. /include/boost/corosio /native /detail /select /select_scheduler.hpp
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1   // 1   //
2   // Copyright (c) 2026 Steve Gerbino 2   // Copyright (c) 2026 Steve Gerbino
3   // Copyright (c) 2026 Michael Vandeberg 3   // Copyright (c) 2026 Michael Vandeberg
4   // 4   //
5   // Distributed under the Boost Software License, Version 1.0. (See accompanying 5   // Distributed under the Boost Software License, Version 1.0. (See accompanying
6   // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) 6   // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
7   // 7   //
8   // Official repository: https://github.com/cppalliance/corosio 8   // Official repository: https://github.com/cppalliance/corosio
9   // 9   //
10   10  
11   #ifndef BOOST_COROSIO_NATIVE_DETAIL_SELECT_SELECT_SCHEDULER_HPP 11   #ifndef BOOST_COROSIO_NATIVE_DETAIL_SELECT_SELECT_SCHEDULER_HPP
12   #define BOOST_COROSIO_NATIVE_DETAIL_SELECT_SELECT_SCHEDULER_HPP 12   #define BOOST_COROSIO_NATIVE_DETAIL_SELECT_SELECT_SCHEDULER_HPP
13   13  
14   #include <boost/corosio/detail/platform.hpp> 14   #include <boost/corosio/detail/platform.hpp>
15   15  
16   #if BOOST_COROSIO_HAS_SELECT 16   #if BOOST_COROSIO_HAS_SELECT
17   17  
18   #include <boost/corosio/detail/config.hpp> 18   #include <boost/corosio/detail/config.hpp>
19   #include <boost/capy/ex/execution_context.hpp> 19   #include <boost/capy/ex/execution_context.hpp>
20   20  
21   #include <boost/corosio/native/detail/reactor/reactor_scheduler.hpp> 21   #include <boost/corosio/native/detail/reactor/reactor_scheduler.hpp>
22   #include <boost/corosio/native/detail/reactor/reactor_signal_pipe.hpp> 22   #include <boost/corosio/native/detail/reactor/reactor_signal_pipe.hpp>
23   23  
24   #include <boost/corosio/native/detail/select/select_traits.hpp> 24   #include <boost/corosio/native/detail/select/select_traits.hpp>
25   #include <boost/corosio/detail/timer_service.hpp> 25   #include <boost/corosio/detail/timer_service.hpp>
26   #include <boost/corosio/native/detail/make_err.hpp> 26   #include <boost/corosio/native/detail/make_err.hpp>
27   27  
28   #include <boost/corosio/detail/except.hpp> 28   #include <boost/corosio/detail/except.hpp>
29   29  
30   #include <sys/select.h> 30   #include <sys/select.h>
31   #include <unistd.h> 31   #include <unistd.h>
32   #include <errno.h> 32   #include <errno.h>
33   #include <fcntl.h> 33   #include <fcntl.h>
34   34  
35   #include <atomic> 35   #include <atomic>
36   #include <chrono> 36   #include <chrono>
37   #include <cstdint> 37   #include <cstdint>
38   #include <limits> 38   #include <limits>
39   #include <mutex> 39   #include <mutex>
40   #include <new> 40   #include <new>
41   #include <unordered_map> 41   #include <unordered_map>
42   42  
43   namespace boost::corosio::detail { 43   namespace boost::corosio::detail {
44   44  
45   struct select_op; 45   struct select_op;
46   46  
47   /** POSIX scheduler using select() for I/O multiplexing. 47   /** POSIX scheduler using select() for I/O multiplexing.
48   48  
49   This scheduler implements the scheduler interface using the POSIX select() 49   This scheduler implements the scheduler interface using the POSIX select()
50   call for I/O event notification. It inherits the shared reactor threading 50   call for I/O event notification. It inherits the shared reactor threading
51   model from reactor_scheduler: signal state machine, inline completion 51   model from reactor_scheduler: signal state machine, inline completion
52   budget, work counting, and the do_one event loop. 52   budget, work counting, and the do_one event loop.
