include/boost/corosio/tcp_server.hpp

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tcp_server.hpp

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1		//
2		// Copyright (c) 2026 Vinnie Falco (vinnie.falco@gmail.com)
3		// Copyright (c) 2026 Michael Vandeberg
4		//
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)
7		//
8		// Official repository: https://github.com/cppalliance/corosio
9		//
10
11		#ifndef BOOST_COROSIO_TCP_SERVER_HPP
12		#define BOOST_COROSIO_TCP_SERVER_HPP
13
14		#include <boost/corosio/detail/config.hpp>
15		#include <boost/corosio/detail/except.hpp>
16		#include <boost/corosio/tcp_acceptor.hpp>
17		#include <boost/corosio/tcp_socket.hpp>
18		#include <boost/corosio/io_context.hpp>
19		#include <boost/corosio/endpoint.hpp>
20		#include <boost/capy/task.hpp>
21		#include <boost/capy/concept/execution_context.hpp>
22		#include <boost/capy/concept/io_awaitable.hpp>
23		#include <boost/capy/concept/executor.hpp>
24		#include <boost/capy/ex/any_executor.hpp>
25		#include <boost/capy/ex/frame_allocator.hpp>
26		#include <boost/capy/ex/io_env.hpp>
27		#include <boost/capy/ex/run_async.hpp>
28
29		#include <coroutine>
30		#include <memory>
31		#include <ranges>
32		#include <vector>
33
34		namespace boost::corosio {
35
36		#ifdef _MSC_VER
37		#pragma warning(push)
38		#pragma warning(disable : 4251) // class needs to have dll-interface
39		#endif
40
41		/** TCP server with pooled workers.
42
43		This class manages a pool of reusable worker objects that handle
44		incoming connections. When a connection arrives, an idle worker
45		is dispatched to handle it. After the connection completes, the
46		worker returns to the pool for reuse, avoiding allocation overhead
47		per connection.
48
49		Workers are set via @ref set_workers as a forward range of
50		pointer-like objects (e.g., `unique_ptr<worker_base>`). The server
51		takes ownership of the container via type erasure.
52
53		@par Thread Safety
54		Distinct objects: Safe.
55		Shared objects: Unsafe.
56
57		@par Lifecycle
58		The server operates in three states:
59
60		- Stopped: Initial state, or after @ref join completes.
61		- Running: After @ref start, actively accepting connections.
62		- Stopping: After @ref stop, draining active work.
63
64		State transitions:
65		@code
66		[Stopped] --start()--> [Running] --stop()--> [Stopping] --join()--> [Stopped]
67		@endcode
68
69		@par Running the Server
70		@par !example running_the_server
71
72		@par Graceful Shutdown
73		To shut down gracefully, call @ref stop then drain the io_context:
74		@par !example graceful_shutdown
75
76		@par Restart After Stop
77		The server can be restarted after a complete shutdown cycle.
78		You must drain the io_context, call @ref join, and restart the
79		io_context itself (`ioc.restart()`) before restarting:
80		@par !example restart_after_stop
81
82		@par WARNING: What NOT to Do
83		- Do NOT call @ref join from inside a worker coroutine (deadlock).
84		- Do NOT call @ref join from a thread running `ioc.run()` (deadlock).
85		- Do NOT call @ref start without completing @ref join after @ref stop.
86		- Do NOT call `ioc.stop()` for graceful shutdown; use @ref stop instead.
87
88		@par Example
89		@par !example custom_worker
90
91		@see worker_base, set_workers, launcher
92		*/
93		class BOOST_COROSIO_DECL tcp_server
94		{
95		public:
96		class worker_base; ///< Abstract base for connection handlers.
97		class launcher; ///< Move-only handle to launch worker coroutines.
