Index/include / boost / corosio / native / detail / select / select_op.hpp

include/boost/corosio/native/detail/select/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_NATIVE_DETAIL_SELECT_SELECT_OP_HPP
11 #define BOOST_COROSIO_NATIVE_DETAIL_SELECT_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/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 <boost/corosio/native/detail/make_err.hpp>
26 #include <boost/corosio/detail/dispatch_coro.hpp>
27 #include <boost/corosio/detail/scheduler_op.hpp>
28 #include <boost/corosio/native/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;
94 class select_acceptor;
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{
129 select_registration_state::unregistered};
130 std::optional<std::stop_callback<canceller>> stop_cb;
131
132 // Prevents use-after-free when socket is closed with pending ops.
133 std::shared_ptr<void> impl_ptr;
134
135 // For stop_token cancellation - pointer to owning socket/acceptor impl.
136 select_socket* socket_impl_ = nullptr;
137 select_acceptor* acceptor_impl_ = nullptr;
138
139 31198x select_op() = default;
140
141 153825x void reset() noexcept
142 {
143 153825x fd = -1;
144 153825x errn = 0;
145 153825x bytes_transferred = 0;
146 153825x cancelled.store(false, std::memory_order_relaxed);
147 153825x registered.store(
148 select_registration_state::unregistered, std::memory_order_relaxed);
149 153825x impl_ptr.reset();
150 153825x socket_impl_ = nullptr;
151 153825x acceptor_impl_ = nullptr;
152 153825x }
153
154 146918x void operator()() override
155 {
156 146918x stop_cb.reset();
157
158 146918x if (ec_out)
159 {
160 146918x if (cancelled.load(std::memory_order_acquire))
161 198x *ec_out = capy::error::canceled;
162 146720x else if (errn != 0)
163 1x *ec_out = make_err(errn);
164 146719x else if (is_read_operation() && bytes_transferred == 0)
165 5x *ec_out = capy::error::eof;
166 else
167 146714x *ec_out = {};
168 }
169
170 146918x if (bytes_out)
171 146918x *bytes_out = bytes_transferred;
172
173 // Move to stack before destroying the frame
174 146918x capy::executor_ref saved_ex(ex);
175 146918x std::coroutine_handle<> saved_h(h);
176 146918x impl_ptr.reset();
177 146918x dispatch_coro(saved_ex, saved_h).resume();
178 146918x }
179
180 73373x virtual bool is_read_operation() const noexcept
181 {
182 73373x return false;
183 }
184 virtual void cancel() noexcept = 0;
185
186 ✗ void destroy() override
187 {
188 ✗ stop_cb.reset();
189 ✗ impl_ptr.reset();
190 ✗ }
191
192 94305x void request_cancel() noexcept
193 {
194 94305x cancelled.store(true, std::memory_order_release);
195 94305x }
196
197 void start(std::stop_token const& token)
198 {
199 cancelled.store(false, std::memory_order_release);
200 stop_cb.reset();
201 socket_impl_ = nullptr;
202 acceptor_impl_ = nullptr;
203
204 if (token.stop_possible())
205 stop_cb.emplace(token, canceller{this});
206 }
207
208 150371x void start(std::stop_token const& token, select_socket* impl)
209 {
210 150371x cancelled.store(false, std::memory_order_release);
211 150371x stop_cb.reset();
212 150371x socket_impl_ = impl;
213 150371x acceptor_impl_ = nullptr;
214
215 150371x if (token.stop_possible())
216 98x stop_cb.emplace(token, canceller{this});
217 150371x }
218
219 3454x void start(std::stop_token const& token, select_acceptor* impl)
220 {
221 3454x cancelled.store(false, std::memory_order_release);
222 3454x stop_cb.reset();
223 3454x socket_impl_ = nullptr;
224 3454x acceptor_impl_ = impl;
225
226 3454x if (token.stop_possible())
227 ✗ stop_cb.emplace(token, canceller{this});
228 3454x }
229
230 153664x void complete(int err, std::size_t bytes) noexcept
231 {
232 153664x errn = err;
233 153664x bytes_transferred = bytes;
234 153664x }
235
236 ✗ virtual void perform_io() noexcept {}
237 };
238
239 struct select_connect_op final : select_op
240 {
241 endpoint target_endpoint;
242
243 3453x void reset() noexcept
244 {
245 3453x select_op::reset();
246 3453x target_endpoint = endpoint{};
