boost/beast/core/async_base.hpp
// // Copyright (c) 2016-2019 Vinnie Falco (vinnie dot falco at gmail 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) // // Official repository: https://github.com/boostorg/beast // #ifndef BOOST_BEAST_CORE_ASYNC_BASE_HPP #define BOOST_BEAST_CORE_ASYNC_BASE_HPP #include <boost/beast/core/detail/config.hpp> #include <boost/beast/core/detail/allocator.hpp> #include <boost/beast/core/detail/async_base.hpp> #include <boost/beast/core/detail/filtering_cancellation_slot.hpp> #include <boost/beast/core/detail/work_guard.hpp> #include <boost/asio/append.hpp> #include <boost/asio/associated_allocator.hpp> #include <boost/asio/associated_cancellation_slot.hpp> #include <boost/asio/associated_executor.hpp> #include <boost/asio/associated_immediate_executor.hpp> #include <boost/asio/bind_executor.hpp> #include <boost/asio/dispatch.hpp> #include <boost/asio/handler_continuation_hook.hpp> #include <boost/asio/post.hpp> #include <boost/core/exchange.hpp> #include <boost/core/empty_value.hpp> #include <utility> namespace boost { namespace beast { /** Base class to assist writing composed operations. A function object submitted to intermediate initiating functions during a composed operation may derive from this type to inherit all of the boilerplate to forward the executor, allocator, and legacy customization points associated with the completion handler invoked at the end of the composed operation. The composed operation must be typical; that is, associated with one executor of an I/O object, and invoking a caller-provided completion handler when the operation is finished. Classes derived from @ref async_base will acquire these properties: @li Ownership of the final completion handler provided upon construction. @li If the final handler has an associated allocator, this allocator will be propagated to the composed operation subclass. Otherwise, the associated allocator will be the type specified in the allocator template parameter, or the default of `std::allocator<void>` if the parameter is omitted. @li If the final handler has an associated executor, then it will be used as the executor associated with the composed operation. Otherwise, the specified `Executor1` will be the type of executor associated with the composed operation. @li An instance of `net::executor_work_guard` for the instance of `Executor1` shall be maintained until either the final handler is invoked, or the operation base is destroyed, whichever comes first. @li Calls to the legacy customization point `asio_handler_is_continuation` which use argument-dependent lookup, will be forwarded to the legacy customization points associated with the handler. @par Example The following code demonstrates how @ref async_base may be be used to assist authoring an asynchronous initiating function, by providing all of the boilerplate to manage the final completion handler in a way that maintains the allocator and executor associations: @code // Asynchronously read into a buffer until the buffer is full, or an error occurs template<class AsyncReadStream, class ReadHandler> typename net::async_result<ReadHandler, void(error_code, std::size_t)>::return_type async_read(AsyncReadStream& stream, net::mutable_buffer buffer, ReadHandler&& handler) { using handler_type = BOOST_ASIO_HANDLER_TYPE(ReadHandler, void(error_code, std::size_t)); using base_type = async_base<handler_type, typename AsyncReadStream::executor_type>; struct op : base_type { AsyncReadStream& stream_; net::mutable_buffer buffer_; std::size_t total_bytes_transferred_; op( AsyncReadStream& stream, net::mutable_buffer buffer, handler_type& handler) : base_type(std::move(handler), stream.get_executor()) , stream_(stream) , buffer_(buffer) , total_bytes_transferred_(0) { (*this)({}, 0, false); // start the operation } void operator()(error_code ec, std::size_t bytes_transferred, bool is_continuation = true) { // Adjust the count of bytes and advance our buffer total_bytes_transferred_ += bytes_transferred; buffer_ = buffer_ + bytes_transferred; // Keep reading until buffer is full or an error occurs if(! ec && buffer_.size() > 0) return stream_.async_read_some(buffer_, std::move(*this)); // Call the completion handler with the result. If `is_continuation` is // false, which happens on the first time through this function, then // `net::post` will be used to call the completion handler, otherwise // the completion handler will be invoked directly. this->complete(is_continuation, ec, total_bytes_transferred_); } }; net::async_completion<ReadHandler, void(error_code, std::size_t)> init{handler}; op(stream, buffer, init.completion_handler); return init.result.get(); } @endcode Data members of composed operations implemented as completion handlers do not have stable addresses, as the composed operation