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Diffstat (limited to 'lib/asio/coroutine.hpp')
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diff --git a/lib/asio/coroutine.hpp b/lib/asio/coroutine.hpp new file mode 100644 index 0000000..cd2d99e --- /dev/null +++ b/lib/asio/coroutine.hpp @@ -0,0 +1,328 @@ +// +// coroutine.hpp +// ~~~~~~~~~~~~~ +// +// Copyright (c) 2003-2018 Christopher M. Kohlhoff (chris at kohlhoff dot com) +// +// Distributed under the Boost Software License, Version 1.0. (See accompanying +// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) +// + +#ifndef ASIO_COROUTINE_HPP +#define ASIO_COROUTINE_HPP + +namespace asio { +namespace detail { + +class coroutine_ref; + +} // namespace detail + +/// Provides support for implementing stackless coroutines. +/** + * The @c coroutine class may be used to implement stackless coroutines. The + * class itself is used to store the current state of the coroutine. + * + * Coroutines are copy-constructible and assignable, and the space overhead is + * a single int. They can be used as a base class: + * + * @code class session : coroutine + * { + * ... + * }; @endcode + * + * or as a data member: + * + * @code class session + * { + * ... + * coroutine coro_; + * }; @endcode + * + * or even bound in as a function argument using lambdas or @c bind(). The + * important thing is that as the application maintains a copy of the object + * for as long as the coroutine must be kept alive. + * + * @par Pseudo-keywords + * + * A coroutine is used in conjunction with certain "pseudo-keywords", which + * are implemented as macros. These macros are defined by a header file: + * + * @code #include <asio/yield.hpp>@endcode + * + * and may conversely be undefined as follows: + * + * @code #include <asio/unyield.hpp>@endcode + * + * <b>reenter</b> + * + * The @c reenter macro is used to define the body of a coroutine. It takes a + * single argument: a pointer or reference to a coroutine object. For example, + * if the base class is a coroutine object you may write: + * + * @code reenter (this) + * { + * ... coroutine body ... + * } @endcode + * + * and if a data member or other variable you can write: + * + * @code reenter (coro_) + * { + * ... coroutine body ... + * } @endcode + * + * When @c reenter is executed at runtime, control jumps to the location of the + * last @c yield or @c fork. + * + * The coroutine body may also be a single statement, such as: + * + * @code reenter (this) for (;;) + * { + * ... + * } @endcode + * + * @b Limitation: The @c reenter macro is implemented using a switch. This + * means that you must take care when using local variables within the + * coroutine body. The local variable is not allowed in a position where + * reentering the coroutine could bypass the variable definition. + * + * <b>yield <em>statement</em></b> + * + * This form of the @c yield keyword is often used with asynchronous operations: + * + * @code yield socket_->async_read_some(buffer(*buffer_), *this); @endcode + * + * This divides into four logical steps: + * + * @li @c yield saves the current state of the coroutine. + * @li The statement initiates the asynchronous operation. + * @li The resume point is defined immediately following the statement. + * @li Control is transferred to the end of the coroutine body. + * + * When the asynchronous operation completes, the function object is invoked + * and @c reenter causes control to transfer to the resume point. It is + * important to remember to carry the coroutine state forward with the + * asynchronous operation. In the above snippet, the current class is a + * function object object with a coroutine object as base class or data member. + * + * The statement may also be a compound statement, and this permits us to + * define local variables with limited scope: + * + * @code yield + * { + * mutable_buffers_1 b = buffer(*buffer_); + * socket_->async_read_some(b, *this); + * } @endcode + * + * <b>yield return <em>expression</em> ;</b> + * + * This form of @c yield is often used in generators or coroutine-based parsers. + * For example, the function object: + * + * @code struct interleave : coroutine + * { + * istream& is1; + * istream& is2; + * char operator()(char c) + * { + * reenter (this) for (;;) + * { + * yield return is1.get(); + * yield return is2.get(); + * } + * } + * }; @endcode + * + * defines a trivial coroutine that interleaves the characters from two input + * streams. + * + * This type of @c yield divides into three logical steps: + * + * @li @c yield saves the current state of the coroutine. + * @li The resume point is defined immediately following the semicolon. + * @li The value of the expression is returned from the function. + * + * <b>yield ;</b> + * + * This form of @c yield is equivalent to the following steps: + * + * @li @c yield saves the current state of the coroutine. + * @li The resume point is defined immediately following the semicolon. + * @li Control is transferred to the end of the