include/boost/corosio/detail/object_pool.hpp

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object_pool.hpp

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1		//
2		// Copyright (c) 2025 Steve Gerbino (steve@gerbino.co)
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_OBJECT_POOL_HPP
11		#define BOOST_COROSIO_DETAIL_OBJECT_POOL_HPP
12
13		#include <boost/corosio/detail/config.hpp>
14		#include <boost/corosio/detail/intrusive.hpp>
15		#include <boost/corosio/detail/object_ref.hpp>
16		#include <atomic>
17		#include <cstddef>
18		#include <initializer_list>
19		#include <limits>
20		#include <mutex>
21		#include <utility>
22		#include <vector>
23
24		namespace boost::corosio::detail {
25
26		/** Per-service recycling pool for I/O implementations.
27
28		Replaces per-impl `shared_ptr` ownership: a service news an impl
29		once, then `recycle()` (the impl's `retire()` action) parks
30		it on the free list and `acquire()` hands it back out, so steady-state
31		construct/destroy allocates nothing. Unbounded; memory is released
32		only on pool destruction. Debug poisoning, where an impl does it,
33		happens in that impl's `retire()`, not here (the pool cannot know
34		which bytes are resettable).
35
36		A service tears down its impls by calling `shutdown()`, which
37		enters shutting-down mode and visits every live impl in one
38		critical section; subsequent zero-ref crossings delete instead of
39		recycling.
40
41		`adopt()` and `remove()` are protected ownership-transfer
42		primitives for sanctioned extensions — a derived pool exposing
43		them to a caller that tracks some of its live impls through its
44		own structure instead of this pool's `live_` list (the timer
45		service, via its expiry heap) — rather than part of the public
46		surface every ordinary service uses.
47
48		@par Thread Safety
49		Distinct objects: Safe. Shared objects: Safe; all operations lock
50		an internal mutex.
51		*/
52		template<class Impl>
53		class object_pool
54		{
55		std::mutex mutex_;
56		intrusive_list<Impl> live_;
57		intrusive_list<Impl> free_;
58		bool shutting_down_ = false;
59
60		/** Marks `refs_` on an impl currently parked in `free_`.
61
62		`intrusive_list::remove()` cannot distinguish "linked in
63		`free_`" from "linked in `live_`" by inspecting the node
64		alone — both lists share one `next_`/`prev_` pair per impl —
65		so `recycle()` cannot safely call `live_.remove()` on an impl
66		it does not already know is live. Stamping this sentinel into
67		`refs_` the moment an impl joins `free_`, and checking it
68		before touching either list, is what lets `recycle()` and
69		`remove()` tell the two cases apart in every build mode, not
70		just under asserts. `acquire()` always overwrites it with 1
71		before the impl is usable again.
72		*/
73		static constexpr std::size_t pooled_sentinel = pooled_refs;
74
75		/// Check whether @p impl is parked in `free_` (`refs_` reads the
76		/// sentinel). Callers must check this before any list operation —
77		/// see the class-level comment on `pooled_sentinel`.
78	86984x	static bool is_pooled(Impl const* impl) noexcept
79		{
80	86984x	return impl->refs_.load(std::memory_order_relaxed) ==
81	86984x	pooled_sentinel;
82		}
83
84		protected:
85		/** Track a fresh or reset impl as live.
86
87		@pre @p impl is not parked in `free_` (`refs_` must not read
88		`pooled_sentinel`). The same list-corruption hazard
89		`recycle()`'s class-level comment documents applies here: a
90		free-listed impl pushed onto `live_` directly, bypassing
91		`acquire()`, would end up linked in both lists through one shared
92		`next_`/`prev_` pair.
93
94		@pre The caller is not racing `shutdown()`. Every caller adopts
95		while constructing an object (`acquire()`, or a derived pool's
96		cache handing a parked impl back out), and construction is
97		expected to stop before the service's own `shutdown()` runs;
98		this is checked only in debug builds because the failure mode
99		is a silently orphaned impl (no close callback), not memory
100		corruption.
