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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 12285 select_op() = default;
139
140 182166 void reset() noexcept
141 {
142 182166 fd = -1;
143 182166 errn = 0;
144 182166 bytes_transferred = 0;
145 182166 cancelled.store(false, std::memory_order_relaxed);
146 182166 registered.store(select_registration_state::unregistered, std::memory_order_relaxed);
147 182166 impl_ptr.reset();
148 182166 socket_impl_ = nullptr;
149 182166 acceptor_impl_ = nullptr;
150 182166 }
151
152 178093 void operator()() override
153 {
154 178093 stop_cb.reset();
155
156
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 178093 if (ec_out)
157 {
158
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 178093 if (cancelled.load(std::memory_order_acquire))
159 197 *ec_out = capy::error::canceled;
160
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 177896 else if (errn != 0)
161 1 *ec_out = make_err(errn);
162
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 177895 else if (is_read_operation() && bytes_transferred == 0)
163 5 *ec_out = capy::error::eof;
164 else
165 177890 *ec_out = {};
166 }
167
168
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 178093 if (bytes_out)
169 178093 *bytes_out = bytes_transferred;
170
171 // Move to stack before destroying the frame
172 178093 capy::executor_ref saved_ex( std::move( ex ) );
173 178093 std::coroutine_handle<> saved_h( std::move( h ) );
174 178093 impl_ptr.reset();
175
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 178093 dispatch_coro(saved_ex, saved_h).resume();
176 178093 }
177
178 88961 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 19091 void request_cancel() noexcept
188 {
189 19091 cancelled.store(true, std::memory_order_release);
190 19091 }
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 180129 void start(std::stop_token token, select_socket_impl* impl)
204 {
205 180129 cancelled.store(false, std::memory_order_release);
206 180129 stop_cb.reset();
207 180129 socket_impl_ = impl;
208 180129 acceptor_impl_ = nullptr;
209
210
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 180129 if (token.stop_possible())
211 98 stop_cb.emplace(token, canceller{this});
212 180129 }
213
214 2037 void start(std::stop_token token, select_acceptor_impl* impl)
215 {
216 2037 cancelled.store(false, std::memory_order_release);
217 2037 stop_cb.reset();
218 2037 socket_impl_ = nullptr;
219 2037 acceptor_impl_ = impl;
220
221
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 2037 if (token.stop_possible())
222 ✗ stop_cb.emplace(token, canceller{this});
223 2037 }
224
225 182006 void complete(int err, std::size_t bytes) noexcept
226 {
227 182006 errn = err;
228 182006 bytes_transferred = bytes;
229 182006 }
230
231 ✗ virtual void perform_io() noexcept {}
232 };
233
234
235 struct select_connect_op : select_op
236 {
237 endpoint target_endpoint;
238
239 2036 void reset() noexcept
240 {
241 2036 select_op::reset();
242 2036 target_endpoint = endpoint{};
243 2036 }
244
245 2036 void perform_io() noexcept override
246 {
247 // connect() completion status is retrieved via SO_ERROR, not return value
248 2036 int err = 0;
249 2036 socklen_t len = sizeof(err);
250
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 2036 if (::getsockopt(fd, SOL_SOCKET, SO_ERROR, &err, &len) < 0)
251 ✗ err = errno;
252 2036 complete(err, 0);
253 2036 }
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 88934 bool is_read_operation() const noexcept override
269 {
270 88934 return !empty_buffer_read;
271 }
272
273 89127 void reset() noexcept
274 {
275 89127 select_op::reset();
276 89127 iovec_count = 0;
277 89127 empty_buffer_read = false;
278 89127 }
279
280 122 void perform_io() noexcept override
281 {
282 122 ssize_t n = ::readv(fd, iovecs, iovec_count);
283
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 122 if (n >= 0)
284 122 complete(0, static_cast<std::size_t>(n));
285 else
286 ✗ complete(errno, 0);
287 122 }
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 88966 void reset() noexcept
300 {
301 88966 select_op::reset();
302 88966 iovec_count = 0;
303 88966 }
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 2037 void reset() noexcept
329 {
330 2037 select_op::reset();
331 2037 accepted_fd = -1;
332 2037 peer_impl = nullptr;
333 2037 impl_out = nullptr;
334 2037 }
335
336 2032 void perform_io() noexcept override
337 {
338 2032 sockaddr_in addr{};
339 2032 socklen_t addrlen = sizeof(addr);
340
341 // Note: select backend uses accept() + fcntl instead of accept4()
342 // for broader POSIX compatibility
343 2032 int new_fd = ::accept(fd, reinterpret_cast<sockaddr*>(&addr), &addrlen);
344
345
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 2032 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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 2032 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 2032 int flags = ::fcntl(new_fd, F_GETFL, 0);
360
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 2032 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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 2032 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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 2032 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 2032 accepted_fd = new_fd;
385 2032 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