include/boost/corosio/detail/object_pool.hpp

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1			//
2			// Copyright (c) 2025 Steve Gerbino ([email protected])
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		5970x	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		94483x	static bool is_pooled(Impl const* impl) noexcept
79			{
80		94483x	return impl->refs_.load(std::memory_order_relaxed) ==
81			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		29406x	void adopt(Impl* impl)
103			{
104		29406x	std::lock_guard lock(mutex_);
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106			{
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108			return;
109			}
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111		29406x	live_.push_back(impl);
112		29406x	}
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		14210x	bool remove(Impl* impl) noexcept
130			{
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132	1/2 ✗ Branch 0 not taken. ✓ Branch 1 taken 14210 times.	14210x	if (is_pooled(impl))
133			{
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135			return false;
136			}
137		14210x	return live_.remove(impl);
138		14210x	}
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		11061x	~object_pool()
152		5090x	{
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155			{
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157			{
158		9311x	impl->refs_.store(pooled_sentinel, std::memory_order_relaxed);
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160			}
161			}
162		11061x	}
163
164		17029x	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		54568x	Impl* acquire(Args&&... args)
191			{
192			Impl* impl;
193			{
194		54568x	std::lock_guard lock(mutex_);
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196		54568x	impl = free_.pop_front();
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198			{
199		35690x	impl->refs_.store(1, std::memory_order_relaxed);
200		35690x	live_.push_back(impl);
201		35690x	}
202		54568x	}
203			// reuse() runs unlocked: acquire()'s precondition excludes a
204			// concurrent shutdown() visiting the impl mid-reset.
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206			{
207		35683x	impl->reuse();
208		35683x	return impl;
209			}
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211		18885x	adopt(impl);
212		18885x	return impl;
213		54568x	}
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		50867x	void recycle(Impl* impl)
246			{
247		50867x	bool do_delete = false;
248			{
249		50867x	std::lock_guard lock(mutex_);
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251			{
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253			return;
254			}
255
256		50867x	bool const was_live = live_.remove(impl);
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258			{
259		5886x	do_delete = was_live;
260		5886x	}
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262			{
263		44981x	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		44981x	free_.push_front(impl);
268		44981x	}
269			else
270			{
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272			do_delete = true;
273			}
274		50867x	}
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277		50867x	}
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		5961x	void shutdown(F&& f)
293			{
294		5961x	std::vector<Impl*> pinned;
295			{
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297		5961x	shutting_down_ = true;
298			// Sized before any pin, so a failure here leaves none held.
299		5961x	std::size_t n = 0;
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303		15098x	[&](Impl* impl)
304			{
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306			{
307		9137x	pinned.push_back(impl);
308		9137x	f(impl);
309		9137x	}
310		9138x	});
311		5963x	}
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313		9137x	release(impl);
314		5963x	}
315			};
316
317			} // namespace boost::corosio::detail
318
319			#endif
320