432 FD_ZERO(&except_fds);
553
554 // Always include the interrupt pipe
555 95496 FD_SET(pipe_fds_[0], &read_fds);
556 95496 int nfds = pipe_fds_[0];
557
558 // Add registered fds
559
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 111650 for (auto& [fd, state] : registered_fds_)
560 {
561
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 16154 if (state.read_op)
562 12930 FD_SET(fd, &read_fds);
563
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 16154 if (state.write_op)
564 {
565 3224 FD_SET(fd, &write_fds);
566 // Also monitor for errors on connect operations
567 3224 FD_SET(fd, &except_fds);
568 }
569
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 16154 if (fd > nfds)
570 12933 nfds = fd;
571 }
572
573 // Convert timeout to timeval
574 struct timeval tv;
575 95496 struct timeval* tv_ptr = nullptr;
576
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 95496 if (effective_timeout_us >= 0)
577 {
578 95461 tv.tv_sec = effective_timeout_us / 1000000;
579 95461 tv.tv_usec = effective_timeout_us % 1000000;
580 95461 tv_ptr = &tv;
581 }
582
583
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 95496 lock.unlock();
584
585
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 95496 int ready = ::select(nfds + 1, &read_fds, &write_fds, &except_fds, tv_ptr);
586 95496 int saved_errno = errno;
587
588 // Process timers outside the lock
589
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 95496 timer_svc_->process_expired();
590
591
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 95496 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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 95496 lock.lock();
596
597 // Drain the interrupt pipe if readable
598
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 95496 if (ready > 0 && FD_ISSET(pipe_fds_[0], &read_fds))
599 {
600 char buf[256];
601
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 20424 while (::read(pipe_fds_[0], buf, sizeof(buf)) > 0) {}
602 }
603
604 // Process I/O completions
605 95496 int completions_queued = 0;
606
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 95496 if (ready > 0)
607 {
608 // Iterate over registered fds (copy keys to avoid iterator invalidation)
609 10212 std::vector<int> fds_to_check;
610
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 10212 fds_to_check.reserve(registered_fds_.size());
611
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 23143 for (auto& [fd, state] : registered_fds_)
612
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 12931 fds_to_check.push_back(fd);
613
614
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 23143 for (int fd : fds_to_check)
615 {
616
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 12931 auto it = registered_fds_.find(fd);
617
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 12931 if (it == registered_fds_.end())
618 ✗ continue;
619
620 12931 auto& state = it->second;
621
622 // Check for errors (especially for connect operations)
623 12931 bool has_error = FD_ISSET(fd, &except_fds);
624
625 // Process read readiness
626
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 12931 if (state.read_op && (FD_ISSET(fd, &read_fds) || has_error))
627 {
628 3301 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 3301 auto prev = op->registered.exchange(
632 select_registration_state::unregistered, std::memory_order_acq_rel);
633
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 3301 if (prev != select_registration_state::unregistered)
634 {
635 3301 state.read_op = nullptr;
636
637
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 3301 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 3301 op->perform_io();
650 }
651
652 3301 completed_ops_.push(op);
653 3301 ++completions_queued;
654 }
655 }
656
657 // Process write readiness
658
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 12931 if (state.write_op && (FD_ISSET(fd, &write_fds) || has_error))
659 {
660 3224 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 3224 auto prev = op->registered.exchange(
664 select_registration_state::unregistered, std::memory_order_acq_rel);
665
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 3224 if (prev != select_registration_state::unregistered)
666 {
667 3224 state.write_op = nullptr;
668
669
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 3224 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 3224 op->perform_io();
682 }
683
684 3224 completed_ops_.push(op);
685 3224 ++completions_queued;
686 }
687 }
688
689 // Clean up empty entries
690
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 12931 if (!state.read_op && !state.write_op)
691
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 6525 registered_fds_.erase(it);
692 }
693 10212 }
694
695
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 95496 if (completions_queued > 0)
696 {
697
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 3304 if (completions_queued == 1)
698 83 wakeup_event_.notify_one();
699 else
700 3221 wakeup_event_.notify_all();
701 }
702 95496 }
703
704 std::size_t
705 171164 select_scheduler::
706 do_one(long timeout_us)
707 {
708
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 171164 std::unique_lock lock(mutex_);
709
710 for (;;)
711 {
712
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 266660 if (stopped_.load(std::memory_order_acquire))
713 ✗ return 0;
714
715 266660 scheduler_op* op = completed_ops_.pop();
716
717
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 266660 if (op == &task_op_)
718 {
719 95585 bool more_handlers = !completed_ops_.empty();
720
721
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 95585 if (!more_handlers)
722 {
723
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 20064 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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 9943 if (timeout_us == 0)
729 {
730 ✗ completed_ops_.push(&task_op_);
731 ✗ return 0;
732 }
733 }
734
735
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 95496 reactor_interrupted_ = more_handlers || timeout_us == 0;
736 95496 reactor_running_ = true;
737
738
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 95496 if (more_handlers && idle_thread_count_ > 0)
739 ✗ wakeup_event_.notify_one();
740
741
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 95496 run_reactor(lock);
742
743 95496 reactor_running_ = false;
744 95496 completed_ops_.push(&task_op_);
745 95496 continue;
746 95496 }
747
748
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 171075 if (op != nullptr)
749 {
750
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 171075 lock.unlock();
751 171075 work_guard g{this};
752
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 171075 (*op)();
753 171075 return 1;
754 171075 }
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 95496 }
769 171164 }
770
771 } // namespace boost::corosio::detail
772
773 #endif // BOOST_COROSIO_HAS_SELECT
774