101		*/
102	32404x	void adopt(Impl* impl)
103		{
104	32404x	std::lock_guard lock(mutex_);
105	32404x	if (is_pooled(impl))
106		{
107	✗	BOOST_COROSIO_ASSERT(false);
108		return;
109		}
110	32404x	BOOST_COROSIO_ASSERT(!shutting_down_);
111	32404x	live_.push_back(impl);
112	32404x	}
113
114		/** Unlink a live impl without recycling (ownership transfer).
115
116		Used by a caller that is taking an impl out of this pool's
117		bookkeeping entirely (a thread-local cache slot, a direct
118		delete during shutdown) rather than parking it on `free_`.
119
120		@pre @p impl is not parked in `free_` (`refs_` must not read
121		`pooled_sentinel`). The same list-corruption hazard
122		`recycle()`'s class-level comment documents applies here: a
123		free-listed impl would be spliced out using `live_`'s
124		boundary pointers instead of `free_`'s.
125
126		@return True if @p impl was linked in `live_` and has now
127		been removed; false if it was already detached.
128		*/
129	18493x	bool remove(Impl* impl) noexcept
130		{
131	18493x	std::lock_guard lock(mutex_);
132	18493x	if (is_pooled(impl))
133		{
134	✗	BOOST_COROSIO_ASSERT(false);
135		return false;
136		}
137	18493x	return live_.remove(impl);
138	18493x	}
139
140		public:
141		/** Destroy the pool, freeing every impl still parked or live.
142
143		A live impl is deleted whatever its count. Keepalives it holds
144		on itself (an embedded op's `object_ref`) are released by its
145		own destructor; the count is pinned first so none of those
146		releases reaches zero and calls `retire()` on an object that
147		is already being destroyed.
148
149		@pre `shutdown()` has already run, or no impls are live.
150		*/
151	8917x	~object_pool()
152		{
153	8917x	BOOST_COROSIO_ASSERT(shutting_down_ \|\| live_.empty());
154	26751x	for (auto* list : {&free_, &live_})
155		{
156	25800x	while (auto* impl = list->pop_front())
157		{
158	7966x	impl->refs_.store(pooled_sentinel, std::memory_order_relaxed);
159	7966x	delete impl;
160		}
161		}
162	8917x	}
163
164	8907x	object_pool() = default;
165		object_pool(object_pool const&) = delete;
166		object_pool& operator=(object_pool const&) = delete;
167
168		/** Pop-or-create an impl with the acquisition protocol applied.
169
170		Recycled impls are `reuse()`-reset and their reference count
171		restored from the pooled sentinel; fresh impls are constructed
172		in place from `args` on the cold path. Either way the impl is
173		adopted as live. This is the sequence every service's
174		`construct()` used to hand-roll. Making the pool itself the
175		only place that calls `new Impl` gives the whole library one
176		audit point for allocation policy.
177
178		@pre Not called concurrently with, or after, `shutdown()` on
179		this pool. Every caller reaches this through a service's
180		`construct()`, and service shutdown order guarantees no
181		`construct()` outlives that service's own `shutdown()`; the
182		window is only asserted in debug builds (see `adopt()`)
183		because the failure mode is an orphaned impl, not corruption.
184
185		@param args Constructor arguments for `Impl` (cold path only).
186
187		@return The acquired impl, `refs_ == 1`, tracked live.
188		*/
189		template<class... Args>
190	39842x	Impl* acquire(Args&&... args)
191		{
192		Impl* impl;
193		{
194	39842x	std::lock_guard lock(mutex_);
195	39842x	BOOST_COROSIO_ASSERT(!shutting_down_);
196	39842x	impl = free_.pop_front();
197	39842x	if (impl)
198		{
199	22182x	impl->refs_.store(1, std::memory_order_relaxed);
200	22182x	live_.push_back(impl);
201		}
202	39842x	}
203		// reuse() runs unlocked: acquire()'s precondition excludes a
204		// concurrent shutdown() visiting the impl mid-reset.
