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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 2348 [](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 161865 select_scheduler::
233 post(scheduler_op* h) const
234 {
235 161865 outstanding_work_.fetch_add(1, std::memory_order_relaxed);
236
237
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 161865 std::unique_lock lock(mutex_);
238 161865 completed_ops_.push(h);
239
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 161865 wake_one_thread_and_unlock(lock);
240 161865 }
241
242 void
243 2345 select_scheduler::
244 on_work_started() noexcept
245 {
246 2345 outstanding_work_.fetch_add(1, std::memory_order_relaxed);
247 2345 }
248
249 void
250 2339 select_scheduler::
251 on_work_finished() noexcept
252 {
253
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 4678 if (outstanding_work_.fetch_sub(1, std::memory_order_acq_rel) == 1)
254 18 stop();
255 2339 }
256
257 bool
258 166506 select_scheduler::
259 running_in_this_thread() const noexcept
260 {
261
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 166506 for (auto* c = context_stack.get(); c != nullptr; c = c->next)
262
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 166351 if (c->key == this)
263 166351 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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 166376 while (do_one(-1))
315
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 166287 if (n != (std::numeric_limits<std::size_t>::max)())
316 166287 ++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 4375 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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 4375 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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 4375 std::lock_guard lock(mutex_);
403
404
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 4375 auto& state = registered_fds_[fd];
405
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 4375 if (events & event_read)
406 2269 state.read_op = op;
407
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 4375 if (events & event_write)
408 2106 state.write_op = op;
409
410
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 4375 if (fd > max_fd_)
411 184 max_fd_ = fd;
412 4375 }
413
414 // Wake the reactor so a thread blocked in select() rebuilds its fd_sets
415 // with the newly registered fd.
416 4375 interrupt_reactor();
417 4375 }
418
419 void
420 4382 select_scheduler::
421 deregister_fd(int fd, int events) const
422 {
423
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 4382 std::lock_guard lock(mutex_);
424
425
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 4382 auto it = registered_fds_.find(fd);
426
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 4382 if (it == registered_fds_.end())
427 4263 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 4382 }
451
452 void
453 4375 select_scheduler::
454 work_started() const noexcept
455 {
456 4375 outstanding_work_.fetch_add(1, std::memory_order_relaxed);
457 4375 }
458
459 void
460 166414 select_scheduler::
461 work_finished() const noexcept
462 {
463
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 332828 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 166414 }
476
477 void
478 7068 select_scheduler::
479 interrupt_reactor() const
480 {
481 7068 char byte = 1;
482
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 7068 [[maybe_unused]] auto r = ::write(pipe_fds_[1], &byte, 1);
483 7068 }
484
485 void
486 162033 select_scheduler::
487 wake_one_thread_and_unlock(std::unique_lock<std::mutex>& lock) const
488 {
489
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 162033 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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 162033 else if (reactor_running_ && !reactor_interrupted_)
496 {
497 // No idle workers but reactor is running - interrupt it
498 207 reactor_interrupted_ = true;
499 207 lock.unlock();
500 207 interrupt_reactor();
501 }
502 else
503 {
504 // No one to wake
505 161826 lock.unlock();
506 }
507 162033 }
508
509 struct work_guard
510 {
511 select_scheduler const* self;
512 316530 ~work_guard() { self->work_finished(); }
513 };
514
515 long
516 6597 select_scheduler::
517 calculate_timeout(long requested_timeout_us) const
518 {
519
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 6597 if (requested_timeout_us == 0)
520 ✗ return 0;
521
522 6597 auto nearest = timer_svc_->nearest_expiry();
523
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 6597 if (nearest == timer_service::time_point::max())
524 36 return requested_timeout_us;
525
526 6561 auto now = std::chrono::steady_clock::now();
527
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 6561 if (nearest <= now)
528 37 return 0;
529
530
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 6524 auto timer_timeout_us = std::chrono::duration_cast<std::chrono::microseconds>(
531
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 13048 nearest - now).count();
532
533
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 6524 if (requested_timeout_us < 0)
534 6524 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 87649 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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 87649 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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 1490033 FD_ZERO(&read_fds);
551
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 1490033 FD_ZERO(&write_fds);
552
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 1490033 FD_ZERO(&except_fds);
553
554 // Always include the interrupt pipe
555 87649 FD_SET(pipe_fds_[0], &read_fds);
556 87649 int nfds = pipe_fds_[0];
557
558 // Add registered fds
559
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 96078 for (auto& [fd, state] : registered_fds_)
560 {
561
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 8429 if (state.read_op)
562 6323 FD_SET(fd, &read_fds);
563
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 8429 if (state.write_op)
564 {
565 2106 FD_SET(fd, &write_fds);
566 // Also monitor for errors on connect operations
567 2106 FD_SET(fd, &except_fds);
568 }
569
