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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 11907 select_op() = default;
138
139 167127 void reset() noexcept
140 {
141 167127 fd = -1;
142 167127 errn = 0;
143 167127 bytes_transferred = 0;
144 167127 cancelled.store(false, std::memory_order_relaxed);
145 167127 registered.store(select_registration_state::unregistered, std::memory_order_relaxed);
146 167127 impl_ptr.reset();
147 167127 socket_impl_ = nullptr;
148 167127 acceptor_impl_ = nullptr;
149 167127 }
150
151 163180 void operator()() override
152 {
153 163180 stop_cb.reset();
154
155
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 163180 if (ec_out)
156 {
157
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 163180 if (cancelled.load(std::memory_order_acquire))
158 201 *ec_out = capy::error::canceled;
159
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 162979 else if (errn != 0)
160 1 *ec_out = make_err(errn);
161
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 162978 else if (is_read_operation() && bytes_transferred == 0)
162 5 *ec_out = capy::error::eof;
163 else
164 162973 *ec_out = {};
165 }
166
167
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 163180 if (bytes_out)
168 163180 *bytes_out = bytes_transferred;
169
170 // Move to stack before destroying the frame
171 163180 capy::executor_ref saved_ex( std::move( ex ) );
172 163180 capy::coro saved_h( std::move( h ) );
173 163180 impl_ptr.reset();
174
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 163180 saved_ex.dispatch( saved_h );
175 163180 }
176
177 81525 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 18282 void request_cancel() noexcept
187 {
188 18282 cancelled.store(true, std::memory_order_release);
189 18282 }
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 165153 void start(std::stop_token token, select_socket_impl* impl)
203 {
204 165153 cancelled.store(false, std::memory_order_release);
205 165153 stop_cb.reset();
206 165153 socket_impl_ = impl;
207 165153 acceptor_impl_ = nullptr;
208
209
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 165153 if (token.stop_possible())
210 99 stop_cb.emplace(token, canceller{this});
211 165153 }
212
213 1974 void start(std::stop_token token, select_acceptor_impl* impl)
214 {
215 1974 cancelled.store(false, std::memory_order_release);
216 1974 stop_cb.reset();
217 1974 socket_impl_ = nullptr;
218 1974 acceptor_impl_ = impl;
219
220
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 1974 if (token.stop_possible())
221 ✗ stop_cb.emplace(token, canceller{this});
222 1974 }
223
224 167008 void complete(int err, std::size_t bytes) noexcept
225 {
226 167008 errn = err;
227 167008 bytes_transferred = bytes;
228 167008 }
229
230 ✗ virtual void perform_io() noexcept {}
231 };
232
233
234 struct select_connect_op : select_op
235 {
236 endpoint target_endpoint;
237
238 1973 void reset() noexcept
239 {
240 1973 select_op::reset();
241 1973 target_endpoint = endpoint{};
242 1973 }
243
244 1973 void perform_io() noexcept override
245 {
246 // connect() completion status is retrieved via SO_ERROR, not return value
247 1973 int err = 0;
248 1973 socklen_t len = sizeof(err);
249
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 1973 if (::getsockopt(fd, SOL_SOCKET, SO_ERROR, &err, &len) < 0)
250 ✗ err = errno;
251 1973 complete(err, 0);
252 1973 }
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 81453 bool is_read_operation() const noexcept override
268 {
269 81453 return !empty_buffer_read;
270 }
271
272 81650 void reset() noexcept
273 {
274 81650 select_op::reset();
275 81650 iovec_count = 0;
276 81650 empty_buffer_read = false;
277 81650 }
278
279 80 void perform_io() noexcept override
280 {
281 80 ssize_t n = ::readv(fd, iovecs, iovec_count);
282
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 80 if (n >= 0)
283 80 complete(0, static_cast<std::size_t>(n));
284 else
285 ✗ complete(errno, 0);
286 80 }
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 81530 void reset() noexcept
299 {
300 81530 select_op::reset();
301 81530 iovec_count = 0;
302 81530 }
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 1974 void reset() noexcept
328 {
329 1974 select_op::reset();
330 1974 accepted_fd = -1;
331 1974 peer_impl = nullptr;
332 1974 impl_out = nullptr;
333 1974 }
334
335 1969 void perform_io() noexcept override
336 {
337 1969 sockaddr_in addr{};
338 1969 socklen_t addrlen = sizeof(addr);
339
340 // Note: select backend uses accept() + fcntl instead of accept4()
341 // for broader POSIX compatibility
342 1969 int new_fd = ::accept(fd, reinterpret_cast<sockaddr*>(&addr), &addrlen);
343
344
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 1969 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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 1969 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 1969 int flags = ::fcntl(new_fd, F_GETFL, 0);
359
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 1969 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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 1969 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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 1969 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 1969 accepted_fd = new_fd;
384 1969 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