205	39842x	if (impl)
206		{
207	22182x	impl->reuse();
208	22182x	return impl;
209		}
210	17660x	impl = new Impl(std::forward<Args>(args)...);
211	17660x	adopt(impl);
212	17660x	return impl;
213		}
214
215		/** Move a zero-ref impl from live to the free list.
216
217		Called from `retire()`, possibly on a scheduler thread.
218		Deletes instead when shutting down. The impl must not be
219		touched after this returns.
220
221		Two accounting bugs are possible here, neither from this
222		pool's own bookkeeping but from a caller's refcounting (e.g.
223		a duplicate `retire()` for one zero-crossing). Both are
224		made safe in every build mode, not just under asserts, because
225		neither can be told apart from the correct case by a
226		`BOOST_COROSIO_ASSERT` alone that still runs the list
227		operations around it:
228
229		- `impl` is already parked in `free_` — the list-corruption
230		hazard the class-level `pooled_sentinel` comment documents.
231		The `is_pooled()` check runs first and returns before either
232		list is touched.
233		- `impl` is fully detached from `live_` and is not the
234		sentinel (not shutting down): `live_.remove()` correctly
235		returns false, but then the impl is reachable through
236		neither `acquire()` nor `shutdown()`'s visit while still
237		allocated — stranded. Deleted instead of left to leak.
238
239		While shutting down, an impl that is no longer in `live_` is
240		left alone rather than deleted: something else already took
241		it out of the pool's bookkeeping (a derived pool's owner, or a
242		duplicate retirement of an impl already deleted), and deleting
243		it again here could be a double free.
244		*/
245	36087x	void recycle(Impl* impl)
246		{
247	36087x	bool do_delete = false;
248		{
249	36087x	std::lock_guard lock(mutex_);
250	36087x	if (is_pooled(impl))
251		{
252	✗	BOOST_COROSIO_ASSERT(false);
253		return;
254		}
255
256	36087x	bool const was_live = live_.remove(impl);
257	36087x	if (shutting_down_)
258		{
259	5946x	do_delete = was_live;
260		}
261	30141x	else if (was_live)
262		{
263	30141x	impl->refs_.store(
264		pooled_sentinel, std::memory_order_relaxed);
265		// LIFO: the most recently retired impl is the
266		// cache-warmest candidate for the next acquire().
267	30141x	free_.push_front(impl);
268		}
269		else
270		{
271	✗	BOOST_COROSIO_ASSERT(false);
272		do_delete = true;
273		}
274	36087x	}
275	36087x	if (do_delete)
276	5946x	delete impl;
277		}
278
279		/** Enter shutdown mode and visit every live impl.
280
281		Sets shutting-down (subsequent zero-crossings delete instead
282		of recycling) and runs `f` on each live impl, all under one
283		critical section. Each visited impl is pinned with a reference
284		for the call and released after the critical section ends, so
285		`f` may hold on to it; an impl whose count already reached zero
286		is retiring and is skipped rather than revived.
287
288		@note The callback runs under the pool's non-recursive mutex;
289		it must not call recycle(), adopt(), or remove() on this pool.
290		*/
291		template<class F>
292	13325x	void shutdown(F&& f)
293		{
294	13325x	std::vector<Impl*> pinned;
295		{
296	13325x	std::lock_guard lock(mutex_);
297	13325x	shutting_down_ = true;
298		// Sized before any pin, so a failure here leaves none held.
299	13325x	std::size_t n = 0;
300	13325x	live_.for_each([&](Impl*) { ++n; });
301	13325x	pinned.reserve(n);
302	13325x	live_.for_each(
303	6x	[&](Impl* impl)
304		{
305	3x	if (try_acquire(impl))
306		{
307	3x	pinned.push_back(impl);
308	3x	f(impl);
309		}
310		});
311	13325x	}
312	22524x	for (auto* impl : pinned)
313	9199x	release(impl);
314	13325x	}
315		};
316
317		} // namespace boost::corosio::detail
318
319		#endif
320