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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/timer_service.hpp"
17 #include "src/detail/make_err.hpp"
18 #include "src/detail/posix/resolver_service.hpp"
19 #include "src/detail/posix/signals.hpp"
20
21 #include <boost/corosio/detail/except.hpp>
22 #include <boost/corosio/detail/thread_local_ptr.hpp>
23
24 #include <chrono>
25 #include <limits>
26
27 #include <errno.h>
28 #include <fcntl.h>
29 #include <sys/select.h>
30 #include <sys/socket.h>
31 #include <unistd.h>
32
33 /*
34 select Scheduler - Single Reactor Model
35 =======================================
36
37 This scheduler mirrors the epoll_scheduler design but uses select() instead
38 of epoll for I/O multiplexing. The thread coordination strategy is identical:
39 one thread becomes the "reactor" while others wait on a condition variable.
40
41 Thread Model
42 ------------
43 - ONE thread runs select() at a time (the reactor thread)
44 - OTHER threads wait on wakeup_event_ (condition variable) for handlers
45 - When work is posted, exactly one waiting thread wakes via notify_one()
46
47 Key Differences from epoll
48 --------------------------
49 - Uses self-pipe instead of eventfd for interruption (more portable)
50 - fd_set rebuilding each iteration (O(n) vs O(1) for epoll)
51 - FD_SETSIZE limit (~1024 fds on most systems)
52 - Level-triggered only (no edge-triggered mode)
53
54 Self-Pipe Pattern
55 -----------------
56 To interrupt a blocking select() call (e.g., when work is posted or a timer
57 expires), we write a byte to pipe_fds_[1]. The read end pipe_fds_[0] is
58 always in the read_fds set, so select() returns immediately. We drain the
59 pipe to clear the readable state.
60
61 fd-to-op Mapping
62 ----------------
63 We use an unordered_map<int, fd_state> to track which operations are
64 registered for each fd. This allows O(1) lookup when select() returns
65 ready fds. Each fd can have at most one read op and one write op registered.
66 */
67
68 namespace boost::corosio::detail {
69
70 namespace {
71
72 struct scheduler_context
73 {
74 select_scheduler const* key;
75 scheduler_context* next;
76 };
77
78 corosio::detail::thread_local_ptr<scheduler_context> context_stack;
79
80 struct thread_context_guard
81 {
82 scheduler_context frame_;
83
84 120 explicit thread_context_guard(
85 select_scheduler const* ctx) noexcept
86 120 : frame_{ctx, context_stack.get()}
87 {
88 120 context_stack.set(&frame_);
89 120 }
90
91 120 ~thread_context_guard() noexcept
92 {
93 120 context_stack.set(frame_.next);
94 120 }
95 };
96
97 } // namespace
98
99 133 select_scheduler::
100 select_scheduler(
101 capy::execution_context& ctx,
102 133 int)
103 133 : pipe_fds_{-1, -1}
104 133 , outstanding_work_(0)
105 133 , stopped_(false)
106 133 , shutdown_(false)
107 133 , max_fd_(-1)
108 133 , reactor_running_(false)
109 133 , reactor_interrupted_(false)
110 266 , idle_thread_count_(0)
111 {
112 // Create self-pipe for interrupting select()
113
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 133 if (::pipe(pipe_fds_) < 0)
114 ✗ detail::throw_system_error(make_err(errno), "pipe");
115
116 // Set both ends to non-blocking and close-on-exec
117
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 399 for (int i = 0; i < 2; ++i)
118 {
119
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 266 int flags = ::fcntl(pipe_fds_[i], F_GETFL, 0);
120
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 266 if (flags == -1)
121 {
122 ✗ int errn = errno;
123 ✗ ::close(pipe_fds_[0]);
124 ✗ ::close(pipe_fds_[1]);
125 ✗ detail::throw_system_error(make_err(errn), "fcntl F_GETFL");
126 }
127
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 266 if (::fcntl(pipe_fds_[i], F_SETFL, flags | O_NONBLOCK) == -1)
128 {
129 ✗ int errn = errno;
130 ✗ ::close(pipe_fds_[0]);
131 ✗ ::close(pipe_fds_[1]);
132 ✗ detail::throw_system_error(make_err(errn), "fcntl F_SETFL");
133 }
134
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 266 if (::fcntl(pipe_fds_[i], F_SETFD, FD_CLOEXEC) == -1)
135 {
136 ✗ int errn = errno;