98
99		private:
100		struct waiter
101		{
102		waiter* next;
103		std::coroutine_handle<> h;
104		capy::continuation cont;
105		worker_base* w;
106		};
107
108		struct impl;
109
110		static impl* make_impl(capy::execution_context& ctx);
111
112		impl* impl_;
113		capy::any_executor ex_;
114		waiter* waiters_ = nullptr;
115		worker_base* idle_head_ = nullptr; // Forward list: available workers
116		worker_base* active_head_ =
117		nullptr; // Doubly linked: workers handling connections
118		worker_base* active_tail_ = nullptr; // Tail for O(1) push_back
119		std::size_t active_accepts_ = 0; // Number of active do_accept coroutines
120		std::shared_ptr<void> storage_; // Owns the worker container (type-erased)
121		bool running_ = false;
122
123		// Idle list (forward/singly linked) - push front, pop front
124	238x	void idle_push(worker_base* w) noexcept
125		{
126	238x	w->next_ = idle_head_;
127	238x	idle_head_ = w;
128	238x	}
129
130	75x	worker_base* idle_pop() noexcept
131		{
132	75x	auto* w = idle_head_;
133	75x	if (w)
134	75x	idle_head_ = w->next_;
135	75x	return w;
136		}
137
138	153x	bool idle_empty() const noexcept
139		{
140	153x	return idle_head_ == nullptr;
141		}
142
143		// Active list (doubly linked) - push back, remove anywhere
144	88x	void active_push(worker_base* w) noexcept
145		{
146	88x	w->next_ = nullptr;
147	88x	w->prev_ = active_tail_;
148	88x	if (active_tail_)
149	4x	active_tail_->next_ = w;
150		else
151	84x	active_head_ = w;
152	88x	active_tail_ = w;
153	88x	}
154
155	153x	void active_remove(worker_base* w) noexcept
156		{
157		// Skip if not in active list (e.g., after failed accept)
158	153x	if (w != active_head_ && w->prev_ == nullptr)
159	65x	return;
160	88x	if (w->prev_)
161	4x	w->prev_->next_ = w->next_;
162		else
163	84x	active_head_ = w->next_;
164	88x	if (w->next_)
165	2x	w->next_->prev_ = w->prev_;
166		else
167	86x	active_tail_ = w->prev_;
168	88x	w->prev_ = nullptr; // Mark as not in active list
169		}
170
171		template<capy::Executor Ex>
172		struct launch_wrapper
173		{
174		struct promise_type
175		{
176		Ex ex; // Executor stored directly in frame (outlives child tasks)
177		capy::io_env env_;
178
179		// For regular coroutines: first arg is executor, second is stop token
180		template<class E, class S, class... Args>
181		requires capy::Executor<std::decay_t<E>>
182		promise_type(E e, S s, Args&&...)
183		: ex(std::move(e))
184		, env_{
185		capy::executor_ref(ex), std::move(s),
186		capy::get_current_frame_allocator()}
187		{
188		}
189
190		// For lambda coroutines: first arg is closure, second is executor, third is stop token
191		template<class Closure, class E, class S, class... Args>
192		requires(!capy::Executor<std::decay_t<Closure>> &&
193		capy::Executor<std::decay_t<E>>)
194	88x	promise_type(Closure&&, E e, S s, Args&&...)
195	88x	: ex(std::move(e))
196	88x	, env_{
197	88x	capy::executor_ref(ex), std::move(s),
198	88x	capy::get_current_frame_allocator()}
199		{
200	88x	}
201
202	88x	launch_wrapper get_return_object() noexcept
203		{
204		return {
205	88x	std::coroutine_handle<promise_type>::from_promise(*this)};
206		}
207	88x	std::suspend_always initial_suspend() noexcept
208		{
209	88x	return {};
210		}
211	88x	std::suspend_never final_suspend() noexcept
212		{
213	88x	return {};
214		}
215	88x	void return_void() noexcept {}
216	✗	void unhandled_exception()
217		{
218		// LCOV_EXCL_START: terminating by contract is not a
219		// coverable outcome.