53   53  
54   The design mirrors epoll_scheduler for behavioral consistency: 54   The design mirrors epoll_scheduler for behavioral consistency:
55   - Same single-reactor thread coordination model 55   - Same single-reactor thread coordination model
56   - Same deferred I/O pattern (reactor marks ready; workers do I/O) 56   - Same deferred I/O pattern (reactor marks ready; workers do I/O)
57   - Same timer integration pattern 57   - Same timer integration pattern
58   58  
59   Known Limitations: 59   Known Limitations:
60   - FD_SETSIZE (~1024) limits maximum concurrent connections 60   - FD_SETSIZE (~1024) limits maximum concurrent connections
61   - O(n) scanning: rebuilds fd_sets each iteration 61   - O(n) scanning: rebuilds fd_sets each iteration
62   - Level-triggered only (no edge-triggered mode) 62   - Level-triggered only (no edge-triggered mode)
63   63  
64   @par Thread Safety 64   @par Thread Safety
65   All public member functions are thread-safe. 65   All public member functions are thread-safe.
66   */ 66   */
67   class BOOST_COROSIO_DECL select_scheduler final : public reactor_scheduler 67   class BOOST_COROSIO_DECL select_scheduler final : public reactor_scheduler
68   { 68   {
69   public: 69   public:
70   /** Construct the scheduler. 70   /** Construct the scheduler.
71   71  
72   Creates a self-pipe for reactor interruption. 72   Creates a self-pipe for reactor interruption.
73   73  
74   @param ctx Reference to the owning execution_context. 74   @param ctx Reference to the owning execution_context.
75   @param concurrency_hint Hint for expected thread count (unused). 75   @param concurrency_hint Hint for expected thread count (unused).
76   */ 76   */
77   select_scheduler(capy::execution_context& ctx, int concurrency_hint = -1); 77   select_scheduler(capy::execution_context& ctx, int concurrency_hint = -1);
78   78  
79   /// Destroy the scheduler. 79   /// Destroy the scheduler.
80   ~select_scheduler() override; 80   ~select_scheduler() override;
81   81  
82   select_scheduler(select_scheduler const&) = delete; 82   select_scheduler(select_scheduler const&) = delete;
83   select_scheduler& operator=(select_scheduler const&) = delete; 83   select_scheduler& operator=(select_scheduler const&) = delete;
84   84  
85   /// Shut down the scheduler, draining pending operations. 85   /// Shut down the scheduler, draining pending operations.
86   void shutdown() override; 86   void shutdown() override;
87   87  
88   /** Return the maximum file descriptor value supported. 88   /** Return the maximum file descriptor value supported.
89   89  
90   Returns FD_SETSIZE - 1, the maximum fd value that can be 90   Returns FD_SETSIZE - 1, the maximum fd value that can be
91   monitored by select(). Operations with fd >= FD_SETSIZE 91   monitored by select(). Operations with fd >= FD_SETSIZE
92   will fail with EINVAL. 92   will fail with EINVAL.
93   93  
94   @return The maximum supported file descriptor value. 94   @return The maximum supported file descriptor value.
95   */ 95   */
96   static constexpr int max_fd() noexcept 96   static constexpr int max_fd() noexcept
97   { 97   {
98   return FD_SETSIZE - 1; 98   return FD_SETSIZE - 1;
99   } 99   }
100   100  
101   /** Register a descriptor for persistent monitoring. 101   /** Register a descriptor for persistent monitoring.
102   102  
103   The fd is added to the registered_descs_ map and will be 103   The fd is added to the registered_descs_ map and will be
104   included in subsequent select() calls. The reactor is 104   included in subsequent select() calls. The reactor is
105   interrupted so a blocked select() rebuilds its fd_sets. 105   interrupted so a blocked select() rebuilds its fd_sets.
106   106  
107   @param fd The file descriptor to register. 107   @param fd The file descriptor to register.
108   @param desc Pointer to descriptor state for this fd. 108   @param desc Pointer to descriptor state for this fd.
109   109  
110   @return The error if the fd cannot be tracked, otherwise a 110   @return The error if the fd cannot be tracked, otherwise a
111   default constructed error code. 111   default constructed error code.
112   */ 112   */
113   std::error_code 113   std::error_code
114   register_descriptor(int fd, reactor_descriptor_state* desc) const; 114   register_descriptor(int fd, reactor_descriptor_state* desc) const;
115   115  
116   /** Deregister a persistently registered descriptor. 116   /** Deregister a persistently registered descriptor.
117   117  
118   @param fd The file descriptor to deregister. 118   @param fd The file descriptor to deregister.
119   */ 119   */
120   void deregister_descriptor(int fd) const; 120   void deregister_descriptor(int fd) const;
121   121  
122   /** Interrupt the reactor so it rebuilds its fd_sets. 122   /** Interrupt the reactor so it rebuilds its fd_sets.