137 ✗ ::close(pipe_fds_[0]);
138 ✗ ::close(pipe_fds_[1]);
139 ✗ detail::throw_system_error(make_err(errn), "fcntl F_SETFD");
140 }
141 }
142
143
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 133 timer_svc_ = &get_timer_service(ctx, *this);
144
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 133 timer_svc_->set_on_earliest_changed(
145 timer_service::callback(
146 this,
147 3757 [](void* p) { static_cast<select_scheduler*>(p)->interrupt_reactor(); }));
148
149 // Initialize resolver service
150
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 133 get_resolver_service(ctx, *this);
151
152 // Initialize signal service
153
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 133 get_signal_service(ctx, *this);
154
155 // Push task sentinel to interleave reactor runs with handler execution
156 133 completed_ops_.push(&task_op_);
157 133 }
158
159 266 select_scheduler::
160 133 ~select_scheduler()
161 {
162
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 133 if (pipe_fds_[0] >= 0)
163 133 ::close(pipe_fds_[0]);
164
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 133 if (pipe_fds_[1] >= 0)
165 133 ::close(pipe_fds_[1]);
166 266 }
167
168 void
169 133 select_scheduler::
170 shutdown()
171 {
172 {
173
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 133 std::unique_lock lock(mutex_);
174 133 shutdown_ = true;
175
176
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 266 while (auto* h = completed_ops_.pop())
177 {
178
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 133 if (h == &task_op_)
179 133 continue;
180 ✗ lock.unlock();
181 ✗ h->destroy();
182 ✗ lock.lock();
183 133 }
184 133 }
185
186 133 outstanding_work_.store(0, std::memory_order_release);
187
188
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 133 if (pipe_fds_[1] >= 0)
189 133 interrupt_reactor();
190
191 133 wakeup_event_.notify_all();
192 133 }
193
194 void
195 4094 select_scheduler::
196 post(std::coroutine_handle<> h) const
197 {
198 struct post_handler final
199 : scheduler_op
200 {
201 std::coroutine_handle<> h_;
202
203 explicit
204 4094 post_handler(std::coroutine_handle<> h)
205 4094 : h_(h)
206 {
207 4094 }
208
209 8188 ~post_handler() = default;
210
211 4094 void operator()() override
212 {
213 4094 auto h = h_;
214
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 4094 delete this;
215
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 4094 h.resume();
216 4094 }
217
218 ✗ void destroy() override
219 {
220 ✗ delete this;
221 ✗ }
222 };
223
224
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 4094 auto ph = std::make_unique<post_handler>(h);
225 4094 outstanding_work_.fetch_add(1, std::memory_order_relaxed);
226
227
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 4094 std::unique_lock lock(mutex_);
228 4094 completed_ops_.push(ph.release());
229
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 4094 wake_one_thread_and_unlock(lock);
230 4094 }
231
232 void
233 159710 select_scheduler::
234 post(scheduler_op* h) const
235 {
236 159710 outstanding_work_.fetch_add(1, std::memory_order_relaxed);
237
238
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 159710 std::unique_lock lock(mutex_);
239 159710 completed_ops_.push(h);
240
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 159710 wake_one_thread_and_unlock(lock);
241 159710 }
242
243 void
244 4331 select_scheduler::
245 on_work_started() noexcept
246 {
247 4331 outstanding_work_.fetch_add(1, std::memory_order_relaxed);
248 4331 }
249
250 void
251 4325 select_scheduler::
252 on_work_finished() noexcept
253 {
254
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 8650 if (outstanding_work_.fetch_sub(1, std::memory_order_acq_rel) == 1)
255 ✗ stop();
256 4325 }
257
258 bool
259 552 select_scheduler::
260 running_in_this_thread() const noexcept
261 {
262
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 552 for (auto* c = context_stack.get(); c != nullptr; c = c->next)
263
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 368 if (c->key == this)
264 368 return true;
265 184 return false;
266 }
267
268 void
