async_read_file.cpp

// The MIT License (MIT)
//
// Copyright (c) 2021 Mateusz Pusz
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.
 
#include <mp-coro/sync_await.h>
#include <mp-coro/task.h>
#include <filesystem>
#include <fstream>
#include <iostream>
#include <source_location>
#include <syncstream>
#include <thread>
 
struct tid_t {
    friend std::ostream &operator<<(std::ostream &os, tid_t) {
        return os << "(tid=" << std::this_thread::get_id() << ')';
    }
};
inline constexpr tid_t tid;
 
std::jthread worker;
 
mp_coro::task<std::string::size_type> async_read_file(const std::filesystem::path &path) {
    struct awaitable {
        awaitable(std::filesystem::path path) : path_(std::move(path)) {}
        static bool await_ready() noexcept { return false; }
        void await_suspend(std::coroutine_handle<> handle) {
            auto work = [handle, this] {
                std::osyncstream(std::cout)
                    << tid << " worker thread: opening file " << path_ << '\n';
                auto stream = std::ifstream(path_);
                std::osyncstream(std::cout) << tid << " worker thread: reading file\n";
                result_.assign(std::istreambuf_iterator<char>(stream),
                               std::istreambuf_iterator<char>());
                std::osyncstream(std::cout) << tid << " worker thread: resuming coro\n";
                handle.resume();
                std::osyncstream(std::cout) << tid << " worker thread: exiting\n";
            };
            worker = std::jthread(work); // TODO: Fix that (replace with a thread pool)
        }
        std::string await_resume() noexcept { return std::move(result_); }
 
      private:
        const std::filesystem::path &path_;
        std::string result_;
    };
 
    std::osyncstream(std::cout) << tid << " readFile(): about to read file async\n";
    const std::string result = co_await awaitable(path);
    std::osyncstream(std::cout) << tid << " readFile(): about to return (size " << result.size()
                                << ")\n";
 
    co_return result.size();
}
 
void f1(const std::filesystem::path &path) {
    auto t = async_read_file(path);
    std::osyncstream(std::cout) << "Result: " << sync_await(t) << '\n';
}
 
mp_coro::task<> f2(const std::filesystem::path &path) {
    auto t = async_read_file(path);
    std::osyncstream(std::cout) << "Result: " << co_await t << '\n';
}
 
int main() {
    try {
        auto path = "/etc/passwd";
        f1(path);
        sync_await(f2(path));
    } catch (const std::exception &ex) {
        std::osyncstream(std::cout) << "Unhandled exception: " << ex.what() << '\n';
    }
}