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libs/corosio/src/corosio/src/detail/select/op.hpp

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libs/corosio/src/corosio/src/detail/select/op.hpp
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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 #ifndef BOOST_COROSIO_DETAIL_SELECT_OP_HPP
11 #define BOOST_COROSIO_DETAIL_SELECT_OP_HPP
12
13 #include <boost/corosio/detail/platform.hpp>
14
15 #if BOOST_COROSIO_HAS_SELECT
16
17 #include <boost/corosio/detail/config.hpp>
18 #include <boost/corosio/io_object.hpp>
19 #include <boost/corosio/endpoint.hpp>
20 #include <boost/capy/ex/executor_ref.hpp>
21 #include <coroutine>
22 #include <boost/capy/error.hpp>
23 #include <system_error>
24
25 #include "src/detail/make_err.hpp"
26 #include "src/detail/dispatch_coro.hpp"
27 #include "src/detail/scheduler_op.hpp"
28 #include "src/detail/endpoint_convert.hpp"
29
30 #include <unistd.h>
31 #include <errno.h>
32 #include <fcntl.h>
33
34 #include <atomic>
35 #include <cstddef>
36 #include <memory>
37 #include <optional>
38 #include <stop_token>
39
40 #include <netinet/in.h>
41 #include <sys/select.h>
42 #include <sys/socket.h>
43 #include <sys/uio.h>
44
45 /*
46 select Operation State
47 ======================
48
49 Each async I/O operation has a corresponding select_op-derived struct that
50 holds the operation's state while it's in flight. The socket impl owns
51 fixed slots for each operation type (conn_, rd_, wr_), so only one
52 operation of each type can be pending per socket at a time.
53
54 This mirrors the epoll_op design for consistency across backends.
55
56 Completion vs Cancellation Race
57 -------------------------------
58 The `registered` atomic uses a tri-state (unregistered, registering,
59 registered) to handle two races: (1) between register_fd() and the
60 reactor seeing an event, and (2) between reactor completion and cancel().
61
62 The registering state closes the window where an event could arrive
63 after register_fd() but before the boolean was set. The reactor and
64 cancel() both treat registering the same as registered when claiming.
65
66 Whoever atomically exchanges to unregistered "claims" the operation
67 and is responsible for completing it. The loser sees unregistered and
68 does nothing. The initiating thread uses compare_exchange to transition
69 from registering to registered; if this fails, the reactor or cancel
70 already claimed the op.
71
72 Impl Lifetime Management
73 ------------------------
74 When cancel() posts an op to the scheduler's ready queue, the socket impl
75 might be destroyed before the scheduler processes the op. The `impl_ptr`
76 member holds a shared_ptr to the impl, keeping it alive until the op
77 completes.
78
79 EOF Detection
80 -------------
81 For reads, 0 bytes with no error means EOF. But an empty user buffer also
82 returns 0 bytes. The `empty_buffer_read` flag distinguishes these cases.
83
84 SIGPIPE Prevention
85 ------------------
86 Writes use sendmsg() with MSG_NOSIGNAL instead of writev() to prevent
87 SIGPIPE when the peer has closed.
88 */
89
90 namespace boost::corosio::detail {
91
92 // Forward declarations for cancellation support
93 class select_socket_impl;
94 class select_acceptor_impl;
95
96 /** Registration state for async operations.
97
98 Tri-state enum to handle the race between register_fd() and
99 run_reactor() seeing an event. Setting REGISTERING before
100 calling register_fd() ensures events delivered during the
101 registration window are not dropped.
102 */
103 enum class select_registration_state : std::uint8_t
104 {
105 unregistered, ///< Not registered with reactor
106 registering, ///< register_fd() called, not yet confirmed
107 registered ///< Fully registered, ready for events
108 };
109
110 struct select_op : scheduler_op
111 {
112 struct canceller
113 {
114 select_op* op;
115 void operator()() const noexcept;
116 };
117
118 std::coroutine_handle<> h;
119 capy::executor_ref ex;
120 std::error_code* ec_out = nullptr;
121 std::size_t* bytes_out = nullptr;
122
123 int fd = -1;
124 int errn = 0;
125 std::size_t bytes_transferred = 0;
126
127 std::atomic<bool> cancelled{false};
128 std::atomic<select_registration_state> registered{select_registration_state::unregistered};
129 std::optional<std::stop_callback<canceller>> stop_cb;
130
131 // Prevents use-after-free when socket is closed with pending ops.