269 3 select_scheduler::
270 stop()
271 {
272 3 bool expected = false;
273
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 3 if (stopped_.compare_exchange_strong(expected, true,
274 std::memory_order_release, std::memory_order_relaxed))
275 {
276 // Wake all threads so they notice stopped_ and exit
277 {
278
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 3 std::lock_guard lock(mutex_);
279 3 wakeup_event_.notify_all();
280 3 }
281
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 3 interrupt_reactor();
282 }
283 3 }
284
285 bool
286 3 select_scheduler::
287 stopped() const noexcept
288 {
289 3 return stopped_.load(std::memory_order_acquire);
290 }
291
292 void
293 34 select_scheduler::
294 restart()
295 {
296 34 stopped_.store(false, std::memory_order_release);
297 34 }
298
299 std::size_t
300 96 select_scheduler::
301 run()
302 {
303
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 96 if (stopped_.load(std::memory_order_acquire))
304 ✗ return 0;
305
306
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 192 if (outstanding_work_.load(std::memory_order_acquire) == 0)
307 {
308 ✗ stop();
309 ✗ return 0;
310 }
311
312 96 thread_context_guard ctx(this);
313
314 96 std::size_t n = 0;
315
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 171078 while (do_one(-1))
316
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 170982 if (n != (std::numeric_limits<std::size_t>::max)())
317 170982 ++n;
318 96 return n;
319 96 }
320
321 std::size_t
322 ✗ select_scheduler::
323 run_one()
324 {
325 ✗ if (stopped_.load(std::memory_order_acquire))
326 ✗ return 0;
327
328 ✗ if (outstanding_work_.load(std::memory_order_acquire) == 0)
329 {
330 ✗ stop();
331 ✗ return 0;
332 }
333
334 ✗ thread_context_guard ctx(this);
335 ✗ return do_one(-1);
336 ✗ }
337
338 std::size_t
339 27 select_scheduler::
340 wait_one(long usec)
341 {
342
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 27 if (stopped_.load(std::memory_order_acquire))
343 ✗ return 0;
344
345
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 54 if (outstanding_work_.load(std::memory_order_acquire) == 0)
346 {
347
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 3 stop();
348 3 return 0;
349 }
350
351 24 thread_context_guard ctx(this);
352
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 24 return do_one(usec);
353 24 }
354
355 std::size_t
356 ✗ select_scheduler::
357 poll()
358 {
359 ✗ if (stopped_.load(std::memory_order_acquire))
360 ✗ return 0;
361
362 ✗ if (outstanding_work_.load(std::memory_order_acquire) == 0)
363 {
364 ✗ stop();
365 ✗ return 0;
366 }
367
368 ✗ thread_context_guard ctx(this);
369
370 ✗ std::size_t n = 0;
371 ✗ while (do_one(0))
372 ✗ if (n != (std::numeric_limits<std::size_t>::max)())
373 ✗ ++n;
374 ✗ return n;
375 ✗ }
376
377 std::size_t
378 ✗ select_scheduler::
379 poll_one()
380 {
381 ✗ if (stopped_.load(std::memory_order_acquire))
382 ✗ return 0;
383
384 ✗ if (outstanding_work_.load(std::memory_order_acquire) == 0)
385 {
386 ✗ stop();
387 ✗ return 0;
388 }
389
390 ✗ thread_context_guard ctx(this);
391 ✗ return do_one(0);
392 ✗ }
393
394 void
395 7363 select_scheduler::
396 register_fd(int fd, select_op* op, int events) const
397 {
398 // Validate fd is within select() limits
399
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 7363 if (fd < 0 || fd >= FD_SETSIZE)
400 ✗ detail::throw_system_error(make_err(EINVAL), "select: fd out of range");
401
402 {
403
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 7363 std::lock_guard lock(mutex_);
404
405
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 7363 auto& state = registered_fds_[fd];
406
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 7363 if (events & event_read)
407 3822 state.read_op = op;
408
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 7363 if (events & event_write)
409 3541 state.write_op = op;
410
411
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 7363 if (fd > max_fd_)
412 227 max_fd_ = fd;
413 7363 }
414
415 // Wake the reactor so a thread blocked in select() rebuilds its fd_sets
416 // with the newly registered fd.