123   123  
124   Called when a write, connect, or write-wait op is registered 124   Called when a write, connect, or write-wait op is registered
125   after the reactor's snapshot was taken. Without this, 125   after the reactor's snapshot was taken. Without this,
126   select() may block not watching for writability on the fd. 126   select() may block not watching for writability on the fd.
127   */ 127   */
128   void notify_reactor() const; 128   void notify_reactor() const;
129   129  
130   /// Watch the read end of the POSIX signal self-pipe (see scheduler.hpp). 130   /// Watch the read end of the POSIX signal self-pipe (see scheduler.hpp).
HITCBC 131   61 [[nodiscard]] std::error_code register_signal_reader(int read_fd) override 131   61 [[nodiscard]] std::error_code register_signal_reader(int read_fd) override
132   { 132   {
HITCBC 133   61 return register_descriptor(read_fd, signal_pipe_reader_.arm()); 133   61 return register_descriptor(read_fd, signal_pipe_reader_.arm());
134   } 134   }
135   135  
136   private: 136   private:
137   void run_task(lock_type& lock, context_type& ctx, long timeout_us) override; 137   void run_task(lock_type& lock, context_type& ctx, long timeout_us) override;
138   void interrupt_reactor() const override; 138   void interrupt_reactor() const override;
139   long calculate_timeout(long requested_timeout_us) const; 139   long calculate_timeout(long requested_timeout_us) const;
140   140  
141   // Watches the global signal self-pipe's read end (armed lazily by 141   // Watches the global signal self-pipe's read end (armed lazily by
142   // register_signal_reader on the first signal registration). 142   // register_signal_reader on the first signal registration).
143   reactor_signal_pipe_reader signal_pipe_reader_; 143   reactor_signal_pipe_reader signal_pipe_reader_;
144   144  
145   // Self-pipe for interrupting select() 145   // Self-pipe for interrupting select()
146   int pipe_fds_[2]; // [0]=read, [1]=write 146   int pipe_fds_[2]; // [0]=read, [1]=write
147   147  
148   // Per-fd tracking for fd_set building 148   // Per-fd tracking for fd_set building
149   mutable std::unordered_map<int, reactor_descriptor_state*> 149   mutable std::unordered_map<int, reactor_descriptor_state*>
150   registered_descs_; 150   registered_descs_;
151   mutable int max_fd_ = -1; 151   mutable int max_fd_ = -1;
152   }; 152   };
153   153  
HITCBC 154   950 inline select_scheduler::select_scheduler(capy::execution_context& ctx, int) 154   950 inline select_scheduler::select_scheduler(capy::execution_context& ctx, int)
HITCBC 155   950 : pipe_fds_{-1, -1} 155   950 : pipe_fds_{-1, -1}
HITCBC 156   950 , max_fd_(-1) 156   950 , max_fd_(-1)
157   { 157   {
HITCBC 158   950 if (::pipe(pipe_fds_) < 0) 158   950 if (::pipe(pipe_fds_) < 0)
HITCBC 159   1 detail::throw_system_error(make_err(errno), "pipe"); 159   1 detail::throw_system_error(make_err(errno), "pipe");
160   160  
HITCBC 161   2838 for (int i = 0; i < 2; ++i) 161   2838 for (int i = 0; i < 2; ++i)
162   { 162   {
HITCBC 163   1895 int flags = ::fcntl(pipe_fds_[i], F_GETFL, 0); 163   1895 int flags = ::fcntl(pipe_fds_[i], F_GETFL, 0);
HITCBC 164   1895 if (flags == -1) 164   1895 if (flags == -1)
165   { 165   {
HITCBC 166   2 int errn = errno; 166   2 int errn = errno;
HITCBC 167   2 ::close(pipe_fds_[0]); 167   2 ::close(pipe_fds_[0]);
HITCBC 168   2 ::close(pipe_fds_[1]); 168   2 ::close(pipe_fds_[1]);
HITCBC 169   2 detail::throw_system_error(make_err(errn), "fcntl F_GETFL"); 169   2 detail::throw_system_error(make_err(errn), "fcntl F_GETFL");
