Re: [Boost-users] Boost ASIO: Problems building a conditional variable using io_context / strand

I encountered this problem too. Chris recommended using a steady_timer to construct an async semaphore. I wasn't happy with that, so wrote an async_semaphore type. Klemens then picked up on that and refined it. https://github.com/klemens-morgenstern/sam On Tue, 23 May 2023 at 00:44, Suraaj K S via Boost-users < boost-users@lists.boost.org> wrote:

Hi all,

In boost-asio, I realized that there was no easy way to have something that resembles a condition variable.

However, I realized that I could get something very similar using stackful coroutines ( https://www.boost.org/doc/libs/1_66_0/doc/html/boost_asio/overview/core/spaw...), while using an io_context as a 'queue' for completion tokens. Below is my approach:

*#include "asio_types.hpp"#include <atomic>#include <iostream>class async_pending_queue {public: async_pending_queue() : pending_handlers_(0), strand_(pending_queue_), wg_(asio::make_work_guard(pending_queue_)) {} template <typename CompletionToken> auto async_submit( CompletionToken &&token, std::function atomic_action = [] {}) { auto init = [this, &atomic_action](auto completion_handler) { auto posted_lambda = [handler = std::move(completion_handler), this]() mutable { pending_handlers_--; asio_sys_err ec; handler(ec); }; post(strand_,std::move(posted_lambda)); pending_handlers_++; atomic_action(); }; return asio::async_initiate(init, token); } int pending_count() { return pending_handlers_.load(); } // It may not run 1 and run 0 bool try_run_one() { auto cnt = pending_queue_.poll_one(); std::cout << "completion token result" << cnt << std::endl; bool ret = (cnt == 1); return ret; }private: std::atomic<unsigned int> pending_handlers_; asio_ioctx pending_queue_; asio_ioctx::strand strand_; decltype(asio::make_work_guard(pending_queue_)) wg_;};* Here, one simply uses calls `my_async_pending_queue.async_submit(yield)`, if calling from a stackful coroutine. The coroutine can be continued by calling `my_async_pending_queue.try_run_one()`.

Using this, I wanted to build a 'memory checker'. It has two functions -> `request_space` and `free_space`. A coroutine calls `request_space`, which may block if there is no space left. Meanwhile, another thread / coroutine can call `free_space`, which will run blocked coroutines if possible.

I built a toy memory checker wrapper as follows:

...

= cnt) { // We bypass the pending queue mem_used_ += cnt; lock_.unlock(); return; } } assert(request_queue_.size() == request_routines_.pending_count()); std::cout << "Pushing " << cnt << std::endl; request_queue_.push(cnt); auto wg = asio::make_work_guard(completion_ioctx_); request_routines_.async_submit(yield, [this] { lock_.unlock(); }); auto

*#ifndef MEM_CHECK_HPP#define MEM_CHECK_HPP#include <cstddef>#include <mutex>#include <queue>#include #include namespace asio = boost::asio;using asio_ioctx = asio::io_context;using asio_sys_err = boost::system::error_code;using asio::yield_context;#include "async_pending_queue.hpp"class MemoryChecker {public: using bytes_cnt = size_t; MemoryChecker(asio_ioctx &ioctx, bytes_cnt total_mem = 1024, bytes_cnt initial_fill = 0); // This requests some space, and possibly yields; void request_space(bytes_cnt cnt, yield_context yield); void free_space(bytes_cnt cnt);private: bytes_cnt get_available_mem(); const bytes_cnt total_mem_; bytes_cnt mem_used_; std::queue request_queue_; async_pending_queue request_routines_; std::mutex lock_; asio_ioctx::strand fifo_strand_; asio_ioctx &completion_ioctx_;};inline MemoryChecker::MemoryChecker(asio_ioctx &ioctx, bytes_cnt total_mem, bytes_cnt initial_fill) : total_mem_(total_mem), mem_used_(initial_fill), request_queue_{}, lock_{}, request_routines_{}, completion_ioctx_(ioctx), fifo_strand_(ioctx) {}inline MemoryChecker::bytes_cnt MemoryChecker::get_available_mem() { assert(total_mem_ >= mem_used_); return total_mem_ - mem_used_;}inline void MemoryChecker::request_space(bytes_cnt cnt, yield_context yield) { // if (cnt > total_mem_) throw logic_error lock_.lock(); assert(cnt <= total_mem_); assert(cnt > 0); if (request_queue_.empty()) { assert(request_routines_.pending_count() == 0); if (get_available_mem() oldest_req{request_queue_.front()}; assert(cnt == oldest_req); request_queue_.pop(); mem_used_ += cnt; assert(request_queue_.size() == request_routines_.pending_count()); asio::post(fifo_strand_, yield);}inline void MemoryChecker::free_space(bytes_cnt cnt) { { std::lock_guardstd::mutex lg{lock_}; mem_used_ -= cnt; // Here, we own the lock, and free as many coroutines as we can while (true) { if (request_queue_.size() == 0) { std::cout << "No pending requests. Bailing" << std::endl; break; } assert(request_queue_.size() == request_routines_.pending_count()); auto oldest_req{request_queue_.front()}; auto available_mem{get_available_mem()}; if (available_mem < oldest_req) { std::cout << "Oldest request is larger than available_mem. Bailing" << std::endl; break; } assert(request_routines_.try_run_one() == true); } }}#endif /* MEM_CHECK_HPP */*

Here is a test program that can run it:

*#include "mem_check.hpp"#include <thread>#include constexpr size_t mc_size{4};asio_ioctx ioctx;size_t total{0};MemoryChecker mc{ioctx, mc_size};void requestor_coroutine(size_t rq,yield_context yield) { asio::steady_timer t(ioctx); while (true) { total += rq; mc.request_space(rq, yield); std::cout << "Got requested space "; asio_sys_err ec; }}int main() { asio::spawn(ioctx, [](yield_context yield) { requestor_coroutine(1,yield); }); asio::spawn(ioctx, [](yield_context yield) { requestor_coroutine(2,yield); }); asio::spawn(ioctx, [](yield_context yield) { requestor_coroutine(3,yield); }); asio::spawn(ioctx, [](yield_context yield) { requestor_coroutine(4,yield); }); std::thread t([] { ioctx.run(); }); while (true) { getchar(); std::cout << total << std::endl; if (total > 0) { std::cout << "freeing" << std::endl; mc.free_space(1); total -= 1; } } t.join();}*

Finally, the problem we face is as follows. When we run the program, the assertion `total_mem >= mem_used_` fails. On some further investigation, I realized that our completion token was being called even when we do not call `try_run_one`, which was very weird.

Finally, somewhat more surprisingly, If I replace *post(stand_,std::move(posted_lambda));* by *post(pending_queue_,std::move(posted_lambda));*, things seem to work. However, the asio documentation says that only strands guarantee a FIFO execution order. I am not sure if using a simple `io_context` will work as a FIFO queue (even though it seems to in these examples).

Any inputs would be helpful - I am happy to hear the problem in this implementation, as well as other implementations (for example, using a std::queue as a proper queue instead of this io_context hack).

This question has also been posted here: https://stackoverflow.com/questions/76310252/c-boost-asio-building-a-conditi... .

Thanks, Suraaj _______________________________________________ Boost-users mailing list Boost-users@lists.boost.org https://lists.boost.org/mailman/listinfo.cgi/boost-users