417 7363 interrupt_reactor();
418 7363 }
419
420 void
421 7294 select_scheduler::
422 deregister_fd(int fd, int events) const
423 {
424
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 7294 std::lock_guard lock(mutex_);
425
426
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 7294 auto it = registered_fds_.find(fd);
427
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 7294 if (it == registered_fds_.end())
428 7133 return;
429
430
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 161 if (events & event_read)
431 161 it->second.read_op = nullptr;
432
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 161 if (events & event_write)
433 ✗ it->second.write_op = nullptr;
434
435 // Remove entry if both are null
436
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 161 if (!it->second.read_op && !it->second.write_op)
437 {
438
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 161 registered_fds_.erase(it);
439
440 // Recalculate max_fd_ if needed
441
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 161 if (fd == max_fd_)
442 {
443 161 max_fd_ = pipe_fds_[0]; // At minimum, the pipe read end
444
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 161 for (auto& [registered_fd, state] : registered_fds_)
445 {
446 ✗ if (registered_fd > max_fd_)
447 ✗ max_fd_ = registered_fd;
448 }
449 }
450 }
451 7294 }
452
453 void
454 7363 select_scheduler::
455 work_started() const noexcept
456 {
457 7363 outstanding_work_.fetch_add(1, std::memory_order_relaxed);
458 7363 }
459
460 void
461 171173 select_scheduler::
462 work_finished() const noexcept
463 {
464
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 342346 if (outstanding_work_.fetch_sub(1, std::memory_order_acq_rel) == 1)
465 {
466 // Last work item completed - wake all threads so they can exit.
467 99 std::unique_lock lock(mutex_);
468 99 wakeup_event_.notify_all();
469
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 99 if (reactor_running_ && !reactor_interrupted_)
470 {
471 ✗ reactor_interrupted_ = true;
472 ✗ lock.unlock();
473 ✗ interrupt_reactor();
474 }
475 99 }
476 171173 }
477
478 void
479 15003 select_scheduler::
480 interrupt_reactor() const
481 {
482 15003 char byte = 1;
483
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 15003 [[maybe_unused]] auto r = ::write(pipe_fds_[1], &byte, 1);
484 15003 }
485
486 void
487 163804 select_scheduler::
488 wake_one_thread_and_unlock(std::unique_lock<std::mutex>& lock) const
489 {
490
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 163804 if (idle_thread_count_ > 0)
491 {
492 // Idle worker exists - wake it via condvar
493 ✗ wakeup_event_.notify_one();
494 ✗ lock.unlock();
495 }
496
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 163804 else if (reactor_running_ && !reactor_interrupted_)
497 {
498 // No idle workers but reactor is running - interrupt it
499 3747 reactor_interrupted_ = true;
500 3747 lock.unlock();
501 3747 interrupt_reactor();
502 }
503 else
504 {
505 // No one to wake
506 160057 lock.unlock();
507 }
508 163804 }
509
510 struct work_guard
511 {
512 select_scheduler const* self;
513 171006 ~work_guard() { self->work_finished(); }
514 };
515
516 long
517 10916 select_scheduler::
518 calculate_timeout(long requested_timeout_us) const
519 {
520
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 10916 if (requested_timeout_us == 0)
521 ✗ return 0;
522
523 10916 auto nearest = timer_svc_->nearest_expiry();
524
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 10916 if (nearest == timer_service::time_point::max())
525 35 return requested_timeout_us;
526
527 10881 auto now = std::chrono::steady_clock::now();
528
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 10881 if (nearest <= now)
529 41 return 0;
530
531
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 10840 auto timer_timeout_us = std::chrono::duration_cast<std::chrono::microseconds>(
532
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 21680 nearest - now).count();
533
534
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 10840 if (requested_timeout_us < 0)
535 10840 return static_cast<long>(timer_timeout_us);
536
537 ✗ return static_cast<long>((std::min)(
538 ✗ static_cast<long long>(requested_timeout_us),
