ConnectionPool.hpp

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#pragma once
 
#include <condition_variable>
#include <cstddef>
#include <memory>
#include <mutex>
#include <string>
#include <vector>
 
#include "DatabaseBackend.hpp"
 
class ConnectionPool;
 
class ConnectionLease {
  public:
    ConnectionLease() noexcept = default;
    ConnectionLease(ConnectionPool& pool, std::unique_ptr<NativeDatabase::Connection> conn) noexcept;
    ConnectionLease(ConnectionLease&& other) noexcept;
    ConnectionLease& operator=(ConnectionLease&& other) noexcept;
    ~ConnectionLease();
 
    ConnectionLease(const ConnectionLease&) = delete;
    ConnectionLease& operator=(const ConnectionLease&) = delete;
 
    [[nodiscard]] explicit operator bool() const noexcept {
        return static_cast<bool>(conn_);
    }
 
    void discard() noexcept {
        recycle_ = false;
    }
 
    [[nodiscard]] NativeDatabase::Connection& get() noexcept {
        return *conn_;
    }
 
  private:
    ConnectionPool* pool_{nullptr};
    std::unique_ptr<NativeDatabase::Connection> conn_;
    bool recycle_{true};
};
 
class ConnectionPool {
  public:
    ConnectionPool() = default;
    ~ConnectionPool();
 
    ConnectionPool(const ConnectionPool&) = delete;
    ConnectionPool& operator=(const ConnectionPool&) = delete;
 
    [[nodiscard]] bool connect(const std::string& connection_string, std::size_t pool_size = 4);
    void disconnect() noexcept;
    [[nodiscard]] bool has_connection() const noexcept;
    [[nodiscard]] ConnectionLease acquire();
 
  private:
    friend class ConnectionLease;
    void release(std::unique_ptr<NativeDatabase::Connection> conn) noexcept;
    void drop_and_replace() noexcept;
 
    std::string connection_string_;
    std::size_t max_size_{4};
    mutable std::mutex mutex_;
    std::condition_variable cv_;
    std::vector<std::unique_ptr<NativeDatabase::Connection>> idle_;
    std::size_t in_use_{0};
    bool connected_{false};
};