mirror of
https://github.com/matrix-construct/construct
synced 2024-06-26 05:48:20 +02:00
151 lines
3 KiB
C++
151 lines
3 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_PEEK_H
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namespace ircd::ctx
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{
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template<class T> class peek;
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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 peek yields a context while other contexts examine the object pointed
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/// to in the peek. 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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/// Consumers get one chance to safely peek the data when a call to wait()
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/// returns. Once the consumer context yields again for any reason the data is
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/// potentially invalid. The data can only be peeked once by the consumer
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/// because the second call to wait() will yield until the next data is
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/// made available by the producer, not the same data.
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///
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/// Producers will share an object during the call to notify(). Once the call
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/// to notify() returns all consumers have peeked the data and the producer is
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/// free to destroy it.
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///
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template<class T>
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class ircd::ctx::peek
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{
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T *t {nullptr};
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dock a, b;
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bool ready() const;
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public:
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size_t waiting() const;
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// Consumer interface;
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template<class time_point> T &wait_until(time_point&&);
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template<class duration> T &wait_for(const duration &);
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T &wait();
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// Producer interface;
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void notify(T &);
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peek() = default;
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~peek() noexcept;
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};
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template<class T>
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ircd::ctx::peek<T>::~peek()
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noexcept
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{
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assert(!waiting());
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}
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template<class T>
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void
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ircd::ctx::peek<T>::notify(T &t)
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{
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const unwind afterward{[this]
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{
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assert(a.empty());
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this->t = nullptr;
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if(!b.empty())
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{
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b.notify_all();
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yield();
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}
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}};
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assert(b.empty());
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this->t = &t;
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a.notify_all();
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yield();
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}
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template<class T>
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T &
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ircd::ctx::peek<T>::wait()
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{
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b.wait([this]
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{
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return !ready();
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});
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a.wait([this]
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{
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return ready();
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});
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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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template<class duration>
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T &
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ircd::ctx::peek<T>::wait_for(const duration &dur)
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{
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return wait_until(now<steady_point>() + dur);
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}
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template<class T>
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template<class time_point>
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T &
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ircd::ctx::peek<T>::wait_until(time_point&& tp)
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{
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if(!b.wait_until(tp, [this]
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{
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return !ready();
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}))
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throw timeout();
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if(!a.wait_until(tp, [this]
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{
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return ready();
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}))
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throw timeout();
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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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size_t
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ircd::ctx::peek<T>::waiting()
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const
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{
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return a.size() + b.size();
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}
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template<class T>
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bool
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ircd::ctx::peek<T>::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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