539 ✗ static_cast<long long>(timer_timeout_us)));
540 }
541
542 void
543 87691 select_scheduler::
544 run_reactor(std::unique_lock<std::mutex>& lock)
545 {
546 // Calculate timeout considering timers, use 0 if interrupted
547
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 87691 long effective_timeout_us = reactor_interrupted_ ? 0 : calculate_timeout(-1);
548
549 // Build fd_sets from registered_fds_
550 fd_set read_fds, write_fds, except_fds;
551
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 1490747 FD_ZERO(&read_fds);
552
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 1490747 FD_ZERO(&write_fds);
553
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 1490747 FD_ZERO(&except_fds);
554
555 // Always include the interrupt pipe
556 87691 FD_SET(pipe_fds_[0], &read_fds);
557 87691 int nfds = pipe_fds_[0];
558
559 // Add registered fds
560
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 105420 for (auto& [fd, state] : registered_fds_)
561 {
562
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 17729 if (state.read_op)
563 14188 FD_SET(fd, &read_fds);
564
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 17729 if (state.write_op)
565 {
566 3541 FD_SET(fd, &write_fds);
567 // Also monitor for errors on connect operations
568 3541 FD_SET(fd, &except_fds);
569 }
570
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 17729 if (fd > nfds)
571 14191 nfds = fd;
572 }
573
574 // Convert timeout to timeval
575 struct timeval tv;
576 87691 struct timeval* tv_ptr = nullptr;
577
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 87691 if (effective_timeout_us >= 0)
578 {
579 87656 tv.tv_sec = effective_timeout_us / 1000000;
580 87656 tv.tv_usec = effective_timeout_us % 1000000;
581 87656 tv_ptr = &tv;
582 }
583
584
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 87691 lock.unlock();
585
586
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 87691 int ready = ::select(nfds + 1, &read_fds, &write_fds, &except_fds, tv_ptr);
587 87691 int saved_errno = errno;
588
589 // Process timers outside the lock
590
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 87691 timer_svc_->process_expired();
591
592
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 87691 if (ready < 0 && saved_errno != EINTR)
593 ✗ detail::throw_system_error(make_err(saved_errno), "select");
594
595 // Re-acquire lock before modifying completed_ops_
596
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 87691 lock.lock();
597
598 // Drain the interrupt pipe if readable
599
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 87691 if (ready > 0 && FD_ISSET(pipe_fds_[0], &read_fds))
600 {
601 char buf[256];
602
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 22494 while (::read(pipe_fds_[0], buf, sizeof(buf)) > 0) {}
603 }
604
605 // Process I/O completions
606 87691 int completions_queued = 0;
607
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 87691 if (ready > 0)
608 {
609 // Iterate over registered fds (copy keys to avoid iterator invalidation)
610 11247 std::vector<int> fds_to_check;
611
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 11247 fds_to_check.reserve(registered_fds_.size());
612
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 25469 for (auto& [fd, state] : registered_fds_)
613
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 14222 fds_to_check.push_back(fd);
614
615
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 25469 for (int fd : fds_to_check)
616 {
617
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 14222 auto it = registered_fds_.find(fd);
618
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 14222 if (it == registered_fds_.end())
619 ✗ continue;
620
621 14222 auto& state = it->second;
622
623 // Check for errors (especially for connect operations)
624 14222 bool has_error = FD_ISSET(fd, &except_fds);
625
626 // Process read readiness
627
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 14222 if (state.read_op && (FD_ISSET(fd, &read_fds) || has_error))
628 {
629 3661 auto* op = state.read_op;
630 // Claim the op by exchanging to unregistered. Both registering and
631 // registered states mean the op is ours to complete.