170   } 170   }
HITCBC 171   1893 if (::fcntl(pipe_fds_[i], F_SETFL, flags | O_NONBLOCK) == -1) 171   1893 if (::fcntl(pipe_fds_[i], F_SETFL, flags | O_NONBLOCK) == -1)
172   { 172   {
HITCBC 173   2 int errn = errno; 173   2 int errn = errno;
HITCBC 174   2 ::close(pipe_fds_[0]); 174   2 ::close(pipe_fds_[0]);
HITCBC 175   2 ::close(pipe_fds_[1]); 175   2 ::close(pipe_fds_[1]);
HITCBC 176   2 detail::throw_system_error(make_err(errn), "fcntl F_SETFL"); 176   2 detail::throw_system_error(make_err(errn), "fcntl F_SETFL");
177   } 177   }
HITCBC 178   1891 if (::fcntl(pipe_fds_[i], F_SETFD, FD_CLOEXEC) == -1) 178   1891 if (::fcntl(pipe_fds_[i], F_SETFD, FD_CLOEXEC) == -1)
179   { 179   {
HITCBC 180   2 int errn = errno; 180   2 int errn = errno;
HITCBC 181   2 ::close(pipe_fds_[0]); 181   2 ::close(pipe_fds_[0]);
HITCBC 182   2 ::close(pipe_fds_[1]); 182   2 ::close(pipe_fds_[1]);
HITCBC 183   2 detail::throw_system_error(make_err(errn), "fcntl F_SETFD"); 183   2 detail::throw_system_error(make_err(errn), "fcntl F_SETFD");
184   } 184   }
185   } 185   }
186   186  
HITCBC 187   943 timer_svc_ = &get_timer_service(ctx, *this); 187   943 timer_svc_ = &get_timer_service(ctx, *this);
HITCBC 188   943 timer_svc_->set_on_earliest_changed( 188   943 timer_svc_->set_on_earliest_changed(
HITCBC 189   3763 timer_service::callback(this, [](void* p) { 189   3830 timer_service::callback(this, [](void* p) {
HITCBC 190   2820 static_cast<select_scheduler*>(p)->interrupt_reactor(); 190   2887 static_cast<select_scheduler*>(p)->interrupt_reactor();
HITCBC 191   2820 })); 191   2887 }));
192   192  
HITCBC 193   943 completed_ops_.push(&task_op_); 193   943 completed_ops_.push(&task_op_);
HITCBC 194   964 } 194   964 }
195   195  
HITCBC 196   1886 inline select_scheduler::~select_scheduler() 196   1886 inline select_scheduler::~select_scheduler()
197   { 197   {
HITCBC 198   943 if (pipe_fds_[0] >= 0) 198   943 if (pipe_fds_[0] >= 0)
HITCBC 199   943 ::close(pipe_fds_[0]); 199   943 ::close(pipe_fds_[0]);
HITCBC 200   943 if (pipe_fds_[1] >= 0) 200   943 if (pipe_fds_[1] >= 0)
HITCBC 201   943 ::close(pipe_fds_[1]); 201   943 ::close(pipe_fds_[1]);
HITCBC 202   1886 } 202   1886 }
203   203  
204   inline void 204   inline void
HITCBC 205   943 select_scheduler::shutdown() 205   943 select_scheduler::shutdown()
206   { 206   {
HITCBC 207   943 shutdown_drain(); 207   943 shutdown_drain();
208   208  
HITCBC 209   943 if (pipe_fds_[1] >= 0) 209   943 if (pipe_fds_[1] >= 0)
HITCBC 210   943 interrupt_reactor(); 210   943 interrupt_reactor();
HITCBC 211   943 } 211   943 }
212   212  
213   inline std::error_code 213   inline std::error_code
HITCBC 214   4955 select_scheduler::register_descriptor( 214   4924 select_scheduler::register_descriptor(
215   int fd, reactor_descriptor_state* desc) const 215   int fd, reactor_descriptor_state* desc) const
216   { 216   {
HITCBC 217   4955 if (fd < 0 || fd >= FD_SETSIZE) 217   4924 if (fd < 0 || fd >= FD_SETSIZE)
HITCBC 218   1 return make_err(EMFILE); 218   1 return make_err(EMFILE);
219   219  
HITCBC 220   4954 desc->registered_events = reactor_event_read | reactor_event_write; 220   4923 desc->registered_events = reactor_event_read | reactor_event_write;
HITCBC 221   4954 desc->fd = fd; 221   4923 desc->fd = fd;
HITCBC 222   4954 desc->scheduler_ = this; 222   4923 desc->scheduler_ = this;
HITCBC 223   4954 desc->mutex.set_enabled(reactor_io_locking_); 223   4923 desc->mutex.set_enabled(reactor_io_locking_);
