resource_pool.hpp

/*
    asynchrony : 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
#include <cstdint>
#ifndef RESOURCE_POOL_HPP
#define RESOURCE_POOL_HPP
 
#include <stdexcept>
#include <mutex>
#include <shared_mutex>
#include <deque>
#include <format>
 
#include "siddiqsoft/RunOnEnd.hpp"
 
namespace siddiqsoft
{
    template <typename T>
        requires std::move_constructible<T>
    class resource_wrap
    {
    protected:
        T rsrc {};

        uint64_t debugId {static_cast<uint64_t>(std::rand())};

        std::function<void(T&&)> putbackCallback {};

        bool isValid {false};
 
    public:
        resource_wrap() = delete;

        resource_wrap(T&& src, std::function<void(T&&)>&& f = {})
            : rsrc(std::move(src))
            , putbackCallback(std::move(f))
            , isValid(true)
        {
#if defined(DEBUG)
            if constexpr (std::is_pointer_v<T>) {
                std::cerr << std::format("  - resource_wrap: debugId:{} {:p}\n", debugId, static_cast<void*>(rsrc));
            }
            else if constexpr (std::is_integral_v<T>) {
                std::cerr << std::format("  - resource_wrap: debugId:{} {}\n", debugId, rsrc);
            }
            else {
                std::cerr << std::format("  - resource_wrap: debugId:{}\n", debugId);
            }
#endif
        }

        resource_wrap(const T&) = delete;

        resource_wrap& operator=(T&& src)
        {
#if defined(DEBUG)
            std::cerr << std::format("  - resource_wrap: move into debugId:{}\n", debugId);
#endif
            rsrc    = std::move(src);
            isValid = true;
            return *this;
        };

        resource_wrap& operator=(const resource_wrap&) = delete;

        auto operator*() -> T& { return rsrc; }

        operator T() { return rsrc; }

        ~resource_wrap()
        {
#if defined(DEBUG)
            std::cerr << std::format("  - ~resource_wrap: putback debugId:{}  isValid:{}\n", debugId, isValid);
#endif
            // Only return resource if it's valid and callback exists
            // This prevents returning uninitialized or moved-out resources to the pool
            if (isValid && putbackCallback) {
                putbackCallback(std::move(rsrc));
                isValid = false;
            }
        }

#if defined(DEBUG)
        void invalidate() { isValid = false; }
#endif
    };

    template <typename T, uint16_t InitCapacity = sizeof(uint8_t)>
        requires((InitCapacity <= sizeof(uint16_t))) && std::move_constructible<T>
    class resource_pool
    {
    private:
        std::deque<T> _pool {};

        std::mutex _poolLock {};
 
    public:
        resource_pool() = default;

        resource_pool(resource_pool&) = delete;

        resource_pool(resource_pool&& src) = default;

        resource_pool& operator=(resource_pool&) = delete;

        resource_pool& operator=(resource_pool&& src) = default;

        ~resource_pool() { clear(); }

        void clear()
        {
            std::scoped_lock<std::mutex> l(_poolLock);
            _pool.clear();
        }

        auto size()
        {
            std::scoped_lock<std::mutex> l(_poolLock);
            return _pool.size();
        }

        [[nodiscard]] auto checkout() -> resource_wrap<T> /* throw() */
        {
            {
                std::scoped_lock<std::mutex> l(_poolLock);
 
                if (!_pool.empty()) {
                    RunOnEnd roe([&]() { _pool.pop_front(); });

                    auto autoReturnResource = [this](T&& rsrc) {
                        this->checkin(std::move(rsrc));
                    };
 
                    return {std::move(_pool.front()), autoReturnResource};
                    // The pop_front() happens within this scope and
                    // within the lock!
                }
            } // scope end
 
            throw std::runtime_error("Empty pool; add something first!");
        }

        void checkin(T&& rsrc)
        {
            std::scoped_lock<std::mutex> l(_poolLock);
            _pool.push_back(std::move(rsrc));
        }
    };
} // namespace siddiqsoft
#endif