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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 <boost/capy/coro.hpp>
22 #include <boost/capy/error.hpp>
23 #include <system_error>
24
25 #include "src/detail/make_err.hpp"
26 #include "src/detail/scheduler_op.hpp"
27 #include "src/detail/endpoint_convert.hpp"
28
29 #include <unistd.h>
30 #include <errno.h>
31 #include <fcntl.h>
32
33 #include <atomic>
34 #include <cstddef>
35 #include <memory>
36 #include <optional>
37 #include <stop_token>
38
39 #include <netinet/in.h>
40 #include <sys/select.h>
41 #include <sys/socket.h>
42 #include <sys/uio.h>
43
44 /*
45 select Operation State
46 ======================
47
48 Each async I/O operation has a corresponding select_op-derived struct that
49 holds the operation's state while it's in flight. The socket impl owns
50 fixed slots for each operation type (conn_, rd_, wr_), so only one
51 operation of each type can be pending per socket at a time.
52
53 This mirrors the epoll_op design for consistency across backends.
54
55 Completion vs Cancellation Race
56 -------------------------------
57 The `registered` atomic uses a tri-state (unregistered, registering,
58 registered) to handle two races: (1) between register_fd() and the
59 reactor seeing an event, and (2) between reactor completion and cancel().
60
61 The registering state closes the window where an event could arrive
62 after register_fd() but before the boolean was set. The reactor and
63 cancel() both treat registering the same as registered when claiming.
64
65 Whoever atomically exchanges to unregistered "claims" the operation
66 and is responsible for completing it. The loser sees unregistered and
67 does nothing. The initiating thread uses compare_exchange to transition
68 from registering to registered; if this fails, the reactor or cancel
69 already claimed the op.
70
71 Impl Lifetime Management
72 ------------------------
73 When cancel() posts an op to the scheduler's ready queue, the socket impl
74 might be destroyed before the scheduler processes the op. The `impl_ptr`
75 member holds a shared_ptr to the impl, keeping it alive until the op
76 completes.
77
78 EOF Detection
79 -------------
80 For reads, 0 bytes with no error means EOF. But an empty user buffer also
81 returns 0 bytes. The `empty_buffer_read` flag distinguishes these cases.
82
83 SIGPIPE Prevention
84 ------------------
85 Writes use sendmsg() with MSG_NOSIGNAL instead of writev() to prevent
86 SIGPIPE when the peer has closed.
87 */
88
89 namespace boost::corosio::detail {
90
91 // Forward declarations for cancellation support
92 class select_socket_impl;
93 class select_acceptor_impl;
94
95 /** Registration state for async operations.
96
97 Tri-state enum to handle the race between register_fd() and
98 run_reactor() seeing an event. Setting REGISTERING before
99 calling register_fd() ensures events delivered during the
100 registration window are not dropped.
101 */
102 enum class select_registration_state : std::uint8_t
103 {
104 unregistered, ///< Not registered with reactor
105 registering, ///< register_fd() called, not yet confirmed
106 registered ///< Fully registered, ready for events
107 };
108
109 struct select_op : scheduler_op
110 {
111 struct canceller
112 {
113 select_op* op;
114 void operator()() const noexcept;
115 };
116
117 capy::coro h;
118 capy::executor_ref ex;
119 std::error_code* ec_out = nullptr;
120 std::size_t* bytes_out = nullptr;
121
122 int fd = -1;
123 int errn = 0;
124 std::size_t bytes_transferred = 0;
125
126 std::atomic<bool> cancelled{false};
127 std::atomic<select_registration_state> registered{select_registration_state::unregistered};
128 std::optional<std::stop_callback<canceller>> stop_cb;
129
130 // Prevents use-after-free when socket is closed with pending ops.
131 std::shared_ptr<void> impl_ptr;
132
133 // For stop_token cancellation - pointer to owning socket/acceptor impl.