object is move constructed upon each call to an initiating function. For most operations this is not a problem. For complex operations requiring stable temporary storage, the class @ref stable_async_base is provided which offers additional functionality: @li The free function @ref allocate_stable may be used to allocate one or more temporary objects associated with the composed operation. @li Memory for stable temporary objects is allocated using the allocator associated with the composed operation. @li Stable temporary objects are automatically destroyed, and the memory freed using the associated allocator, either before the final completion handler is invoked (a Networking requirement) or when the composed operation is destroyed, whichever occurs first. @par Temporary Storage Example The following example demonstrates how a composed operation may store a temporary object. @code @endcode @tparam Handler The type of the completion handler to store. This type must meet the requirements of <em>CompletionHandler</em>. @tparam Executor1 The type of the executor used when the handler has no associated executor. An instance of this type must be provided upon construction. The implementation will maintain an executor work guard and a copy of this instance. @tparam Allocator The allocator type to use if the handler does not have an associated allocator. If this parameter is omitted, then `std::allocator<void>` will be used. If the specified allocator is not default constructible, an instance of the type must be provided upon construction. @see stable_async_base */ template< class Handler, class Executor1, class Allocator = std::allocator<void> > class async_base #if ! BOOST_BEAST_DOXYGEN : private boost::empty_value<Allocator> #endif { static_assert( net::is_executor<Executor1>::value || net::execution::is_executor<Executor1>::value, "Executor type requirements not met"); Handler h_; detail::select_work_guard_t<Executor1> wg1_; net::cancellation_type act_{net::cancellation_type::terminal}; public: /** The type of executor associated with this object. If a class derived from @ref boost::beast::async_base is a completion handler, then the associated executor of the derived class will be this type. */ using executor_type = #if BOOST_BEAST_DOXYGEN __implementation_defined__; #else typename net::associated_executor< Handler, typename detail::select_work_guard_t<Executor1>::executor_type >::type; #endif /** The type of the immediate executor associated with this object. If a class derived from @ref boost::beast::async_base is a completion handler, then the associated immediage executor of the derived class will be this type. */ using immediate_executor_type = #if BOOST_BEAST_DOXYGEN __implementation_defined__; #else typename net::associated_immediate_executor< Handler, typename detail::select_work_guard_t<Executor1>::executor_type >::type; #endif private: virtual void before_invoke_hook() { } public: /** Constructor @param handler The final completion handler. The type of this object must meet the requirements of <em>CompletionHandler</em>. The implementation takes ownership of the handler by performing a decay-copy. @param ex1 The executor associated with the implied I/O object target of the operation. The implementation shall maintain an executor work guard for the lifetime of the operation, or until the final completion handler is invoked, whichever is shorter. @param alloc The allocator to be associated with objects derived from this class. If `Allocator` is default-constructible, this parameter is optional and may be omitted. */ #if BOOST_BEAST_DOXYGEN template<class Handler_> async_base( Handler&& handler, Executor1 const& ex1, Allocator const& alloc = Allocator()); #else template< class Handler_, class = typename std::enable_if< ! std::is_same<typename std::decay<Handler_>::type, async_base >::value>::type > async_base( Handler_&& handler, Executor1 const& ex1) : h_(std::forward<Handler_>(handler)) , wg1_(detail::make_work_guard(ex1)) { } template<class Handler_> async_base( Handler_&& handler, Executor1 const& ex1, Allocator const& alloc) : boost::empty_value<Allocator>( boost::empty_init_t{}, alloc) , h_(std::forward<Handler_>(handler)) , wg1_(ex1) { } #endif /// Move Constructor async_base(async_base&& other) = default; virtual ~async_base() = default; async_base(async_base const&) = delete; async_base& operator=(async_base const&) = delete; /** The type of allocator associated with this object. If a class derived from @ref boost::beast::async_base is a completion handler, then the associated allocator of the derived class will be this type. */ using allocator_type = net::associated_allocator_t<Handler, Allocator>; /** Returns the allocator associated with this object. If a class derived from @ref boost::beast::async_base is a completion