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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 3616 [](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 3951 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 3951 post_handler(std::coroutine_handle<> h)
205 3951 : h_(h)
206 {
207 3951 }
208
209 7902 ~post_handler() = default;
210
211 3951 void operator()() override
212 {
213 3951 auto h = h_;
214
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 3951 delete this;
215
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 3951 h.resume();
216 3951 }
217
218 ✗ void destroy() override
219 {
220 ✗ delete this;
221 ✗ }
222 };
223
224
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 3951 auto ph = std::make_unique<post_handler>(h);
225 3951 outstanding_work_.fetch_add(1, std::memory_order_relaxed);
226
227
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 3951 std::unique_lock lock(mutex_);
228 3951 completed_ops_.push(ph.release());
229
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 3951 wake_one_thread_and_unlock(lock);
230 3951 }
231
232 void
233 178081 select_scheduler::
234 post(scheduler_op* h) const
235 {
236 178081 outstanding_work_.fetch_add(1, std::memory_order_relaxed);
237
238
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 178081 std::unique_lock lock(mutex_);
239 178081 completed_ops_.push(h);
240
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 178081 wake_one_thread_and_unlock(lock);
241 178081 }
242
243 void
244 4183 select_scheduler::
245 on_work_started() noexcept
246 {
247 4183 outstanding_work_.fetch_add(1, std::memory_order_relaxed);
248 4183 }
249
250 void
251 4177 select_scheduler::
252 on_work_finished() noexcept
253 {
254
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 8354 if (outstanding_work_.fetch_sub(1, std::memory_order_acq_rel) == 1)
255 ✗ stop();
256 4177 }
257
258 bool
259 545 select_scheduler::
260 running_in_this_thread() const noexcept
261 {
262
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 545 for (auto* c = context_stack.get(); c != nullptr; c = c->next)
263
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 361 if (c->key == this)
264 361 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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 189027 while (do_one(-1))
316
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 188931 if (n != (std::numeric_limits<std::size_t>::max)())
317 188931 ++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 7082 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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 7082 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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 7082 std::lock_guard lock(mutex_);
404
405
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 7082 auto& state = registered_fds_[fd];
406
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 7082 if (events & event_read)
407 3679 state.read_op = op;
408
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 7082 if (events & event_write)
409 3403 state.write_op = op;
410
411
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 7082 if (fd > max_fd_)
412 225 max_fd_ = fd;
413 7082 }
414
415 // Wake the reactor so a thread blocked in select() rebuilds its fd_sets
416 // with the newly registered fd.
417 7082 interrupt_reactor();
418 7082 }
419
420 void
421 7016 select_scheduler::
422 deregister_fd(int fd, int events) const
423 {
424
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 7016 std::lock_guard lock(mutex_);
425
426
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 7016 auto it = registered_fds_.find(fd);
427
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 7016 if (it == registered_fds_.end())
428 6857 return;
429
430
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 159 if (events & event_read)
431 159 it->second.read_op = nullptr;
432
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 159 if (events & event_write)
433 ✗ it->second.write_op = nullptr;
434
435 // Remove entry if both are null
436
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 159 if (!it->second.read_op && !it->second.write_op)
437 {
438
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 159 registered_fds_.erase(it);
439
440 // Recalculate max_fd_ if needed
441
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 159 if (fd == max_fd_)
442 {
443 158 max_fd_ = pipe_fds_[0]; // At minimum, the pipe read end
444
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 158 for (auto& [registered_fd, state] : registered_fds_)
445 {
446 ✗ if (registered_fd > max_fd_)
447 ✗ max_fd_ = registered_fd;
448 }
449 }
450 }
451 7016 }
452
453 void
454 7082 select_scheduler::
455 work_started() const noexcept
456 {
457 7082 outstanding_work_.fetch_add(1, std::memory_order_relaxed);
458 7082 }
459
460 void
461 189120 select_scheduler::
462 work_finished() const noexcept
463 {
464
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 378240 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 189120 }
477
478 void
479 14440 select_scheduler::
480 interrupt_reactor() const
481 {
482 14440 char byte = 1;
483
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 14440 [[maybe_unused]] auto r = ::write(pipe_fds_[1], &byte, 1);
484 14440 }
485
486 void
487 182032 select_scheduler::
488 wake_one_thread_and_unlock(std::unique_lock<std::mutex>& lock) const
489 {
490
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 182032 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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 182032 else if (reactor_running_ && !reactor_interrupted_)
497 {
498 // No idle workers but reactor is running - interrupt it
499 3606 reactor_interrupted_ = true;
500 3606 lock.unlock();
501 3606 interrupt_reactor();
502 }
503 else
504 {
505 // No one to wake
506 178426 lock.unlock();
507 }
508 182032 }
509
510 struct work_guard
511 {
512 select_scheduler const* self;
513 188955 ~work_guard() { self->work_finished(); }
514 };
515
516 long
517 10501 select_scheduler::
518 calculate_timeout(long requested_timeout_us) const
519 {
520
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 10501 if (requested_timeout_us == 0)
521 ✗ return 0;
522
523 10501 auto nearest = timer_svc_->nearest_expiry();
524
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 10501 if (nearest == timer_service::time_point::max())
525 35 return requested_timeout_us;
526
527 10466 auto now = std::chrono::steady_clock::now();
528
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 10466 if (nearest <= now)
529 32 return 0;
530
531
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 10434 auto timer_timeout_us = std::chrono::duration_cast<std::chrono::microseconds>(
532
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 20868 nearest - now).count();
533
534
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 10434 if (requested_timeout_us < 0)
535 10434 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 96058 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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 96058 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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 1632986 FD_ZERO(&read_fds);