HITCBC 224   4954 desc->ready_events_.store(0, std::memory_order_relaxed); 224   4923 desc->ready_events_.store(0, std::memory_order_relaxed);
225   225  
226   { 226   {
HITCBC 227   4954 conditionally_enabled_mutex::scoped_lock lock(desc->mutex); 227   4923 conditionally_enabled_mutex::scoped_lock lock(desc->mutex);
HITCBC 228   4954 desc->impl_ref_.reset(); 228   4923 desc->impl_ref_.reset();
HITCBC 229   4954 desc->read_ready = false; 229   4923 desc->read_ready = false;
HITCBC 230   4954 desc->write_ready = false; 230   4923 desc->write_ready = false;
HITCBC 231   4954 } 231   4923 }
232   232  
233   { 233   {
HITCBC 234   4954 mutex_type::scoped_lock lock(mutex_); 234   4923 mutex_type::scoped_lock lock(mutex_);
235   try 235   try
236   { 236   {
HITCBC 237   4954 registered_descs_[fd] = desc; 237   4923 registered_descs_[fd] = desc;
238   } 238   }
HITCBC 239   1 catch (std::bad_alloc const&) 239   1 catch (std::bad_alloc const&)
240   { 240   {
HITCBC 241   1 return make_err(ENOMEM); 241   1 return make_err(ENOMEM);
HITCBC 242   1 } 242   1 }
HITCBC 243   4953 if (fd > max_fd_) 243   4922 if (fd > max_fd_)
HITCBC 244   4903 max_fd_ = fd; 244   4872 max_fd_ = fd;
HITCBC 245   4954 } 245   4923 }
246   246  
HITCBC 247   4953 interrupt_reactor(); 247   4922 interrupt_reactor();
HITCBC 248   4953 return {}; 248   4922 return {};
249   } 249   }
250   250  
251   inline void 251   inline void
HITCBC 252   4893 select_scheduler::deregister_descriptor(int fd) const 252   4862 select_scheduler::deregister_descriptor(int fd) const
253   { 253   {
HITCBC 254   4893 mutex_type::scoped_lock lock(mutex_); 254   4862 mutex_type::scoped_lock lock(mutex_);
255   255  
HITCBC 256   4893 auto it = registered_descs_.find(fd); 256   4862 auto it = registered_descs_.find(fd);
HITCBC 257   4893 if (it == registered_descs_.end()) 257   4862 if (it == registered_descs_.end())
MISUBC 258   ✗ return; 258   ✗ return;
259   259  
HITCBC 260   4893 registered_descs_.erase(it); 260   4862 registered_descs_.erase(it);
261   261  
HITCBC 262   4893 if (fd == max_fd_) 262   4862 if (fd == max_fd_)
263   { 263   {
HITCBC 264   4559 max_fd_ = pipe_fds_[0]; 264   4527 max_fd_ = pipe_fds_[0];
HITCBC 265   8712 for (auto& [registered_fd, state] : registered_descs_) 265   8648 for (auto& [registered_fd, state] : registered_descs_)
266   { 266   {
HITCBC 267   4153 if (registered_fd > max_fd_) 267   4121 if (registered_fd > max_fd_)
HITCBC 268   4060 max_fd_ = registered_fd; 268   4028 max_fd_ = registered_fd;
269   } 269   }
270   } 270   }
HITCBC 271   4893 } 271   4862 }
272   272  
273   inline void 273   inline void
HITCBC 274   2205 select_scheduler::notify_reactor() const 274   2190 select_scheduler::notify_reactor() const
275   { 275   {
HITCBC 276   2205 interrupt_reactor(); 276   2190 interrupt_reactor();
HITCBC 277   2205 } 277   2190 }
278   278  
279   inline void 279   inline void
HITCBC 280   12647 select_scheduler::interrupt_reactor() const 280   12602 select_scheduler::interrupt_reactor() const
281   { 281   {
HITCBC 282   12647 char byte = 1; 282   12602 char byte = 1;
HITCBC 283   12647 [[maybe_unused]] auto r = ::write(pipe_fds_[1], &byte, 1); 283   12602 [[maybe_unused]] auto r = ::write(pipe_fds_[1], &byte, 1);
HITCBC 284   12647 } 284   12602 }
285   285  
286   inline long 286   inline long
HITCBC 287   272683 select_scheduler::calculate_timeout(long requested_timeout_us) const 287   277282 select_scheduler::calculate_timeout(long requested_timeout_us) const
288   { 288   {
HITCBC 289   272683 if (requested_timeout_us == 0) 289   277282 if (requested_timeout_us == 0)
