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construct/include/ircd/ctx/view.h

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2018-02-04 03:22:01 +01:00
// Matrix Construct
//
// Copyright (C) Matrix Construct Developers, Authors & Contributors
// Copyright (C) 2016-2018 Jason Volk <jason@zemos.net>
//
// Permission to use, copy, modify, and/or distribute this software for any
// purpose with or without fee is hereby granted, provided that the above
// copyright notice and this permission notice is present in all copies. The
// full license for this software is available in the LICENSE file.
#pragma once
#define HAVE_IRCD_CTX_VIEW_H
namespace ircd::ctx
{
template<class T> class view;
}
/// Device for a context to share data on its stack with others while yielding
///
/// The view yields a context while other contexts examine the object pointed
/// to in the view. This allows a producing context to construct something
/// on its stack and then wait for the consuming contexts to do something with
/// that data before the producer resumes and potentially destroys the data.
/// This creates a very simple and lightweight single-producer/multi-consumer
/// queue mechanism using only context switching.
///
/// The producer is blocked until all consumers are finished with their view.
/// The consumers acquire the unique_lock before passing it to the call to wait().
/// wait() returns with a view of the object under unique_lock. Once the
/// consumer releases the unique_lock the viewed object is not safe for them.
///
template<class T>
class ircd::ctx::view
:public mutex
{
T *t {nullptr};
dock q;
size_t waiting {0};
bool ready() const;
public:
// Consumer interface;
template<class time_point> T &wait_until(std::unique_lock<view> &, time_point&&);
template<class duration> T &wait_for(std::unique_lock<view> &, const duration &);
T &wait(std::unique_lock<view> &);
// Producer interface;
void notify(T &);
view() = default;
~view() noexcept;
};
template<class T>
ircd::ctx::view<T>::~view()
noexcept
{
assert(!waiting);
}
template<class T>
void
ircd::ctx::view<T>::notify(T &t)
{
if(!waiting)
return;
this->t = &t;
q.notify_all();
q.wait([this] { return !waiting; });
const std::lock_guard<view> lock{*this};
this->t = nullptr;
assert(!waiting);
q.notify_all();
}
template<class T>
T &
ircd::ctx::view<T>::wait(std::unique_lock<view> &lock)
{
for(assert(lock.owns_lock()); ready(); lock.lock())
{
lock.unlock();
q.wait();
}
const unwind ul{[this]
{
--waiting;
q.notify_all();
}};
for(++waiting; !ready(); lock.lock())
{
lock.unlock();
q.wait();
}
assert(t != nullptr);
return *t;
}
template<class T>
template<class duration>
T &
ircd::ctx::view<T>::wait_for(std::unique_lock<view> &lock,
const duration &dur)
{
return wait_until(now<steady_point>() + dur);
}
template<class T>
template<class time_point>
T &
ircd::ctx::view<T>::wait_until(std::unique_lock<view> &lock,
time_point&& tp)
{
for(assert(lock.owns_lock()); ready(); lock.lock())
{
lock.unlock();
q.wait_until(tp);
}
const unwind ul{[this]
{
--waiting;
q.notify_all();
}};
for(++waiting; !ready(); lock.lock())
{
lock.unlock();
q.wait_until(tp);
}
assert(t != nullptr);
return *t;
}
template<class T>
bool
ircd::ctx::view<T>::ready()
const
{
return t != nullptr;
}