220	−	std::terminate();
221		// LCOV_EXCL_STOP
222		}
223
224		// Inject io_env for IoAwaitable
225		template<capy::IoAwaitable Awaitable>
226	176x	auto await_transform(Awaitable&& a)
227		{
228		using AwaitableT = std::decay_t<Awaitable>;
229		struct adapter
230		{
231		AwaitableT aw;
232		capy::io_env const* env;
233
234		bool await_ready()
235		{
236		return aw.await_ready();
237		}
238		decltype(auto) await_resume()
239		{
240		return aw.await_resume();
241		}
242
243		auto await_suspend(std::coroutine_handle<promise_type> h)
244		{
245		return aw.await_suspend(h, env);
246		}
247		};
248	264x	return adapter{std::forward<Awaitable>(a), &env_};
249	88x	}
250		};
251
252		std::coroutine_handle<promise_type> h;
253
254	88x	launch_wrapper(std::coroutine_handle<promise_type> handle) noexcept
255	88x	: h(handle)
256		{
257	88x	}
258
259	88x	~launch_wrapper()
260		{
261	88x	if (h)
262	✗	h.destroy();
263	88x	}
264
265		launch_wrapper(launch_wrapper&& o) noexcept
266		: h(std::exchange(o.h, nullptr))
267		{
268		}
269
270		launch_wrapper(launch_wrapper const&) = delete;
271		launch_wrapper& operator=(launch_wrapper const&) = delete;
272		launch_wrapper& operator=(launch_wrapper&&) = delete;
273		};
274
275		// Named functor to avoid incomplete lambda type in coroutine promise
276		template<class Executor>
277		struct launch_coro
278		{
279	88x	launch_wrapper<Executor> operator()(
280		Executor,
281		std::stop_token,
282		tcp_server* self,
283		capy::task<void> t,
284		worker_base* wp)
285		{
286		// Executor and stop token stored in promise via constructor
287		co_await std::move(t);
288		co_await self->push(*wp); // worker goes back to idle list
289	176x	}
290		};
291
292		class push_awaitable
293		{
294		tcp_server& self_;
295		worker_base& w_;
296		capy::continuation cont_;
297
298		public:
299	145x	push_awaitable(tcp_server& self, worker_base& w) noexcept
300	145x	: self_(self)
301	145x	, w_(w)
302		{
303	145x	}
304
305	145x	bool await_ready() const noexcept
306		{
307	145x	return false;
308		}
309
310		std::coroutine_handle<>
311	145x	await_suspend(std::coroutine_handle<> h, capy::io_env const*) noexcept
312		{
313		// Symmetric transfer to server's executor
314	145x	cont_.h = h;
315	145x	return self_.ex_.dispatch(cont_);
316		}
317
318	145x	void await_resume() noexcept
319		{
320		// Running on server executor - safe to modify lists
321		// Remove from active (if present), then wake waiter or add to idle
322	145x	self_.active_remove(&w_);
323	145x	if (self_.waiters_)
324		{
325	76x	auto* wait = self_.waiters_;
326	76x	self_.waiters_ = wait->next;
327	76x	wait->w = &w_;
328	76x	wait->cont.h = wait->h;
329	76x	self_.ex_.post(wait->cont);
330		}
331		else
332		{
333	69x	self_.idle_push(&w_);
334		}
335	145x	}
336		};
337
338		class pop_awaitable
339		{
340		tcp_server& self_;
341		waiter wait_;
342
343		public:
344	153x	pop_awaitable(tcp_server& self) noexcept : self_(self), wait_{} {}
345
346	153x	bool await_ready() const noexcept
347		{
348	153x	return !self_.idle_empty();
349		}
350
351		bool
352	78x	await_suspend(std::coroutine_handle<> h, capy::io_env const*) noexcept
353		{
354		// Running on server executor (do_accept runs there)
355	78x	wait_.h = h;
356	78x	wait_.w = nullptr;
357	78x	wait_.next = self_.waiters_;
358	78x	self_.waiters_ = &wait_;
359	78x	return true;
360		}
361
362	153x	worker_base& await_resume() noexcept