134 select_socket_impl* socket_impl_ = nullptr;
135 select_acceptor_impl* acceptor_impl_ = nullptr;
136
137 11090 select_op() = default;
138
139 172749 void reset() noexcept
140 {
141 172749 fd = -1;
142 172749 errn = 0;
143 172749 bytes_transferred = 0;
144 172749 cancelled.store(false, std::memory_order_relaxed);
145 172749 registered.store(select_registration_state::unregistered, std::memory_order_relaxed);
146 172749 impl_ptr.reset();
147 172749 socket_impl_ = nullptr;
148 172749 acceptor_impl_ = nullptr;
149 172749 }
150
151 169078 void operator()() override
152 {
153 169078 stop_cb.reset();
154
155
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 169078 if (ec_out)
156 {
157
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 169078 if (cancelled.load(std::memory_order_acquire))
158 198 *ec_out = capy::error::canceled;
159
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 168880 else if (errn != 0)
160 1 *ec_out = make_err(errn);
161
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 168879 else if (is_read_operation() && bytes_transferred == 0)
162 5 *ec_out = capy::error::eof;
163 else
164 168874 *ec_out = {};
165 }
166
167
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 169078 if (bytes_out)
168 169078 *bytes_out = bytes_transferred;
169
170 // Move to stack before destroying the frame
171 169078 capy::executor_ref saved_ex( std::move( ex ) );
172 169078 capy::coro saved_h( std::move( h ) );
173 169078 impl_ptr.reset();
174
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 169078 saved_ex.dispatch( saved_h );
175 169078 }
176
177 84475 virtual bool is_read_operation() const noexcept { return false; }
178 virtual void cancel() noexcept = 0;
179
180 ✗ void destroy() override
181 {
182 ✗ stop_cb.reset();
183 ✗ impl_ptr.reset();
184 ✗ }
185
186 17055 void request_cancel() noexcept
187 {
188 17055 cancelled.store(true, std::memory_order_release);
189 17055 }
190
191 void start(std::stop_token token)
192 {
193 cancelled.store(false, std::memory_order_release);
194 stop_cb.reset();
195 socket_impl_ = nullptr;
196 acceptor_impl_ = nullptr;
197
198 if (token.stop_possible())
199 stop_cb.emplace(token, canceller{this});
200 }
201
202 170913 void start(std::stop_token token, select_socket_impl* impl)
203 {
204 170913 cancelled.store(false, std::memory_order_release);
205 170913 stop_cb.reset();
206 170913 socket_impl_ = impl;
207 170913 acceptor_impl_ = nullptr;
208
209
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 170913 if (token.stop_possible())
210 98 stop_cb.emplace(token, canceller{this});
211 170913 }
212
213 1836 void start(std::stop_token token, select_acceptor_impl* impl)
214 {
215 1836 cancelled.store(false, std::memory_order_release);
216 1836 stop_cb.reset();
217 1836 socket_impl_ = nullptr;
218 1836 acceptor_impl_ = impl;
219
220
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 1836 if (token.stop_possible())
221 ✗ stop_cb.emplace(token, canceller{this});
222 1836 }
223
224 172632 void complete(int err, std::size_t bytes) noexcept
225 {
226 172632 errn = err;
227 172632 bytes_transferred = bytes;
228 172632 }
229
230 ✗ virtual void perform_io() noexcept {}
231 };
232
233
234 struct select_connect_op : select_op
235 {
236 endpoint target_endpoint;
237
238 1835 void reset() noexcept
239 {
240 1835 select_op::reset();
241 1835 target_endpoint = endpoint{};
242 1835 }
243
244 1835 void perform_io() noexcept override
245 {
246 // connect() completion status is retrieved via SO_ERROR, not return value
247 1835 int err = 0;
248 1835 socklen_t len = sizeof(err);
249
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 1835 if (::getsockopt(fd, SOL_SOCKET, SO_ERROR, &err, &len) < 0)
250 ✗ err = errno;
251 1835 complete(err, 0);
252 1835 }
253
254 // Defined in sockets.cpp where select_socket_impl is complete