handler, then the object returned from this function will be used as the associated allocator of the derived class. */ allocator_type get_allocator() const noexcept { return net::get_associated_allocator(h_, boost::empty_value<Allocator>::get()); } /** Returns the executor associated with this object. If a class derived from @ref boost::beast::async_base is a completion handler, then the object returned from this function will be used as the associated executor of the derived class. */ executor_type get_executor() const noexcept { return net::get_associated_executor( h_, wg1_.get_executor()); } /** Returns the immediate executor associated with this handler. If the handler has none it returns asios default immediate executor based on the executor of the object. If a class derived from @ref boost::beast::async_base is a completion handler, then the object returned from this function will be used as the associated immediate executor of the derived class. */ immediate_executor_type get_immediate_executor() const noexcept { return net::get_associated_immediate_executor( h_, wg1_.get_executor()); } /** The type of cancellation_slot associated with this object. If a class derived from @ref async_base is a completion handler, then the associated cancellation_slot of the derived class will be this type. The default type is a filtering cancellation slot, that only allows terminal cancellation. */ using cancellation_slot_type = beast::detail::filtering_cancellation_slot<net::associated_cancellation_slot_t<Handler>>; /** Returns the cancellation_slot associated with this object. If a class derived from @ref async_base is a completion handler, then the object returned from this function will be used as the associated cancellation_slot of the derived class. */ cancellation_slot_type get_cancellation_slot() const noexcept { return cancellation_slot_type(act_, net::get_associated_cancellation_slot(h_, net::cancellation_slot())); } /// Set the allowed cancellation types, default is `terminal`. void set_allowed_cancellation( net::cancellation_type allowed_cancellation_types = net::cancellation_type::terminal) { act_ = allowed_cancellation_types; } /// Returns the handler associated with this object Handler const& handler() const noexcept { return h_; } /** Returns ownership of the handler associated with this object This function is used to transfer ownership of the handler to the caller, by move-construction. After the move, the only valid operations on the base object are move construction and destruction. */ Handler release_handler() { return std::move(h_); } /** Invoke the final completion handler, maybe using post. This invokes the final completion handler with the specified arguments forwarded. It is undefined to call either of @ref boost::beast::async_base::complete or @ref boost::beast::async_base::complete_now more than once. Any temporary objects allocated with @ref boost::beast::allocate_stable will be automatically destroyed before the final completion handler is invoked. @param is_continuation If this value is `false`, then the handler will be submitted to the to the immediate executor using `net::dispatch`. If the handler has no immediate executor, this will submit to the executor via `net::post`. Otherwise the handler will be invoked as if by calling @ref boost::beast::async_base::complete_now. @param args A list of optional parameters to invoke the handler with. The completion handler must be invocable with the parameter list, or else a compilation error will result. */ template<class... Args> void complete(bool is_continuation, Args&&... args) { this->before_invoke_hook(); if(! is_continuation) { auto const ex = this->get_immediate_executor(); net::dispatch( ex, net::append(std::move(h_), std::forward<Args>(args)...)); wg1_.reset(); } else { wg1_.reset(); h_(std::forward<Args>(args)...); } } /** Invoke the final completion handler. This invokes the final completion handler with the specified arguments forwarded. It is undefined to call either of @ref boost::beast::async_base::complete or @ref boost::beast::async_base::complete_now more than once. Any temporary objects allocated with @ref boost::beast::allocate_stable will be automatically destroyed before the final completion handler is invoked. @param args A list of optional parameters to invoke the handler with. The completion handler must be invocable with the parameter list, or else a compilation error will result. */ template<class... Args> void complete_now(Args&&... args) { this->before_invoke_hook(); wg1_.reset(); h_(std::forward<Args>(args)...); } #if ! BOOST_BEAST_DOXYGEN Handler* get_legacy_handler_pointer() noexcept { return std::addressof(h_); } #endif }; //------------------------------------------------------------------------------ /** Base class to provide completion