363		{
364		// Running on server executor
365	153x	if (wait_.w)
366	78x	return *wait_.w; // Woken by push_awaitable
367	75x	return *self_.idle_pop();
368		}
369		};
370
371	145x	push_awaitable push(worker_base& w)
372		{
373	145x	return push_awaitable{*this, w};
374		}
375
376		// Synchronous version for destructor/guard paths
377		// Must be called from server executor context
378	8x	void push_sync(worker_base& w) noexcept
379		{
380	8x	active_remove(&w);
381	8x	if (waiters_)
382		{
383	2x	auto* wait = waiters_;
384	2x	waiters_ = wait->next;
385	2x	wait->w = &w;
386	2x	wait->cont.h = wait->h;
387	2x	ex_.post(wait->cont);
388		}
389		else
390		{
391	6x	idle_push(&w);
392		}
393	8x	}
394
395	153x	pop_awaitable pop()
396		{
397	153x	return pop_awaitable{*this};
398		}
399
400		capy::task<void> do_accept(tcp_acceptor& acc);
401
402		public:
403		/** Abstract base class for connection handlers.
404
405		Derive from this class to implement custom connection handling.
406		Each worker owns a socket and is reused across multiple
407		connections to avoid per-connection allocation.
408
409		@see tcp_server, launcher
410		*/
411		class BOOST_COROSIO_DECL worker_base
412		{
413		// Ordered largest to smallest for optimal packing
414		std::stop_source stop_; // ~16 bytes
415		worker_base* next_ = nullptr; // 8 bytes - used by idle and active lists
416		worker_base* prev_ = nullptr; // 8 bytes - used only by active list
417
418		friend class tcp_server;
419
420		public:
421		/// Construct a worker.
422		worker_base();
423
424		/// Destroy the worker.
425		virtual ~worker_base();
426
427		/** Handle an accepted connection.
428
429		Called when this worker is dispatched to handle a new
430		connection. The implementation must invoke the launcher
431		exactly once to start the handling coroutine.
432
433		@param launch Handle to launch the connection coroutine.
434		*/
435		virtual void run(launcher launch) = 0;
436
437		/// Return the socket used for connections.
438		virtual corosio::tcp_socket& socket() = 0;
439		};
440
441		/** Move-only handle to launch a worker coroutine.
442
443		Passed to @ref worker_base::run to start the connection-handling
444		coroutine. The launcher ensures the worker returns to the idle
445		pool when the coroutine completes or if launching fails.
446
447		The launcher must be invoked exactly once via `operator()`.
448		If destroyed without invoking, the worker is returned to the
449		idle pool automatically.
450
451		@see worker_base::run
452		*/
453		class BOOST_COROSIO_DECL launcher
454		{
455		tcp_server* srv_;
456		worker_base* w_;
457
458		friend class tcp_server;
459
460	96x	launcher(tcp_server& srv, worker_base& w) noexcept : srv_(&srv), w_(&w)
461		{
462	96x	}
463
464		public:
465		/// Return the worker to the pool if not launched.
466	98x	~launcher()
467		{
468	98x	if (w_)
469	8x	srv_->push_sync(*w_);
470	98x	}
471
472	2x	launcher(launcher&& o) noexcept
473	2x	: srv_(o.srv_)
474	2x	, w_(std::exchange(o.w_, nullptr))
475		{
476	2x	}
477		launcher(launcher const&) = delete;
478		launcher& operator=(launcher const&) = delete;
479		launcher& operator=(launcher&&) = delete;
480
481		/** Launch the connection-handling coroutine.
482
483		Starts the given coroutine on the specified executor. When
484		the coroutine completes, the worker is automatically returned
485		to the idle pool.