255 void operator()() override;
256 void cancel() noexcept override;
257 };
258
259
260 struct select_read_op : select_op
261 {
262 static constexpr std::size_t max_buffers = 16;
263 iovec iovecs[max_buffers];
264 int iovec_count = 0;
265 bool empty_buffer_read = false;
266
267 84404 bool is_read_operation() const noexcept override
268 {
269 84404 return !empty_buffer_read;
270 }
271
272 84598 void reset() noexcept
273 {
274 84598 select_op::reset();
275 84598 iovec_count = 0;
276 84598 empty_buffer_read = false;
277 84598 }
278
279 78 void perform_io() noexcept override
280 {
281 78 ssize_t n = ::readv(fd, iovecs, iovec_count);
282
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 78 if (n >= 0)
283 78 complete(0, static_cast<std::size_t>(n));
284 else
285 ✗ complete(errno, 0);
286 78 }
287
288 void cancel() noexcept override;
289 };
290
291
292 struct select_write_op : select_op
293 {
294 static constexpr std::size_t max_buffers = 16;
295 iovec iovecs[max_buffers];
296 int iovec_count = 0;
297
298 84480 void reset() noexcept
299 {
300 84480 select_op::reset();
301 84480 iovec_count = 0;
302 84480 }
303
304 ✗ void perform_io() noexcept override
305 {
306 ✗ msghdr msg{};
307 ✗ msg.msg_iov = iovecs;
308 ✗ msg.msg_iovlen = static_cast<std::size_t>(iovec_count);
309
310 ✗ ssize_t n = ::sendmsg(fd, &msg, MSG_NOSIGNAL);
311 ✗ if (n >= 0)
312 ✗ complete(0, static_cast<std::size_t>(n));
313 else
314 ✗ complete(errno, 0);
315 ✗ }
316
317 void cancel() noexcept override;
318 };
319
320
321 struct select_accept_op : select_op
322 {
323 int accepted_fd = -1;
324 io_object::io_object_impl* peer_impl = nullptr;
325 io_object::io_object_impl** impl_out = nullptr;
326
327 1836 void reset() noexcept
328 {
329 1836 select_op::reset();
330 1836 accepted_fd = -1;
331 1836 peer_impl = nullptr;
332 1836 impl_out = nullptr;
333 1836 }
334
335 1831 void perform_io() noexcept override
336 {
337 1831 sockaddr_in addr{};
338 1831 socklen_t addrlen = sizeof(addr);
339
340 // Note: select backend uses accept() + fcntl instead of accept4()
341 // for broader POSIX compatibility
342 1831 int new_fd = ::accept(fd, reinterpret_cast<sockaddr*>(&addr), &addrlen);
343
344
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 1831 if (new_fd >= 0)
345 {
346 // Reject fds that exceed select()'s FD_SETSIZE limit.
347 // Better to fail now than during later async operations.
348
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 1831 if (new_fd >= FD_SETSIZE)
349 {
350 ✗ ::close(new_fd);
351 ✗ complete(EINVAL, 0);
352 ✗ return;
353 }
354
355 // Set non-blocking and close-on-exec flags.
356 // A non-blocking socket is essential for the async reactor;
357 // if we can't configure it, fail rather than risk blocking.
358 1831 int flags = ::fcntl(new_fd, F_GETFL, 0);
359
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 1831 if (flags == -1)
360 {
361 ✗ int err = errno;
362 ✗ ::close(new_fd);
363 ✗ complete(err, 0);
364 ✗ return;
365 }
366
367
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 1831 if (::fcntl(new_fd, F_SETFL, flags | O_NONBLOCK) == -1)
368 {
369 ✗ int err = errno;
370 ✗ ::close(new_fd);
371 ✗ complete(err, 0);
372 ✗ return;
373 }
374
375
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 1831 if (::fcntl(new_fd, F_SETFD, FD_CLOEXEC) == -1)
376 {
377 ✗ int err = errno;
378 ✗ ::close(new_fd);
379 ✗ complete(err, 0);
380 ✗ return;
381 }
382
383 1831 accepted_fd = new_fd;
384 1831 complete(0, 0);
385 }
386 else
387 {
388 ✗ complete(errno, 0);
389 }
390 }
391
392 // Defined in acceptors.cpp where select_acceptor_impl is complete
393 void operator()() override;
394 void cancel() noexcept override;
395 };
396
397 } // namespace boost::corosio::detail
398
399 #endif // BOOST_COROSIO_HAS_SELECT
400
401 #endif // BOOST_COROSIO_DETAIL_SELECT_OP_HPP
402