handler boilerplate for composed operations. A function object submitted to intermediate initiating functions during a composed operation may derive from this type to inherit all of the boilerplate to forward the executor, allocator, and legacy customization points associated with the completion handler invoked at the end of the composed operation. The composed operation must be typical; that is, associated with one executor of an I/O object, and invoking a caller-provided completion handler when the operation is finished. Classes derived from @ref async_base will acquire these properties: @li Ownership of the final completion handler provided upon construction. @li If the final handler has an associated allocator, this allocator will be propagated to the composed operation subclass. Otherwise, the associated allocator will be the type specified in the allocator template parameter, or the default of `std::allocator<void>` if the parameter is omitted. @li If the final handler has an associated executor, then it will be used as the executor associated with the composed operation. Otherwise, the specified `Executor1` will be the type of executor associated with the composed operation. @li An instance of `net::executor_work_guard` for the instance of `Executor1` shall be maintained until either the final handler is invoked, or the operation base is destroyed, whichever comes first. Data members of composed operations implemented as completion handlers do not have stable addresses, as the composed operation object is move constructed upon each call to an initiating function. For most operations this is not a problem. For complex operations requiring stable temporary storage, the class @ref stable_async_base is provided which offers additional functionality: @li The free function @ref beast::allocate_stable may be used to allocate one or more temporary objects associated with the composed operation. @li Memory for stable temporary objects is allocated using the allocator associated with the composed operation. @li Stable temporary objects are automatically destroyed, and the memory freed using the associated allocator, either before the final completion handler is invoked (a Networking requirement) or when the composed operation is destroyed, whichever occurs first. @par Example The following code demonstrates how @ref stable_async_base may be be used to assist authoring an asynchronous initiating function, by providing all of the boilerplate to manage the final completion handler in a way that maintains the allocator and executor associations. Furthermore, the operation shown allocates temporary memory using @ref beast::allocate_stable for the timer and message, whose addresses must not change between intermediate operations: @code // Asynchronously send a message multiple times, once per second template <class AsyncWriteStream, class T, class WriteHandler> auto async_write_messages( AsyncWriteStream& stream, T const& message, std::size_t repeat_count, WriteHandler&& handler) -> typename net::async_result< typename std::decay<WriteHandler>::type, void(error_code)>::return_type { using handler_type = typename net::async_completion<WriteHandler, void(error_code)>::completion_handler_type; using base_type = stable_async_base<handler_type, typename AsyncWriteStream::executor_type>; struct op : base_type, boost::asio::coroutine { // This object must have a stable address struct temporary_data { // Although std::string is in theory movable, most implementations // use a "small buffer optimization" which means that we might // be submitting a buffer to the write operation and then // moving the string, invalidating the buffer. To prevent // undefined behavior we store the string object itself at // a stable location. std::string const message; net::steady_timer timer; temporary_data(std::string message_, net::io_context& ctx) : message(std::move(message_)) , timer(ctx) { } }; AsyncWriteStream& stream_; std::size_t repeats_; temporary_data& data_; op(AsyncWriteStream& stream, std::size_t repeats, std::string message, handler_type& handler) : base_type(std::move(handler), stream.get_executor()) , stream_(stream) , repeats_(repeats) , data_(allocate_stable<temporary_data>(*this, std::move(message), stream.get_executor().context())) { (*this)(); // start the operation } // Including this file provides the keywords for macro-based coroutines #include <boost/asio/yield.hpp> void operator()(error_code ec = {}, std::size_t = 0) { reenter(*this) { // If repeats starts at 0 then we must complete immediately. But // we can't call the final handler from inside the initiating // function, so we post our intermediate handler first. We use // net::async_write with an empty buffer instead of calling // net::post to avoid an extra function template instantiation, to // keep compile times lower and make the resulting executable smaller. yield net::async_write(stream_, net::const_buffer{}, std::move(*this)); while(! ec && repeats_-- > 0) { // Send the string. We construct a `const_buffer` here to guarantee // that we do not create an additional function template instantation // of net::async_write, since we already instantiated it above for // net::const_buffer. yield net::async_write(stream_, net::const_buffer(net::buffer(data_.message)), std::move(*this)); if(ec) break; // Set the timer and wait data_.timer.expires_after(std::chrono::seconds(1)); yield data_.timer.async_wait(std::move(*this)); } } // The base class destroys the temporary data automatically, // before invoking the final completion handler this->complete_now(ec); } // Including this file undefines the macros for the coroutines #include <boost/asio/unyield.hpp> }; net::async_completion<WriteHandler, void(error_code)> completion(handler); std::ostringstream os; os << message; op(stream, repeat_count, os.str(), completion.completion_handler); return completion.result.get(); } @endcode @tparam Handler The type of the completion handler to store. This type must meet the requirements of <em>CompletionHandler</em>. @tparam Executor1 The type of the executor used when the handler has no associated executor. An instance of this type must be provided upon construction. The implementation will maintain an executor work guard and a copy of this instance. @tparam Allocator The allocator type to use if the handler does not have an associated allocator. If this parameter is omitted, then `std::allocator<void>` will be used. If the specified allocator is not default constructible, an instance of the type must be provided upon construction. @see allocate_stable, async_base */ template< class Handler, class Executor1, class Allocator = std::allocator<void> > class stable_async_base : public async_base< Handler, Executor1, Allocator> { detail::stable_base* list_ = nullptr; void before_invoke_hook() override { detail::stable_base::destroy_list(list_); } public: /** Constructor @param handler The final completion handler. The type of this object must meet the requirements of <em>CompletionHandler</em>. The implementation takes ownership of the handler by performing a decay-copy. @param ex1 The executor associated with the implied I/O object target of the operation. The implementation shall maintain an executor work guard for the lifetime of the operation, or until the final completion handler is invoked, whichever is shorter. @param alloc The allocator to be associated with objects derived from this class. If `Allocator` is default-constructible, this parameter is optional and may be omitted. */ #if BOOST_BEAST_DOXYGEN template<class Handler> stable_async_base( Handler&& handler, Executor1 const& ex1, Allocator const& alloc = Allocator()); #else template< class Handler_, class = typename std::enable_if< ! std::is_same<typename std::decay<Handler_>::type, stable_async_base >::value>::type > stable_async_base( Handler_&& handler, Executor1 const& ex1) : async_base< Handler, Executor1, Allocator>( std::forward<Handler_>(handler), ex1) { } template<class Handler_> stable_async_base( Handler_&& handler, Executor1 const& ex1, Allocator const& alloc) : async_base< Handler, Executor1, Allocator>( std::forward<Handler_>(handler), ex1, alloc) { } #endif /// Move Constructor stable_async_base(stable_async_base&& other) : async_base<Handler, Executor1, Allocator>( std::move(other)) , list_(boost::exchange(other.list_, nullptr)) { } /** Destructor If the completion handler was not invoked, then any state objects allocated with @ref allocate_stable will be destroyed here. */ ~stable_async_base() { detail::stable_base::destroy_list(list_); } /** Allocate a temporary object to hold operation state. The object will be destroyed just before the completion handler is invoked, or when the operation base is destroyed. */ template< class State, class Handler_, class Executor1_, class Allocator_, class... Args> friend State& allocate_stable( stable_async_base< Handler_, Executor1_, Allocator_>& base, Args&&... args); }; /** Allocate a temporary object to hold stable asynchronous operation state. The object will be destroyed just before the completion handler is invoked, or when the base is destroyed. @tparam State The type of object to allocate. @param base The helper to allocate from. @param args An optional list of parameters to forward to the constructor of the object being allocated. @see stable_async_base */ template< class State, class Handler, class Executor1, class Allocator, class... Args> State& allocate_stable( stable_async_base< Handler, Executor1, Allocator>& base, Args&&... args); } // beast } // boost #include <boost/beast/core/impl/async_base.hpp> #endif