coroutine body. + * + * This form might be applied when coroutines are used for cooperative + * threading and scheduling is explicitly managed. For example: + * + * @code struct task : coroutine + * { + * ... + * void operator()() + * { + * reenter (this) + * { + * while (... not finished ...) + * { + * ... do something ... + * yield; + * ... do some more ... + * yield; + * } + * } + * } + * ... + * }; + * ... + * task t1, t2; + * for (;;) + * { + * t1(); + * t2(); + * } @endcode + * + * <b>yield break ;</b> + * + * The final form of @c yield is used to explicitly terminate the coroutine. + * This form is comprised of two steps: + * + * @li @c yield sets the coroutine state to indicate termination. + * @li Control is transferred to the end of the coroutine body. + * + * Once terminated, calls to is_complete() return true and the coroutine cannot + * be reentered. + * + * Note that a coroutine may also be implicitly terminated if the coroutine + * body is exited without a yield, e.g. by return, throw or by running to the + * end of the body. + * + * <b>fork <em>statement</em></b> + * + * The @c fork pseudo-keyword is used when "forking" a coroutine, i.e. splitting + * it into two (or more) copies. One use of @c fork is in a server, where a new + * coroutine is created to handle each client connection: + * + * @code reenter (this) + * { + * do + * { + * socket_.reset(new tcp::socket(io_context_)); + * yield acceptor->async_accept(*socket_, *this); + * fork server(*this)(); + * } while (is_parent()); + * ... client-specific handling follows ... + * } @endcode + * + * The logical steps involved in a @c fork are: + * + * @li @c fork saves the current state of the coroutine. + * @li The statement creates a copy of the coroutine and either executes it + * immediately or schedules it for later execution. + * @li The resume point is defined immediately following the semicolon. + * @li For the "parent", control immediately continues from the next line. + * + * The functions is_parent() and is_child() can be used to differentiate + * between parent and child. You would use these functions to alter subsequent + * control flow. + * + * Note that @c fork doesn't do the actual forking by itself. It is the + * application's responsibility to create a clone of the coroutine and call it. + * The clone can be called immediately, as above, or scheduled for delayed + * execution using something like io_context::post(). + * + * @par Alternate macro names + * + * If preferred, an application can use macro names that follow a more typical + * naming convention, rather than the pseudo-keywords. These are: + * + * @li @c ASIO_CORO_REENTER instead of @c reenter + * @li @c ASIO_CORO_YIELD instead of @c yield + * @li @c ASIO_CORO_FORK instead of @c fork + */ +class coroutine +{ +public: + /// Constructs a coroutine in its initial state. + coroutine() : value_(0) {} + + /// Returns true if the coroutine is the child of a fork. + bool is_child() const { return value_ < 0; } + + /// Returns true if the coroutine is the parent of a fork. + bool is_parent() const { return !is_child(); } + + /// Returns true if the coroutine has reached its terminal state. + bool is_complete() const { return value_ == -1; } + +private: + friend class detail::coroutine_ref; + int value_; +}; + + +namespace detail { + +class coroutine_ref +{ +public: + coroutine_ref(coroutine& c) : value_(c.value_), modified_(false) {} + coroutine_ref(coroutine* c) : value_(c->value_), modified_(false) {} + ~coroutine_ref() { if (!modified_) value_ = -1; } + operator int() const { return value_; } + int& operator=(int v) { modified_ = true; return value_ = v; } +private: + void operator=(const coroutine_ref&); + int& value_; + bool modified_; +}; + +} // namespace detail +} // namespace asio + +#define ASIO_CORO_REENTER(c) \ + switch (::asio::detail::coroutine_ref _coro_value = c) \ + case -1: if (_coro_value) \ + { \ + goto terminate_coroutine; \ + terminate_coroutine: \ + _coro_value = -1; \ + goto bail_out_of_coroutine; \ + bail_out_of_coroutine: \ + break; \ + } \ + else /* fall-through */ case 0: + +#define ASIO_CORO_YIELD_IMPL(n) \ + for (_coro_value = (n);;) \ + if (_coro_value == 0) \ + { \ + case (n): ; \ + break; \ + } \ + else \ + switch (_coro_value ? 0 : 1) \ + for (;;) \ + /* fall-through */ case -1: if (_coro_value) \ + goto terminate_coroutine; \ + else for (;;) \ + /* fall-through */ case 1: if (_coro_value) \ + goto bail_out_of_coroutine; \ + else /* fall-through */ case 0: + +#define ASIO_CORO_FORK_IMPL(n) \ + for (_coro_value = -(n);; _coro_value = (n)) \ + if (_coro_value == (n)) \ + { \ + case -(n): ; \ + break; \ + } \ + else + +#if defined(_MSC_VER) +# define ASIO_CORO_YIELD ASIO_CORO_YIELD_IMPL(__COUNTER__ + 1) +# define ASIO_CORO_FORK ASIO_CORO_FORK_IMPL(__COUNTER__ + 1) +#else // defined(_MSC_VER) +# define ASIO_CORO_YIELD ASIO_CORO_YIELD_IMPL(__LINE__) +# define ASIO_CORO_FORK ASIO_CORO_FORK_IMPL(__LINE__) +#endif // defined(_MSC_VER) + +#endif // ASIO_COROUTINE_HPP |