simple_pool.hpp

/*
    basic-pool : Add asynchrony to your apps
 
    BSD 3-Clause License
 
    Copyright (c) 2021, Siddiq Software LLC
    All rights reserved.
 
    Redistribution and use in source and binary forms, with or without
    modification, are permitted provided that the following conditions are met:
 
    1. Redistributions of source code must retain the above copyright notice, this
       list of conditions and the following disclaimer.
 
    2. Redistributions in binary form must reproduce the above copyright notice,
       this list of conditions and the following disclaimer in the documentation
       and/or other materials provided with the distribution.
 
    3. Neither the name of the copyright holder nor the names of its
       contributors may be used to endorse or promote products derived from
       this software without specific prior written permission.
 
    THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
    AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
    IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
    DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
    FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
    DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
    SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
    CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
    OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
    OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
 
#pragma once
#ifndef SIMPLE_POOL_HPP
#define SIMPLE_POOL_HPP
 
#include "simple_worker.hpp"
#include <optional>
#include <latch>
#include <exception>
#include <atomic>
#include <utility>
 
#include "siddiqsoft/WaitableQueue.hpp"
#include "siddiqsoft/RunOnEnd.hpp"
 
namespace siddiqsoft
{
    template <typename T, uint16_t N = 0>
        requires std::is_move_constructible_v<T>
    struct simple_pool
    {
        static constexpr std::chrono::milliseconds DEFAULT_WAIT_FOR_NEXT_ITEM_MS {1500};

        simple_pool(simple_pool&&) = delete;

        simple_pool& operator=(simple_pool&&) = delete;

        simple_pool(simple_pool&) = delete;

        simple_pool& operator=(simple_pool&) = delete;
 

        ~simple_pool()
        {
            // Compared to skipping the following code, we save at least about 100ms
            // of idle time waiting for the threads to be signalled by default.
            // Request all threads to stop first
            for (auto& t : workers) {
                t.request_stop();
            }
 
            // Then wake them up and join
            for (auto& t : workers) {
                signal.release();
                if (t.joinable()) t.join();
            }
        }

        simple_pool(std::function<void(T&&)> c)
            : callback(std::move(c))
        {
            // *CRITICAL*
            // This is step is *critical* otherwise we will end up moving threads as we add elements to the vector.
            workers.reserve((N > 0) ? N : std::thread::hardware_concurrency());
 
            // Create as many threads as reported by the system..
            for (unsigned i = 0; i < ((N > 0) ? N : std::thread::hardware_concurrency()); i++) {
                // Add the thread with the main driver
                workers.emplace_back([&](std::stop_token st) {
                    // The driver runs forever until signalled to stop
                    // Tries to get next item ready in the queue (for max 1s cycle)
                    // If we have an item, invoke the callback with the item
                    while (!st.stop_requested()) {
                        try {
                            // The getNextItem performs the wait on the signal and if it expires, returns empty.
                            // If there is an item, it will get that item (minimizing move) and performs the pop
                            // and returns the item so we can invoke the callback outside the lock.
                            if (auto item = getNextItem(); item.has_value() && !st.stop_requested() && callback) {
                                // Delegate to the callback outside the lock
                                callback(std::move(*item));
                            }
                        }
                        catch (const std::exception& ex) {
                            // We swallow exceptions from the callback to avoid thread termination and log it if needed.
                            std::cerr << std::format("Ignoring Exception in simple_worker callback: {}", ex.what());
                        }
                    } // while ..continue until we're asked to stop
                });
            }
        }

        void queue(T&& item)
        {
            // With this interface, we can peform a perfect forward of the r-value from the caller into the
            // items internal container without the complexity of lambda capture forwards.
            items.emplace(std::forward<T>(item));
 
            // Use atomic fetch_add with release semantics to ensure thread-safe updates
            queueCounter.fetch_add(1, std::memory_order_release);
            signal.release();
        }
 
#if defined(NLOHMANN_JSON_VERSION_MAJOR)
        auto toJson() const -> nlohmann::json
        {
            const auto sz = items.size();
            return nlohmann::json {{"_typver", "siddiqsoft.asynchrony-lib.simple_pool/0.10"},
                                   {"workersSize", workers.size()},
                                   {"dequeSize", sz},
                                   {"queueCounter", queueCounter.load(std::memory_order_acquire)},
                                   {"waitInterval", DEFAULT_WAIT_FOR_NEXT_ITEM_MS.count()}};
        }
#endif
 
#ifdef _DEBUG
    public:
        std::atomic_uint64_t queueCounter {0};
#else
    private:
        std::atomic_uint64_t queueCounter {0};
#endif
 
    private:
        std::vector<std::jthread> workers {};

        std::function<void(T&&)> callback;

        std::counting_semaphore<> signal {0};

        siddiqsoft::WaitableQueue<T> items {};

        std::optional<T> getNextItem(const std::chrono::milliseconds& delta = DEFAULT_WAIT_FOR_NEXT_ITEM_MS)
        {
            return items.tryWaitItem(delta);
        }
    };
 
#if defined(NLOHMANN_JSON_VERSION_MAJOR)
    template <typename T, uint16_t N = 0>
    static auto to_json(nlohmann::json& dest, const siddiqsoft::simple_pool<T, N>& src) -> void const
    {
        dest = src.toJson();
    }
#endif
 
} // namespace siddiqsoft
#endif // !SIMPLE_POOL_HPP