132 std::shared_ptr<void> impl_ptr;
133
134 // For stop_token cancellation - pointer to owning socket/acceptor impl.
135 select_socket_impl* socket_impl_ = nullptr;
136 select_acceptor_impl* acceptor_impl_ = nullptr;
137
138 20487 select_op() = default;
139
140 181363 void reset() noexcept
141 {
142 181363 fd = -1;
143 181363 errn = 0;
144 181363 bytes_transferred = 0;
145 181363 cancelled.store(false, std::memory_order_relaxed);
146 181363 registered.store(select_registration_state::unregistered, std::memory_order_relaxed);
147 181363 impl_ptr.reset();
148 181363 socket_impl_ = nullptr;
149 181363 acceptor_impl_ = nullptr;
150 181363 }
151
152 174556 void operator()() override
153 {
154 174556 stop_cb.reset();
155
156
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 174556 if (ec_out)
157 {
158
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 174556 if (cancelled.load(std::memory_order_acquire))
159 196 *ec_out = capy::error::canceled;
160
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 174360 else if (errn != 0)
161 1 *ec_out = make_err(errn);
162
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 174359 else if (is_read_operation() && bytes_transferred == 0)
163 5 *ec_out = capy::error::eof;
164 else
165 174354 *ec_out = {};
166 }
167
168
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 174556 if (bytes_out)
169 174556 *bytes_out = bytes_transferred;
170
171 // Move to stack before destroying the frame
172 174556 capy::executor_ref saved_ex( std::move( ex ) );
173 174556 std::coroutine_handle<> saved_h( std::move( h ) );
174 174556 impl_ptr.reset();
175
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 174556 dispatch_coro(saved_ex, saved_h).resume();
176 174556 }
177
178 87193 virtual bool is_read_operation() const noexcept { return false; }
179 virtual void cancel() noexcept = 0;
180
181 ✗ void destroy() override
182 {
183 ✗ stop_cb.reset();
184 ✗ impl_ptr.reset();
185 ✗ }
186
187 31388 void request_cancel() noexcept
188 {
189 31388 cancelled.store(true, std::memory_order_release);
190 31388 }
191
192 void start(std::stop_token token)
193 {
194 cancelled.store(false, std::memory_order_release);
195 stop_cb.reset();
196 socket_impl_ = nullptr;
197 acceptor_impl_ = nullptr;
198
199 if (token.stop_possible())
200 stop_cb.emplace(token, canceller{this});
201 }
202
203 177959 void start(std::stop_token token, select_socket_impl* impl)
204 {
205 177959 cancelled.store(false, std::memory_order_release);
206 177959 stop_cb.reset();
207 177959 socket_impl_ = impl;
208 177959 acceptor_impl_ = nullptr;
209
210
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 177959 if (token.stop_possible())
211 98 stop_cb.emplace(token, canceller{this});
212 177959 }
213
214 3404 void start(std::stop_token token, select_acceptor_impl* impl)
215 {
216 3404 cancelled.store(false, std::memory_order_release);
217 3404 stop_cb.reset();
218 3404 socket_impl_ = nullptr;
219 3404 acceptor_impl_ = impl;
220
221
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 3404 if (token.stop_possible())
222 ✗ stop_cb.emplace(token, canceller{this});
223 3404 }
224
225 181204 void complete(int err, std::size_t bytes) noexcept
226 {
227 181204 errn = err;
228 181204 bytes_transferred = bytes;
229 181204 }
230
231 ✗ virtual void perform_io() noexcept {}
232 };
233
234
235 struct select_connect_op : select_op
236 {
237 endpoint target_endpoint;
238
239 3403 void reset() noexcept
240 {
241 3403 select_op::reset();
242 3403 target_endpoint = endpoint{};
243 3403 }
244
245 3403 void perform_io() noexcept override
246 {
247 // connect() completion status is retrieved via SO_ERROR, not return value
248 3403 int err = 0;
249 3403 socklen_t len = sizeof(err);
250
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 3403 if (::getsockopt(fd, SOL_SOCKET, SO_ERROR, &err, &len) < 0)
251 ✗ err = errno;
252 3403 complete(err, 0);
