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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 23094 select_op() = default;
138
139 240194 void reset() noexcept
140 {
141 240194 fd = -1;
142 240194 errn = 0;
143 240194 bytes_transferred = 0;
144 240194 cancelled.store(false, std::memory_order_relaxed);
145 240194 registered.store(select_registration_state::unregistered, std::memory_order_relaxed);
146 240194 impl_ptr.reset();
147 240194 socket_impl_ = nullptr;
148 240194 acceptor_impl_ = nullptr;
149 240194 }
150
151 232522 void operator()() override
152 {
153 232522 stop_cb.reset();
154
155
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 232522 if (ec_out)
156 {
157
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 232522 if (cancelled.load(std::memory_order_acquire))
158 204 *ec_out = capy::error::canceled;
159
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 232318 else if (errn != 0)
160 1 *ec_out = make_err(errn);
161
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 232317 else if (is_read_operation() && bytes_transferred == 0)
162 5 *ec_out = capy::error::eof;
163 else
164 232312 *ec_out = {};
165 }
166
167
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 232522 if (bytes_out)
168 232522 *bytes_out = bytes_transferred;
169
170 // Move to stack before destroying the frame
171 232522 capy::executor_ref saved_ex( std::move( ex ) );
172 232522 capy::coro saved_h( std::move( h ) );
173 232522 impl_ptr.reset();
174
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 232522 saved_ex.dispatch( saved_h );
175 232522 }
176
177 116195 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 35074 void request_cancel() noexcept
187 {
188 35074 cancelled.store(true, std::memory_order_release);
189 35074 }
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 236357 void start(std::stop_token token, select_socket_impl* impl)
203 {
204 236357 cancelled.store(false, std::memory_order_release);
205 236357 stop_cb.reset();
206 236357 socket_impl_ = impl;
207 236357 acceptor_impl_ = nullptr;
208
209
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 236357 if (token.stop_possible())
210 100 stop_cb.emplace(token, canceller{this});
211 236357 }
212
213 3837 void start(std::stop_token token, select_acceptor_impl* impl)
214 {
215 3837 cancelled.store(false, std::memory_order_release);
216 3837 stop_cb.reset();
217 3837 socket_impl_ = nullptr;
218 3837 acceptor_impl_ = impl;
219
220
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 3837 if (token.stop_possible())
221 ✗ stop_cb.emplace(token, canceller{this});
222 3837 }
223
224 240073 void complete(int err, std::size_t bytes) noexcept
225 {
226 240073 errn = err;
227 240073 bytes_transferred = bytes;
228 240073 }
229
230 ✗ virtual void perform_io() noexcept {}
231 };
232
233
234 struct select_connect_op : select_op
235 {
236 endpoint target_endpoint;
237
238 3835 void reset() noexcept
239 {
240 3835 select_op::reset();
241 3835 target_endpoint = endpoint{};
242 3835 }
243
244 3835 void perform_io() noexcept override
245 {
246 // connect() completion status is retrieved via SO_ERROR, not return value
247 3835 int err = 0;
248 3835 socklen_t len = sizeof(err);
249
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 3835 if (::getsockopt(fd, SOL_SOCKET, SO_ERROR, &err, &len) < 0)
250 ✗ err = errno;
251 3835 complete(err, 0);
252 3835 }
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 116122 bool is_read_operation() const noexcept override
268 {
269 116122 return !empty_buffer_read;
270 }
271
272 116322 void reset() noexcept
273 {
274 116322 select_op::reset();
275 116322 iovec_count = 0;
276 116322 empty_buffer_read = false;
277 116322 }
278
279 81 void perform_io() noexcept override
280 {
281 81 ssize_t n = ::readv(fd, iovecs, iovec_count);
282
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 81 if (n >= 0)
283 81 complete(0, static_cast<std::size_t>(n));
284 else
285 ✗ complete(errno, 0);
286 81 }
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 116200 void reset() noexcept
299 {
300 116200 select_op::reset();
301 116200 iovec_count = 0;
302 116200 }
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 3837 void reset() noexcept
328 {
329 3837 select_op::reset();
330 3837 accepted_fd = -1;
331 3837 peer_impl = nullptr;
332 3837 impl_out = nullptr;
333 3837 }
334
335 3832 void perform_io() noexcept override
336 {
337 3832 sockaddr_in addr{};
338 3832 socklen_t addrlen = sizeof(addr);
339
340 // Note: select backend uses accept() + fcntl instead of accept4()
341 // for broader POSIX compatibility
342 3832 int new_fd = ::accept(fd, reinterpret_cast<sockaddr*>(&addr), &addrlen);
343
344
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 3832 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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 3832 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 3832 int flags = ::fcntl(new_fd, F_GETFL, 0);
359
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 3832 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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 3832 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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 3832 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 3832 accepted_fd = new_fd;
384 3832 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