290   return 0; // LCOV_EXCL_LINE run_task passes 0 via task_interrupted_, never through this argument 290   return 0; // LCOV_EXCL_LINE run_task passes 0 via task_interrupted_, never through this argument
291   291  
HITCBC 292   272683 auto nearest = timer_svc_->nearest_expiry(); 292   277282 auto nearest = timer_svc_->nearest_expiry();
HITCBC 293   272683 if (nearest == timer_service::time_point::max()) 293   277282 if (nearest == timer_service::time_point::max())
HITCBC 294   1144 return requested_timeout_us; 294   1095 return requested_timeout_us;
295   295  
HITCBC 296   271539 auto now = std::chrono::steady_clock::now(); 296   276187 auto now = std::chrono::steady_clock::now();
HITCBC 297   271539 if (nearest <= now) 297   276187 if (nearest <= now)
HITCBC 298   399 return 0; 298   416 return 0;
299   299  
300   auto timer_timeout_us = 300   auto timer_timeout_us =
HITCBC 301   271140 std::chrono::duration_cast<std::chrono::microseconds>(nearest - now) 301   275771 std::chrono::duration_cast<std::chrono::microseconds>(nearest - now)
HITCBC 302   271140 .count(); 302   275771 .count();
303   303  
HITCBC 304   271140 constexpr auto long_max = 304   275771 constexpr auto long_max =
305   static_cast<long long>((std::numeric_limits<long>::max)()); 305   static_cast<long long>((std::numeric_limits<long>::max)());
306   auto capped_timer_us = 306   auto capped_timer_us =
HITCBC 307   271140 (std::min)((std::max)(static_cast<long long>(timer_timeout_us), 307   275771 (std::min)((std::max)(static_cast<long long>(timer_timeout_us),
HITCBC 308   271140 static_cast<long long>(0)), 308   275771 static_cast<long long>(0)),
HITCBC 309   271140 long_max); 309   275771 long_max);
310   310  
HITCBC 311   271140 if (requested_timeout_us < 0) 311   275771 if (requested_timeout_us < 0)
HITCBC 312   271138 return static_cast<long>(capped_timer_us); 312   275769 return static_cast<long>(capped_timer_us);
313   313  
314   return static_cast<long>( 314   return static_cast<long>(
HITCBC 315   2 (std::min)(static_cast<long long>(requested_timeout_us), 315   2 (std::min)(static_cast<long long>(requested_timeout_us),
HITCBC 316   2 capped_timer_us)); 316   2 capped_timer_us));
317   } 317   }
318   318  
319   inline void 319   inline void
HITCBC 320   297635 select_scheduler::run_task(lock_type& lock, context_type& ctx, long timeout_us) 320   302709 select_scheduler::run_task(lock_type& lock, context_type& ctx, long timeout_us)
321   { 321   {
322   long effective_timeout_us = 322   long effective_timeout_us =
HITCBC 323   297635 task_interrupted_ ? 0 : calculate_timeout(timeout_us); 323   302709 task_interrupted_ ? 0 : calculate_timeout(timeout_us);
324   324  
325   // Snapshot registered descriptors while holding lock. 325   // Snapshot registered descriptors while holding lock.
326   // Record which fds need write monitoring to avoid a hot loop: 326   // Record which fds need write monitoring to avoid a hot loop:
327   // select is level-triggered so writable sockets (nearly always 327   // select is level-triggered so writable sockets (nearly always
328   // writable) would cause select() to return immediately every 328   // writable) would cause select() to return immediately every
329   // iteration if unconditionally added to write_fds. Membership 329   // iteration if unconditionally added to write_fds. Membership
330   // stays opt-in: a parked write wait opts in the same way a 330   // stays opt-in: a parked write wait opts in the same way a
331   // parked write or connect op does. 331   // parked write or connect op does.