253 3403 }
254
255 // Defined in sockets.cpp where select_socket_impl is complete
256 void operator()() override;
257 void cancel() noexcept override;
258 };
259
260
261 struct select_read_op : select_op
262 {
263 static constexpr std::size_t max_buffers = 16;
264 iovec iovecs[max_buffers];
265 int iovec_count = 0;
266 bool empty_buffer_read = false;
267
268 87166 bool is_read_operation() const noexcept override
269 {
270 87166 return !empty_buffer_read;
271 }
272
273 87358 void reset() noexcept
274 {
275 87358 select_op::reset();
276 87358 iovec_count = 0;
277 87358 empty_buffer_read = false;
278 87358 }
279
280 121 void perform_io() noexcept override
281 {
282 121 ssize_t n = ::readv(fd, iovecs, iovec_count);
283
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 121 if (n >= 0)
284 121 complete(0, static_cast<std::size_t>(n));
285 else
286 ✗ complete(errno, 0);
287 121 }
288
289 void cancel() noexcept override;
290 };
291
292
293 struct select_write_op : select_op
294 {
295 static constexpr std::size_t max_buffers = 16;
296 iovec iovecs[max_buffers];
297 int iovec_count = 0;
298
299 87198 void reset() noexcept
300 {
301 87198 select_op::reset();
302 87198 iovec_count = 0;
303 87198 }
304
305 ✗ void perform_io() noexcept override
306 {
307 ✗ msghdr msg{};
308 ✗ msg.msg_iov = iovecs;
309 ✗ msg.msg_iovlen = static_cast<std::size_t>(iovec_count);
310
311 ✗ ssize_t n = ::sendmsg(fd, &msg, MSG_NOSIGNAL);
312 ✗ if (n >= 0)
313 ✗ complete(0, static_cast<std::size_t>(n));
314 else
315 ✗ complete(errno, 0);
316 ✗ }
317
318 void cancel() noexcept override;
319 };
320
321
322 struct select_accept_op : select_op
323 {
324 int accepted_fd = -1;
325 io_object::io_object_impl* peer_impl = nullptr;
326 io_object::io_object_impl** impl_out = nullptr;
327
328 3404 void reset() noexcept
329 {
330 3404 select_op::reset();
331 3404 accepted_fd = -1;
332 3404 peer_impl = nullptr;
333 3404 impl_out = nullptr;
334 3404 }
335
336 3399 void perform_io() noexcept override
337 {
338 3399 sockaddr_in addr{};
339 3399 socklen_t addrlen = sizeof(addr);
340
341 // Note: select backend uses accept() + fcntl instead of accept4()
342 // for broader POSIX compatibility
343 3399 int new_fd = ::accept(fd, reinterpret_cast<sockaddr*>(&addr), &addrlen);
344
345
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 3399 if (new_fd >= 0)
346 {
347 // Reject fds that exceed select()'s FD_SETSIZE limit.
348 // Better to fail now than during later async operations.
349
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 3399 if (new_fd >= FD_SETSIZE)
350 {
351 ✗ ::close(new_fd);
352 ✗ complete(EINVAL, 0);
353 ✗ return;
354 }
355
356 // Set non-blocking and close-on-exec flags.
357 // A non-blocking socket is essential for the async reactor;
358 // if we can't configure it, fail rather than risk blocking.
359 3399 int flags = ::fcntl(new_fd, F_GETFL, 0);
360
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 3399 if (flags == -1)
361 {
362 ✗ int err = errno;
363 ✗ ::close(new_fd);
364 ✗ complete(err, 0);
365 ✗ return;
366 }
367
368
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 3399 if (::fcntl(new_fd, F_SETFL, flags | O_NONBLOCK) == -1)
369 {
370 ✗ int err = errno;
371 ✗ ::close(new_fd);
372 ✗ complete(err, 0);
373 ✗ return;
374 }
375
376
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 3399 if (::fcntl(new_fd, F_SETFD, FD_CLOEXEC) == -1)
377 {
378 ✗ int err = errno;
379 ✗ ::close(new_fd);
380 ✗ complete(err, 0);
381 ✗ return;
382 }
383
384 3399 accepted_fd = new_fd;
385 3399 complete(0, 0);
386 }
387 else
388 {
389 ✗ complete(errno, 0);
390 }
391 }
392
393 // Defined in acceptors.cpp where select_acceptor_impl is complete
394 void operator()() override;
395 void cancel() noexcept override;
396 };
397
398 } // namespace boost::corosio::detail
399
400 #endif // BOOST_COROSIO_HAS_SELECT
401
402 #endif // BOOST_COROSIO_DETAIL_SELECT_OP_HPP
403