247 3453x }
248
249 3453x void perform_io() noexcept override
250 {
251 // connect() completion status is retrieved via SO_ERROR, not return value
252 3453x int err = 0;
253 3453x socklen_t len = sizeof(err);
254 3453x if (::getsockopt(fd, SOL_SOCKET, SO_ERROR, &err, &len) < 0)
255 ✗ err = errno;
256 3453x complete(err, 0);
257 3453x }
258
259 // Defined in sockets.cpp where select_socket is complete
260 void operator()() override;
261 void cancel() noexcept override;
262 };
263
264 struct select_read_op final : select_op
265 {
266 static constexpr std::size_t max_buffers = 16;
267 iovec iovecs[max_buffers];
268 int iovec_count = 0;
269 bool empty_buffer_read = false;
270
271 73346x bool is_read_operation() const noexcept override
272 {
273 73346x return !empty_buffer_read;
274 }
275
276 73540x void reset() noexcept
277 {
278 73540x select_op::reset();
279 73540x iovec_count = 0;
280 73540x empty_buffer_read = false;
281 73540x }
282
283 123x void perform_io() noexcept override
284 {
285 123x ssize_t n = ::readv(fd, iovecs, iovec_count);
286 123x if (n >= 0)
287 123x complete(0, static_cast<std::size_t>(n));
288 else
289 ✗ complete(errno, 0);
290 123x }
291
292 void cancel() noexcept override;
293 };
294
295 struct select_write_op final : select_op
296 {
297 static constexpr std::size_t max_buffers = 16;
298 iovec iovecs[max_buffers];
299 int iovec_count = 0;
300
301 73378x void reset() noexcept
302 {
303 73378x select_op::reset();
304 73378x iovec_count = 0;
305 73378x }
306
307 ✗ void perform_io() noexcept override
308 {
309 ✗ msghdr msg{};
310 ✗ msg.msg_iov = iovecs;
311 ✗ msg.msg_iovlen = static_cast<std::size_t>(iovec_count);
312
313 ✗ ssize_t n = ::sendmsg(fd, &msg, MSG_NOSIGNAL);
314 ✗ if (n >= 0)
315 ✗ complete(0, static_cast<std::size_t>(n));
316 else
317 ✗ complete(errno, 0);
318 ✗ }
319
320 void cancel() noexcept override;
321 };
322
323 struct select_accept_op final : select_op
324 {
325 int accepted_fd = -1;
326 io_object::implementation* peer_impl = nullptr;
327 io_object::implementation** impl_out = nullptr;
328
329 3454x void reset() noexcept
330 {
331 3454x select_op::reset();
332 3454x accepted_fd = -1;
333 3454x peer_impl = nullptr;
334 3454x impl_out = nullptr;
335 3454x }
336
337 3449x void perform_io() noexcept override
338 {
339 3449x sockaddr_storage addr_storage{};
340 3449x socklen_t addrlen = sizeof(addr_storage);
341
342 // Note: select backend uses accept() + fcntl instead of accept4()
343 // for broader POSIX compatibility
344 int new_fd =
345 3449x ::accept(fd, reinterpret_cast<sockaddr*>(&addr_storage), &addrlen);
346
347 3449x if (new_fd >= 0)
348 {
349 // Reject fds that exceed select()'s FD_SETSIZE limit.
350 // Better to fail now than during later async operations.
351 3449x if (new_fd >= FD_SETSIZE)
352 {
353 ✗ ::close(new_fd);
354 ✗ complete(EINVAL, 0);
355 ✗ return;
356 }
357
358 // Set non-blocking and close-on-exec flags.
359 // A non-blocking socket is essential for the async reactor;
360 // if we can't configure it, fail rather than risk blocking.
361 3449x int flags = ::fcntl(new_fd, F_GETFL, 0);
362 3449x if (flags == -1)
363 {
364 ✗ int err = errno;
365 ✗ ::close(new_fd);
366 ✗ complete(err, 0);
367 ✗ return;
368 }
369
370 3449x if (::fcntl(new_fd, F_SETFL, flags | O_NONBLOCK) == -1)
371 {
372 ✗ int err = errno;
373 ✗ ::close(new_fd);
374 ✗ complete(err, 0);
375 ✗ return;
376 }
377
378 3449x if (::fcntl(new_fd, F_SETFD, FD_CLOEXEC) == -1)
379 {
380 ✗ int err = errno;
381 ✗ ::close(new_fd);
382 ✗ complete(err, 0);
383 ✗ return;
384 }
385
386 3449x accepted_fd = new_fd;
387 3449x complete(0, 0);
388 }
389 else
390 {
391 ✗ complete(errno, 0);
392 }
393 }
394
395 // Defined in acceptors.cpp where select_acceptor is complete
396 void operator()() override;
397 void cancel() noexcept override;
398 };
399
400 } // namespace boost::corosio::detail
401
402 #endif // BOOST_COROSIO_HAS_SELECT
403
404 #endif // BOOST_COROSIO_NATIVE_DETAIL_SELECT_SELECT_OP_HPP
405