332   struct fd_entry 332   struct fd_entry
333   { 333   {
334   int fd; 334   int fd;
335   reactor_descriptor_state* desc; 335   reactor_descriptor_state* desc;
336   bool needs_write; 336   bool needs_write;
337   }; 337   };
338   fd_entry snapshot[FD_SETSIZE]; 338   fd_entry snapshot[FD_SETSIZE];
HITCBC 339   297635 int snapshot_count = 0; 339   302709 int snapshot_count = 0;
340   340  
HITCBC 341   802744 for (auto& [fd, desc] : registered_descs_) 341   799843 for (auto& [fd, desc] : registered_descs_)
342   { 342   {
HITCBC 343   505109 if (snapshot_count < FD_SETSIZE) 343   497134 if (snapshot_count < FD_SETSIZE)
344   { 344   {
HITCBC 345   505109 conditionally_enabled_mutex::scoped_lock desc_lock(desc->mutex); 345   497134 conditionally_enabled_mutex::scoped_lock desc_lock(desc->mutex);
HITCBC 346   505109 snapshot[snapshot_count].fd = fd; 346   497134 snapshot[snapshot_count].fd = fd;
HITCBC 347   505109 snapshot[snapshot_count].desc = desc; 347   497134 snapshot[snapshot_count].desc = desc;
HITCBC 348   505109 snapshot[snapshot_count].needs_write = 348   497134 snapshot[snapshot_count].needs_write =
HITCBC 349   505109 (desc->write_op || desc->connect_op || desc->wait_write_op); 349   497134 (desc->write_op || desc->connect_op || desc->wait_write_op);
HITCBC 350   505109 ++snapshot_count; 350   497134 ++snapshot_count;
HITCBC 351   505109 } 351   497134 }
352   } 352   }
353   353  
HITCBC 354   297635 if (lock.owns_lock()) 354   302709 if (lock.owns_lock())
HITCBC 355   272684 lock.unlock(); 355   277283 lock.unlock();
356   356  
HITCBC 357   297635 task_cleanup on_exit{this, &lock, ctx}; 357   302709 task_cleanup on_exit{this, &lock, ctx};
358   358  
359   fd_set read_fds, write_fds, except_fds; 359   fd_set read_fds, write_fds, except_fds;
HITCBC 360   5059795 FD_ZERO(&read_fds); 360   5146053 FD_ZERO(&read_fds);
HITCBC 361   5059795 FD_ZERO(&write_fds); 361   5146053 FD_ZERO(&write_fds);
HITCBC 362   5059795 FD_ZERO(&except_fds); 362   5146053 FD_ZERO(&except_fds);
363   363  
HITCBC 364   297635 FD_SET(pipe_fds_[0], &read_fds); 364   302709 FD_SET(pipe_fds_[0], &read_fds);
HITCBC 365   297635 int nfds = pipe_fds_[0]; 365   302709 int nfds = pipe_fds_[0];
366   366  
HITCBC 367   802744 for (int i = 0; i < snapshot_count; ++i) 367   799843 for (int i = 0; i < snapshot_count; ++i)
368   { 368   {
HITCBC 369   505109 int fd = snapshot[i].fd; 369   497134 int fd = snapshot[i].fd;
HITCBC 370   505109 FD_SET(fd, &read_fds); 370   497134 FD_SET(fd, &read_fds);
HITCBC 371   505109 if (snapshot[i].needs_write) 371   497134 if (snapshot[i].needs_write)
HITCBC 372   13038 FD_SET(fd, &write_fds); 372   13322 FD_SET(fd, &write_fds);
HITCBC 373   505109 FD_SET(fd, &except_fds); 373   497134 FD_SET(fd, &except_fds);
HITCBC 374   505109 if (fd > nfds) 374   497134 if (fd > nfds)
HITCBC 375   296994 nfds = fd; 375   302101 nfds = fd;
376   } 376   }
377   377  
378   struct timeval tv; 378   struct timeval tv;
HITCBC 379   297635 struct timeval* tv_ptr = nullptr; 379   302709 struct timeval* tv_ptr = nullptr;
HITCBC 380   297635 if (effective_timeout_us >= 0) 380   302709 if (effective_timeout_us >= 0)
381   { 381   {
HITCBC 382   296807 tv.tv_sec = effective_timeout_us / 1000000; 382   301929 tv.tv_sec = effective_timeout_us / 1000000;
HITCBC 383   296807 tv.tv_usec = effective_timeout_us % 1000000; 383   301929 tv.tv_usec = effective_timeout_us % 1000000;
HITCBC 384   296807 tv_ptr = &tv; 384   301929 tv_ptr = &tv;
385   } 385   }
386   386  
HITCBC 387   297635 int ready = ::select(nfds + 1, &read_fds, &write_fds, &except_fds, tv_ptr); 387   302709 int ready = ::select(nfds + 1, &read_fds, &write_fds, &except_fds, tv_ptr);
388   388  
389   // EINTR: signal interrupted select(), just retry. 389   // EINTR: signal interrupted select(), just retry.
390   // EBADF: an fd was closed between snapshot and select(); retry 390   // EBADF: an fd was closed between snapshot and select(); retry
391   // with a fresh snapshot from registered_descs_. 391   // with a fresh snapshot from registered_descs_.
392   // Both fall through with no ready descriptors rather than 392   // Both fall through with no ready descriptors rather than
393   // returning: the caller handed this function an owned lock that 393   // returning: the caller handed this function an owned lock that
394   // only the epilogue below re-acquires. 394   // only the epilogue below re-acquires.