632 3661 auto prev = op->registered.exchange(
633 select_registration_state::unregistered, std::memory_order_acq_rel);
634
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 3661 if (prev != select_registration_state::unregistered)
635 {
636 3661 state.read_op = nullptr;
637
638
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 3661 if (has_error)
639 {
640 ✗ int errn = 0;
641 ✗ socklen_t len = sizeof(errn);
642 ✗ if (::getsockopt(fd, SOL_SOCKET, SO_ERROR, &errn, &len) < 0)
643 ✗ errn = errno;
644 ✗ if (errn == 0)
645 ✗ errn = EIO;
646 ✗ op->complete(errn, 0);
647 }
648 else
649 {
650 3661 op->perform_io();
651 }
652
653 3661 completed_ops_.push(op);
654 3661 ++completions_queued;
655 }
656 }
657
658 // Process write readiness
659
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 14222 if (state.write_op && (FD_ISSET(fd, &write_fds) || has_error))
660 {
661 3541 auto* op = state.write_op;
662 // Claim the op by exchanging to unregistered. Both registering and
663 // registered states mean the op is ours to complete.
664 3541 auto prev = op->registered.exchange(
665 select_registration_state::unregistered, std::memory_order_acq_rel);
666
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 3541 if (prev != select_registration_state::unregistered)
667 {
668 3541 state.write_op = nullptr;
669
670
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 3541 if (has_error)
671 {
672 ✗ int errn = 0;
673 ✗ socklen_t len = sizeof(errn);
674 ✗ if (::getsockopt(fd, SOL_SOCKET, SO_ERROR, &errn, &len) < 0)
675 ✗ errn = errno;
676 ✗ if (errn == 0)
677 ✗ errn = EIO;
678 ✗ op->complete(errn, 0);
679 }
680 else
681 {
682 3541 op->perform_io();
683 }
684
685 3541 completed_ops_.push(op);
686 3541 ++completions_queued;
687 }
688 }
689
690 // Clean up empty entries
691
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 14222 if (!state.read_op && !state.write_op)
692
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 7202 registered_fds_.erase(it);
693 }
694 11247 }
695
696
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 87691 if (completions_queued > 0)
697 {
698
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 3664 if (completions_queued == 1)
699 126 wakeup_event_.notify_one();
700 else
701 3538 wakeup_event_.notify_all();
702 }
703 87691 }
704
705 std::size_t
706 171102 select_scheduler::
707 do_one(long timeout_us)
708 {
709
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 171102 std::unique_lock lock(mutex_);
710
711 for (;;)
712 {
713
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 258793 if (stopped_.load(std::memory_order_acquire))
714 ✗ return 0;
715
716 258793 scheduler_op* op = completed_ops_.pop();
717
718
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 258793 if (op == &task_op_)
719 {
720 87787 bool more_handlers = !completed_ops_.empty();
721
722
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 87787 if (!more_handlers)
723 {
724
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 22024 if (outstanding_work_.load(std::memory_order_acquire) == 0)
725 {
726 96 completed_ops_.push(&task_op_);
727 96 return 0;
728 }
729
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 10916 if (timeout_us == 0)
730 {
731 ✗ completed_ops_.push(&task_op_);
732 ✗ return 0;
733 }
734 }
735
736
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 87691 reactor_interrupted_ = more_handlers || timeout_us == 0;
737 87691 reactor_running_ = true;
738
739
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 87691 if (more_handlers && idle_thread_count_ > 0)
740 ✗ wakeup_event_.notify_one();
741
742
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 87691 run_reactor(lock);
743
744 87691 reactor_running_ = false;
745 87691 completed_ops_.push(&task_op_);
746 87691 continue;
747 87691 }
748
749
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 171006 if (op != nullptr)
750 {
751
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 171006 lock.unlock();
752 171006 work_guard g{this};
753
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 171006 (*op)();
754 171006 return 1;
755 171006 }
756
757 ✗ if (outstanding_work_.load(std::memory_order_acquire) == 0)
758 ✗ return 0;
759
760 ✗ if (timeout_us == 0)
761 ✗ return 0;
762
763 ✗ ++idle_thread_count_;
764 ✗ if (timeout_us < 0)
765 ✗ wakeup_event_.wait(lock);
766 else
767 ✗ wakeup_event_.wait_for(lock, std::chrono::microseconds(timeout_us));
768 ✗ --idle_thread_count_;
769 87691 }
770 171102 }
771
772 } // namespace boost::corosio::detail
773
774 #endif // BOOST_COROSIO_HAS_SELECT
775