mirror of
https://github.com/matrix-construct/construct
synced 2024-12-26 15:33:54 +01:00
171 lines
3.8 KiB
C++
171 lines
3.8 KiB
C++
// Matrix Construct
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//
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// Copyright (C) Matrix Construct Developers, Authors & Contributors
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// Copyright (C) 2016-2018 Jason Volk <jason@zemos.net>
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//
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// Permission to use, copy, modify, and/or distribute this software for any
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// purpose with or without fee is hereby granted, provided that the above
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// copyright notice and this permission notice is present in all copies. The
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// full license for this software is available in the LICENSE file.
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#pragma once
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#define HAVE_IRCD_CTX_VIEW_H
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namespace ircd::ctx
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{
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template<class T, class mutex = mutex> class view;
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template<class T> using shared_view = view<T, shared_mutex>;
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}
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/// Device for a context to share data on its stack with others while yielding
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///
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/// The view yields a context while other contexts examine the object pointed
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/// to in the view. This allows a producing context to construct something
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/// on its stack and then wait for the consuming contexts to do something with
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/// that data before the producer resumes and potentially destroys the data.
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/// This creates a very simple and lightweight single-producer/multi-consumer
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/// queue mechanism using only context switching.
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///
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/// The producer is blocked until all consumers are finished with their view.
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/// The consumers lock the mutex before passing it to the call to wait().
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/// wait() returns with a view of the object under lock. Once the consumer
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/// releases their lock the viewed object is not safe for them.
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///
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template<class T,
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class mutex>
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class ircd::ctx::view
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:public mutex
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{
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dock q;
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T *t {nullptr};
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size_t wanting {0};
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size_t waiting {0};
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bool ready() const;
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public:
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// Consumer interface;
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template<class lock, class time_point> T &wait_until(lock &, time_point&&);
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template<class lock, class duration> T &wait_for(lock &, const duration &);
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template<class lock> T &wait(lock &);
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// Producer interface;
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void operator()(T &);
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view() = default;
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~view() noexcept;
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};
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template<class T,
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class mutex>
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ircd::ctx::view<T, mutex>::~view()
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noexcept
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{
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assert(!waiting);
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assert(!wanting);
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}
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template<class T,
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class mutex>
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void
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ircd::ctx::view<T, mutex>::operator()(T &t)
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{
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const auto produce{[this]
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(T *const &t, size_t &semaphore)
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{
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q.wait([this, &semaphore]
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{
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return semaphore == 0;
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});
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const std::lock_guard<view> lock{*this};
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this->t = t;
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q.notify_all();
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}};
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produce(&t, wanting);
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produce(nullptr, waiting);
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}
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template<class T,
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class mutex>
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template<class lock>
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T &
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ircd::ctx::view<T, mutex>::wait(lock &l)
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{
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const auto consume{[this, &l]
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(const bool &r, size_t &semaphore)
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{
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const unwind uw{[this, &l, &semaphore]
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{
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--semaphore;
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q.notify_all();
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}};
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++semaphore;
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assert(l.owns_lock());
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const unlock_guard<lock> ul{l};
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q.wait([this, &r]
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{
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return ready() == r;
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});
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}};
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consume(false, wanting);
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consume(true, waiting);
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assert(t != nullptr);
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return *t;
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}
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template<class T,
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class mutex>
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template<class lock,
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class duration>
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T &
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ircd::ctx::view<T, mutex>::wait_for(lock &l,
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const duration &dur)
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{
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return wait_until(l, now<steady_point>() + dur);
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}
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template<class T,
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class mutex>
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template<class lock,
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class time_point>
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T &
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ircd::ctx::view<T, mutex>::wait_until(lock &l,
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time_point&& tp)
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{
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const auto consume{[this, &l, &tp]
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(const bool &r, size_t &semaphore)
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{
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const unwind uw{[this, &l, &semaphore]
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{
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--semaphore;
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q.notify_all();
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}};
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++semaphore;
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assert(l.owns_lock());
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const unlock_guard<lock> ul{l};
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if(!q.wait_until(tp, [this, &r]
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{
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return ready() == r;
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}))
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throw timeout{};
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}};
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consume(false, wanting);
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consume(true, waiting);
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assert(t != nullptr);
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return *t;
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}
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template<class T,
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class mutex>
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bool
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ircd::ctx::view<T, mutex>::ready()
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const
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{
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return t != nullptr;
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}
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