486
487		@param ex The executor to run the coroutine on.
488		@param task The coroutine to execute.
489
490		@throws std::logic_error If this launcher was already invoked.
491		*/
492		template<class Executor>
493	90x	void operator()(Executor const& ex, capy::task<void> task)
494		{
495	90x	if (!w_)
496	2x	detail::throw_logic_error(); // launcher already invoked
497
498	88x	auto* w = std::exchange(w_, nullptr);
499
500		// Worker is being dispatched - add to active list
501	88x	srv_->active_push(w);
502
503		// Return worker to pool if coroutine setup throws
504		struct guard_t
505		{
506		tcp_server* srv;
507		worker_base* w;
508	88x	~guard_t()
509		{
510	88x	if (w)
511	✗	srv->push_sync(*w);
512	88x	}
513	88x	} guard{srv_, w};
514
515		// Reset worker's stop source for this connection
516	88x	w->stop_ = {};
517	88x	auto st = w->stop_.get_token();
518
519	88x	auto wrapper =
520	88x	launch_coro<Executor>{}(ex, st, srv_, std::move(task), w);
521
522		// Executor and stop token stored in promise via constructor
523	88x	ex.post(std::exchange(wrapper.h, nullptr)); // Release before post
524	88x	guard.w = nullptr; // Success - dismiss guard
525	88x	}
526		};
527
528		/** Construct a TCP server.
529
530		@tparam Ctx Execution context type satisfying ExecutionContext.
531		@tparam Ex Executor type satisfying Executor.
532
533		@param ctx The execution context for socket operations.
534		@param ex The executor for dispatching coroutines.
535
536		@par Example
537		@par !example tcp_server
538		*/
539		template<capy::ExecutionContext Ctx, capy::Executor Ex>
540	73x	tcp_server(Ctx& ctx, Ex ex) : impl_(make_impl(ctx))
541	73x	, ex_(std::move(ex))
542		{
543	73x	}
544
545		public:
546		/// Destroy the server, stopping all accept loops.
547		~tcp_server();
548
549		tcp_server(tcp_server const&) = delete;
550		tcp_server& operator=(tcp_server const&) = delete;
551
552		/** Move construct from another server.
553
554		@param o The source server. After the move, @p o is
555		in a valid but unspecified state.
556		*/
557		tcp_server(tcp_server&& o) noexcept;
558
559		/** Move assign from another server.
560
561		@param o The source server. After the move, @p o is
562		in a valid but unspecified state.
563
564		@return `*this`.
565		*/
566		tcp_server& operator=(tcp_server&& o) noexcept;
567
568		/** Bind to a local endpoint.
569
570		Creates an acceptor listening on the specified endpoint.
571		Multiple endpoints can be bound by calling this method
572		multiple times before @ref start.
573
574		@param ep The local endpoint to bind to.
575
576		@return The error code if binding fails.
577		*/
578		[[nodiscard]] std::error_code bind(endpoint ep);
579
580		/** Set the worker pool.
581
582		Replaces any existing workers with the given range. Any
583		previous workers are released and the idle/active lists
584		are cleared before populating with new workers.
585
586		@tparam Range Forward range of pointer-like objects to worker_base.
587
588		@param workers Range of workers to manage. Each element must
589		support `std::to_address()` yielding `worker_base*`.
590
591		@par Example
592		@par !example set_workers
593		*/
594		template<std::ranges::forward_range Range>
595		requires std::convertible_to<
596		decltype(std::to_address(
597		std::declval<std::ranges::range_value_t<Range>&>())),
598		worker_base*>
599	73x	void set_workers(Range&& workers)
600		{
601		// Clear existing state
602	73x	storage_.reset();
603	73x	idle_head_ = nullptr;
604	73x	active_head_ = nullptr;
605	73x	active_tail_ = nullptr;
606
607		// Take ownership and populate idle list
608		using StorageType = std::decay_t<Range>;
609	73x	auto* p = new StorageType(std::forward<Range>(workers));
610	73x	storage_ = std::shared_ptr<void>(
611	73x	p, [](void* ptr) { delete static_cast<StorageType*>(ptr); });
612	236x	for (auto&& elem : static_cast<StorageType>(p))
613	163x	idle_push(std::to_address(elem));
614	73x	}
615
616		/** Start accepting connections.