HITCBC 395   297635 if (ready < 0) 395   302709 if (ready < 0)
396   { 396   {
HITCBC 397   3 if (errno != EINTR && errno != EBADF) 397   3 if (errno != EINTR && errno != EBADF)
HITCBC 398   1 detail::throw_system_error(make_err(errno), "select"); 398   1 detail::throw_system_error(make_err(errno), "select");
HITCBC 399   2 ready = 0; 399   2 ready = 0;
400   } 400   }
401   401  
402   // Process timers outside the lock 402   // Process timers outside the lock
HITCBC 403   297634 timer_svc_->process_expired(); 403   302708 timer_svc_->process_expired();
404   404  
HITCBC 405   297634 ready_queue local_ops; 405   302708 ready_queue local_ops;
406   406  
HITCBC 407   297634 if (ready > 0) 407   302708 if (ready > 0)
408   { 408   {
HITCBC 409   280671 if (FD_ISSET(pipe_fds_[0], &read_fds)) 409   285928 if (FD_ISSET(pipe_fds_[0], &read_fds))
410   { 410   {
411   char buf[256]; 411   char buf[256];
HITCBC 412   11990 while (::read(pipe_fds_[0], buf, sizeof(buf)) > 0) 412   11794 while (::read(pipe_fds_[0], buf, sizeof(buf)) > 0)
413   { 413   {
414   } 414   }
415   } 415   }
416   416  
HITCBC 417   735326 for (int i = 0; i < snapshot_count; ++i) 417   737617 for (int i = 0; i < snapshot_count; ++i)
418   { 418   {
HITCBC 419   454655 int fd = snapshot[i].fd; 419   451689 int fd = snapshot[i].fd;
HITCBC 420   454655 reactor_descriptor_state* desc = snapshot[i].desc; 420   451689 reactor_descriptor_state* desc = snapshot[i].desc;
421   421  
HITCBC 422   454655 std::uint32_t flags = 0; 422   451689 std::uint32_t flags = 0;
HITCBC 423   454655 if (FD_ISSET(fd, &read_fds)) 423   451689 if (FD_ISSET(fd, &read_fds))
HITCBC 424   279362 flags |= reactor_event_read; 424   284668 flags |= reactor_event_read;
HITCBC 425   454655 if (FD_ISSET(fd, &write_fds)) 425   451689 if (FD_ISSET(fd, &write_fds))
HITCBC 426   2197 flags |= reactor_event_write; 426   2182 flags |= reactor_event_write;
HITCBC 427   454655 if (FD_ISSET(fd, &except_fds)) 427   451689 if (FD_ISSET(fd, &except_fds))
HITCBC 428   16 flags |= reactor_event_error; 428   16 flags |= reactor_event_error;
429   429  
HITCBC 430   454655 if (flags == 0) 430   451689 if (flags == 0)
HITCBC 431   173104 continue; 431   164848 continue;
432   432  
HITCBC 433   281551 desc->add_ready_events(flags); 433   286841 desc->add_ready_events(flags);
434   434  
HITCBC 435   281551 bool expected = false; 435   286841 bool expected = false;
HITCBC 436   281551 if (desc->is_enqueued_.compare_exchange_strong( 436   286841 if (desc->is_enqueued_.compare_exchange_strong(
437   expected, true, std::memory_order_release, 437   expected, true, std::memory_order_release,
438   std::memory_order_relaxed)) 438   std::memory_order_relaxed))
439   { 439   {
HITCBC 440   281551 local_ops.push(desc); 440   286841 local_ops.push(desc);
441   } 441   }
442   } 442   }
443   } 443   }
444   444  
HITCBC 445   297634 lock.lock(); 445   302708 lock.lock();
446   446  
HITCBC 447   297634 completed_ops_.splice(local_ops); 447   302708 completed_ops_.splice(local_ops);
HITCBC 448   297635 } 448   302709 }
449   449  
450   } // namespace boost::corosio::detail 450   } // namespace boost::corosio::detail
451   451  
452   #endif // BOOST_COROSIO_HAS_SELECT 452   #endif // BOOST_COROSIO_HAS_SELECT
453   453  
454   #endif // BOOST_COROSIO_NATIVE_DETAIL_SELECT_SELECT_SCHEDULER_HPP 454   #endif // BOOST_COROSIO_NATIVE_DETAIL_SELECT_SELECT_SCHEDULER_HPP