construct/include/ircd/ctx/continuation.h

// 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_CONTINUATION_H

// This file is not included with the IRCd standard include stack because
// it requires symbols we can't forward declare without boost headers. It
// is part of the <ircd/asio.h> stack which can be included in your
// definition file if necessary.

namespace ircd::ctx
{
	struct continuation;

	using yield_context = boost::asio::yield_context;
	using interruptor = std::function<void (ctx *const &)>;
	using predicate = std::function<bool ()>;
}

namespace ircd
{
	using ctx::continuation;
}

/// This object must be placed on the stack when the context is yielding (INTERNAL)
///
/// The instance contains references to some callables which must remain valid.
///
/// - predicate (NOT YET USED)
/// A wakeup condition. This should be a simple boolean function which
/// tests whether the context should be woken up. The continuation references
/// this to convey the condition to a scheduler which may test many predicates
/// while contexts are asleep and then determine a schedule. This is an
/// alternative to waking up contexts first to test their predicates.
///
/// - interruptor
/// An interruption action. This is called when a context cannot wakeup on its
/// own after receiving an interruption without help from this action. Common
/// use for this is with yields to asio.
///
struct [[gnu::visibility("hidden")]]
ircd::ctx::continuation
{
	static const predicate asio_predicate;
	static const predicate true_predicate;
	static const predicate false_predicate;
	static const interruptor noop_interruptor;

	const void *const frame_address;
	const void *const return_address;
	const interruptor *const intr;
	const predicate *const pred;
	const size_t uncaught_exceptions;
	ctx *const self;

	operator const boost::asio::yield_context &() const noexcept;
	operator boost::asio::yield_context &() noexcept;

	void enter();
	void leave() noexcept;

	template<class yield_closure>
	continuation(const predicate &,
	             const interruptor &,
	             yield_closure&&);

	continuation(continuation &&) = delete;
	continuation(const continuation &) = delete;
	continuation &operator=(continuation &&) = delete;
	continuation &operator=(const continuation &) = delete;
};

template<class yield_closure>
inline
__attribute__((always_inline))
ircd::ctx::continuation::continuation(const predicate &pred,
                                      const interruptor &intr,
                                      yield_closure&& closure)
:frame_address
{
	__builtin_frame_address(0)
}
,return_address
{
	__builtin_return_address(0)
}
,intr
{
	&intr
}
,pred
{
	&pred
}
,uncaught_exceptions
{
	exception_handler::uncaught_exceptions(0)
}
,self
{
	ircd::ctx::current
}
{
	leave();

	// Run the provided routine which performs the actual context switch.
	// Everything happening in this closure is no longer considered part
	// of this context, but it is technically operating on this stack.
	std::exception_ptr eptr; try
	{
		closure(static_cast<yield_context &>(*this));
	}
	catch(...)
	{
		eptr = std::current_exception();
	}

	enter();
	if(unlikely(eptr))
		std::rethrow_exception(eptr);
}

inline void *
__attribute__((always_inline))
boost::coroutines::detail::coroutine_context::jump(coroutine_context &other,
                                                   void *param)
{
	data_t data
	{
		this, param
	};

	context::detail::transfer_t t
	{
		context::detail::jump_fcontext(other.ctx_, &data)
	};

	data_t *const td
	{
		static_cast<data_t *>(t.data)
	};

	td->from->ctx_ = t.fctx;
	return td->data;
}