552
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 1632986 FD_ZERO(&write_fds);
553
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 1632986 FD_ZERO(&except_fds);
554
555 // Always include the interrupt pipe
556 96058 FD_SET(pipe_fds_[0], &read_fds);
557 96058 int nfds = pipe_fds_[0];
558
559 // Add registered fds
560
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 113099 for (auto& [fd, state] : registered_fds_)
561 {
562
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 17041 if (state.read_op)
563 13638 FD_SET(fd, &read_fds);
564
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 17041 if (state.write_op)
565 {
566 3403 FD_SET(fd, &write_fds);
567 // Also monitor for errors on connect operations
568 3403 FD_SET(fd, &except_fds);
569 }
570
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 17041 if (fd > nfds)
571 13642 nfds = fd;
572 }
573
574 // Convert timeout to timeval
575 struct timeval tv;
576 96058 struct timeval* tv_ptr = nullptr;
577
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 96058 if (effective_timeout_us >= 0)
578 {
579 96023 tv.tv_sec = effective_timeout_us / 1000000;
580 96023 tv.tv_usec = effective_timeout_us % 1000000;
581 96023 tv_ptr = &tv;
582 }
583
584
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 96058 lock.unlock();
585
586
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 96058 int ready = ::select(nfds + 1, &read_fds, &write_fds, &except_fds, tv_ptr);
587 96058 int saved_errno = errno;
588
589 // Process timers outside the lock
590
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 96058 timer_svc_->process_expired();
591
592
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 96058 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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 96058 lock.lock();
597
598 // Drain the interrupt pipe if readable
599
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 96058 if (ready > 0 && FD_ISSET(pipe_fds_[0], &read_fds))
600 {
601 char buf[256];
602
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 21654 while (::read(pipe_fds_[0], buf, sizeof(buf)) > 0) {}
603 }
604
605 // Process I/O completions
606 96058 int completions_queued = 0;
607
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 96058 if (ready > 0)
608 {
609 // Iterate over registered fds (copy keys to avoid iterator invalidation)
610 10827 std::vector<int> fds_to_check;
611
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 10827 fds_to_check.reserve(registered_fds_.size());
612
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 24499 for (auto& [fd, state] : registered_fds_)
613
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 13672 fds_to_check.push_back(fd);
614
615
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 24499 for (int fd : fds_to_check)
616 {
617
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 13672 auto it = registered_fds_.find(fd);
618
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 13672 if (it == registered_fds_.end())
619 ✗ continue;
620
621 13672 auto& state = it->second;
622
623 // Check for errors (especially for connect operations)
624 13672 bool has_error = FD_ISSET(fd, &except_fds);
625
626 // Process read readiness
627
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 13672 if (state.read_op && (FD_ISSET(fd, &read_fds) || has_error))
628 {
629 3520 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 3520 auto prev = op->registered.exchange(
633 select_registration_state::unregistered, std::memory_order_acq_rel);
634
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 3520 if (prev != select_registration_state::unregistered)
635 {
636 3520 state.read_op = nullptr;
637
638
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 3520 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 3520 op->perform_io();
651 }
652
653 3520 completed_ops_.push(op);
654 3520 ++completions_queued;
655 }
656 }
657
658 // Process write readiness
659
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 13672 if (state.write_op && (FD_ISSET(fd, &write_fds) || has_error))
660 {
661 3403 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 3403 auto prev = op->registered.exchange(
665 select_registration_state::unregistered, std::memory_order_acq_rel);
666
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 3403 if (prev != select_registration_state::unregistered)
667 {
668 3403 state.write_op = nullptr;
669
670
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 3403 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 3403 op->perform_io();
683 }
684
685 3403 completed_ops_.push(op);
686 3403 ++completions_queued;
687 }
688 }
689
690 // Clean up empty entries
691
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 13672 if (!state.read_op && !state.write_op)
692
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 6923 registered_fds_.erase(it);
693 }
694 10827 }
695
696
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 96058 if (completions_queued > 0)
697 {
698
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 3524 if (completions_queued == 1)
699 125 wakeup_event_.notify_one();
700 else
701 3399 wakeup_event_.notify_all();
702 }
703 96058 }
704
705 std::size_t
706 189051 select_scheduler::
707 do_one(long timeout_us)
708 {
709
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 189051 std::unique_lock lock(mutex_);
710
711 for (;;)
712 {
713
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 285109 if (stopped_.load(std::memory_order_acquire))
714 ✗ return 0;
715
716 285109 scheduler_op* op = completed_ops_.pop();
717
718
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 285109 if (op == &task_op_)
719 {
720 96154 bool more_handlers = !completed_ops_.empty();
721
722
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 96154 if (!more_handlers)
723 {
724
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 21194 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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 10501 if (timeout_us == 0)
730 {
731 ✗ completed_ops_.push(&task_op_);
732 ✗ return 0;
733 }
734 }
735
736
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 96058 reactor_interrupted_ = more_handlers || timeout_us == 0;
737 96058 reactor_running_ = true;
738
739
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 96058 if (more_handlers && idle_thread_count_ > 0)
740 ✗ wakeup_event_.notify_one();
741
742
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 96058 run_reactor(lock);
743
744 96058 reactor_running_ = false;
745 96058 completed_ops_.push(&task_op_);
746 96058 continue;
747 96058 }
748
749
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 188955 if (op != nullptr)
750 {
751
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 188955 lock.unlock();
752 188955 work_guard g{this};
753
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 188955 (*op)();
754 188955 return 1;
755 188955 }
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 96058 }
770 189051 }
771
772 } // namespace boost::corosio::detail
773
774 #endif // BOOST_COROSIO_HAS_SELECT
775