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 8429 if (fd > nfds)
570 6327 nfds = fd;
571 }
572
573 // Convert timeout to timeval
574 struct timeval tv;
575 87649 struct timeval* tv_ptr = nullptr;
576
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 87649 if (effective_timeout_us >= 0)
577 {
578 87613 tv.tv_sec = effective_timeout_us / 1000000;
579 87613 tv.tv_usec = effective_timeout_us % 1000000;
580 87613 tv_ptr = &tv;
581 }
582
583
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 87649 lock.unlock();
584
585
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 87649 int ready = ::select(nfds + 1, &read_fds, &write_fds, &except_fds, tv_ptr);
586 87649 int saved_errno = errno;
587
588 // Process timers outside the lock
589
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 87649 timer_svc_->process_expired();
590
591
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 87649 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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 87649 lock.lock();
596
597 // Drain the interrupt pipe if readable
598
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 87649 if (ready > 0 && FD_ISSET(pipe_fds_[0], &read_fds))
599 {
600 char buf[256];
601
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 8826 while (::read(pipe_fds_[0], buf, sizeof(buf)) > 0) {}
602 }
603
604 // Process I/O completions
605 87649 int completions_queued = 0;
606
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 87649 if (ready > 0)
607 {
608 // Iterate over registered fds (copy keys to avoid iterator invalidation)
609 4416 std::vector<int> fds_to_check;
610
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 4416 fds_to_check.reserve(registered_fds_.size());
611
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 10747 for (auto& [fd, state] : registered_fds_)
612
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 6331 fds_to_check.push_back(fd);
613
614
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 10747 for (int fd : fds_to_check)
615 {
616
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 6331 auto it = registered_fds_.find(fd);
617
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 6331 if (it == registered_fds_.end())
618 ✗ continue;
619
620 6331 auto& state = it->second;
621
622 // Check for errors (especially for connect operations)
623 6331 bool has_error = FD_ISSET(fd, &except_fds);
624
625 // Process read readiness
626
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 6331 if (state.read_op && (FD_ISSET(fd, &read_fds) || has_error))
627 {
628 2150 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 2150 auto prev = op->registered.exchange(
632 select_registration_state::unregistered, std::memory_order_acq_rel);
633
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 2150 if (prev != select_registration_state::unregistered)
634 {
635 2150 state.read_op = nullptr;
636
637
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 2150 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 2150 op->perform_io();
650 }
651
652 2150 completed_ops_.push(op);
653 2150 ++completions_queued;
654 }
655 }
656
657 // Process write readiness
658
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 6331 if (state.write_op && (FD_ISSET(fd, &write_fds) || has_error))
659 {
660 2106 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 2106 auto prev = op->registered.exchange(
664 select_registration_state::unregistered, std::memory_order_acq_rel);
665
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 2106 if (prev != select_registration_state::unregistered)
666 {
667 2106 state.write_op = nullptr;
668
669
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 2106 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 2106 op->perform_io();
682 }
683
684 2106 completed_ops_.push(op);
685 2106 ++completions_queued;
686 }
687 }
688
689 // Clean up empty entries
690
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 6331 if (!state.read_op && !state.write_op)
691
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 4256 registered_fds_.erase(it);
692 }
693 4416 }
694
695
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 87649 if (completions_queued > 0)
696 {
697
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 2154 if (completions_queued == 1)
698 52 wakeup_event_.notify_one();
699 else
700 2102 wakeup_event_.notify_all();
701 }
702 87649 }
703
704 std::size_t
705 166379 select_scheduler::
706 do_one(long timeout_us)
707 {
708
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 166379 std::unique_lock lock(mutex_);
709
710 for (;;)
711 {
712
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 254028 if (stopped_.load(std::memory_order_acquire))
713 18 return 0;
714
715 254010 scheduler_op* op = completed_ops_.pop();
716
717
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 254010 if (op == &task_op_)
718 {
719 87721 bool more_handlers = !completed_ops_.empty();
720
721
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 87721 if (!more_handlers)
722 {
723
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 13338 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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 6597 if (timeout_us == 0)
729 {
730 ✗ completed_ops_.push(&task_op_);
731 ✗ return 0;
732 }
733 }
734
735
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 87649 reactor_interrupted_ = more_handlers || timeout_us == 0;
736 87649 reactor_running_ = true;
737
738
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 87649 if (more_handlers && idle_thread_count_ > 0)
739 ✗ wakeup_event_.notify_one();
740
741
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 87649 run_reactor(lock);
742
743 87649 reactor_running_ = false;
744 87649 completed_ops_.push(&task_op_);
745 87649 continue;
746 87649 }
747
748
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 166289 if (op != nullptr)
749 {
750
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 166289 lock.unlock();
751 166289 work_guard g{this};
752
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 166289 (*op)();
753 166289 return 1;
754 166289 }
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 87649 }
769 166379 }
770
771 } // namespace boost::corosio::detail
772
773 #endif // BOOST_COROSIO_HAS_SELECT
774