617
618		Launches accept loops for all bound endpoints. Incoming
619		connections are dispatched to idle workers from the pool.
620
621		Calling `start()` on an already-running server has no effect.
622
623		@par Preconditions
624		- At least one endpoint bound via @ref bind.
625		- Workers provided via @ref set_workers.
626		- If restarting, @ref join must have completed first, and the
627		io_context must have been restarted (`ioc.restart()`).
628
629		@par Effects
630		Creates one accept coroutine per bound endpoint. Each coroutine
631		runs on the server's executor, waiting for connections and
632		dispatching them to idle workers.
633
634		@par Restart Sequence
635		To restart after stopping, complete the full shutdown cycle:
636		@par !example start
637
638		@par Thread Safety
639		Not thread safe.
640
641		@throws std::logic_error If a previous session has not been
642		joined (accept loops still active).
643		*/
644		void start();
645
646		/** Return the local endpoint for the i-th bound port.
647
648		@param index Zero-based index into the list of bound ports.
649
650		@return The local endpoint, or a default-constructed endpoint
651		if @p index is out of range or the acceptor is not open.
652		*/
653		endpoint local_endpoint(std::size_t index = 0) const noexcept;
654
655		/** Stop accepting connections.
656
657		Requests the accept loops' stop token and requests cancellation
658		of active workers via their stop tokens. The acceptors are not
659		closed; a suspended accept completes once more before its loop
660		observes the stop token and ends.
661
662		This function returns immediately; it does not wait for workers
663		to finish. Pending I/O operations complete asynchronously.
664
665		Calling `stop()` on a non-running server has no effect.
666
667		@par Effects
668		- Requests stop on the accept loops' stop token. The acceptors
669		are not closed; a pending accept completes once more before
670		the accept loop ends.
671		- Requests stop on each active worker's stop token.
672		- Workers observing their stop token should exit promptly.
673
674		@par Postconditions
675		No new connections will be accepted. Active workers continue
676		until they observe their stop token or complete naturally.
677
678		@par What Happens Next
679		After calling `stop()`:
680		1. Let `ioc.run()` return (drains pending completions).
681		2. Call @ref join to wait for accept loops to finish.
682		3. Only then is it safe to restart or destroy the server.
683
684		@par Thread Safety
685		Not thread safe.
686
687		@see join, start
688		*/
689		void stop();
690
691		/** Block until all accept loops complete.
692
693		Blocks the calling thread until all accept coroutines launched
694		by @ref start have finished executing. This synchronizes the
695		shutdown sequence, ensuring the server is fully stopped before
696		restarting or destroying it.
697
698		@par Preconditions
699		@ref stop has been called and `ioc.run()` has returned.
700
701		@par Postconditions
702		All accept loops have completed. The server is in the stopped
703		state and may be restarted via @ref start.
704
705		@par Example (Correct Usage)
706		@par !example correct_usage
707
708		@par WARNING: Deadlock Scenario
709		Calling `join()` from inside a worker coroutine deadlocks:
710
711		@par !example deadlock_scenarios
712
713		@par Thread Safety
714		May be called from any thread, but will deadlock if called
715		from within the io_context event loop or from a worker coroutine.
716
717		@see stop, start
718		*/
719		void join();
720
721		private:
722		capy::task<> do_stop();
723		};
724
725		#ifdef _MSC_VER
726		#pragma warning(pop)
727		#endif
728
729		} // namespace boost::corosio
730
731		#endif
732