/*
 * Copyright (C) 2021-present ScyllaDB
 */

/*
 * SPDX-License-Identifier: LicenseRef-ScyllaDB-Source-Available-1.0
 */

#pragma once

#include "utils/log.hh"

#include "seastarx.hh"
#include "utils/updateable_value.hh"
#include "utils/scoped_item_list.hh"

#include <cstdint>

#include <seastar/core/file-types.hh>
#include <seastar/core/future.hh>
#include <seastar/core/gate.hh>
#include <seastar/util/noncopyable_function.hh>
#include <seastar/net/api.hh>
#include <seastar/net/tls.hh>
#include <seastar/core/semaphore.hh>

namespace generic_server {

class server;

// A generic TCP connection.
//
// This class is used in tandem with the `server`class to implement a protocol
// specific TCP connection.
//
// Protocol specific classes are expected to override the `process_request`
// member function to perform request processing. This base class provides a
// `_read_buf` and a `_write_buf` for reading requests and writing responses.
class connection {
    friend class server;
public:
    using connection_process_loop = noncopyable_function<future<> ()>;
    using execute_under_tenant_type = noncopyable_function<future<> (connection_process_loop)>;
    bool _tenant_switch = false;
    struct cpu_concurrency_t {
        named_semaphore& semaphore;
        semaphore_units<named_semaphore_exception_factory> units;
        bool stopped;
    };
    cpu_concurrency_t _conns_cpu_concurrency;
    execute_under_tenant_type _execute_under_current_tenant = no_tenant();
protected:
    server& _server;
    utils::scoped_item_list<std::reference_wrapper<connection>>::handle _connections_list_entry;

    connected_socket _fd;
    input_stream<char> _read_buf;
    output_stream<char> _write_buf;
    future<> _ready_to_respond = make_ready_future<>();
    seastar::named_gate _pending_requests_gate;
    seastar::gate::holder _hold_server;

private:
    future<> process_until_tenant_switch();
    bool shutdown_input();
    bool shutdown_output();
public:
    connection(server& server, connected_socket&& fd, named_semaphore& sem, semaphore_units<named_semaphore_exception_factory> initial_sem_units);
    virtual ~connection();

    virtual future<> process();

    virtual void handle_error(future<>&& f) = 0;

    virtual future<> process_request() = 0;

    virtual void on_connection_ready();

    virtual void shutdown();

    void switch_tenant(execute_under_tenant_type execute);

    static execute_under_tenant_type no_tenant();
};

struct config {
    utils::updateable_value<uint32_t> uninitialized_connections_semaphore_cpu_concurrency;
    utils::updateable_value<uint32_t> shutdown_timeout_in_seconds;
};

// A generic TCP socket server.
//
// This class can be used as a base for a protocol specific TCP socket server
// that listens to incoming connections and processes requests coming over the
// connection.
//
// The provides a `listen` member function that creates a TCP server socket and
// registers it to the Seastar reactor. The class also provides a `stop` member
// function that can be used to safely stop the server.
//
// Protocol specific classes that inherit `server` are expected to also inherit
// a connection class from `connection` and override the `make_connection` member
// function to create a protocol specific connection upon `accept`.
class server {
    friend class connection;

protected:
    sstring _server_name;
    logging::logger& _logger;
    seastar::named_gate _gate;
    future<> _all_connections_stopped = make_ready_future<>();
    uint64_t _total_connections = 0;
    uint64_t _shed_connections = 0;
    uint64_t _blocked_connections = 0;
    future<> _listeners_stopped = make_ready_future<>();
    utils::scoped_item_list<std::reference_wrapper<connection>> _connections_list;
    std::vector<server_socket> _listeners;
    shared_ptr<seastar::tls::server_credentials> _credentials;
    seastar::abort_source _abort_source;
private:
    utils::updateable_value<uint32_t> _conns_cpu_concurrency;
    utils::observer<uint32_t> _conns_cpu_concurrency_observer;
    uint32_t _prev_conns_cpu_concurrency;
    named_semaphore _conns_cpu_concurrency_semaphore;
    std::chrono::seconds _shutdown_timeout;
public:
    server(const sstring& server_name, logging::logger& logger, config cfg);

    virtual ~server() = default;

    // Makes sure listening sockets no longer generate new connections and aborts the
    // connected sockets, so that new requests are not served and existing requests don't
    // send responses back.
    //
    // It does _not_ wait for any internal activity started by the established connections
    // to finish. It's the .stop() method that does it
    future<> shutdown();
    future<> stop();

    future<> listen(socket_address addr,
        std::shared_ptr<seastar::tls::credentials_builder> creds,
        bool is_shard_aware, bool keepalive,
        std::optional<file_permissions> unix_domain_socket_permissions,
        std::function<server&()> get_shard_instance = {}
        );

    future<> do_accepts(int which, bool keepalive, socket_address server_addr);

protected:
    virtual seastar::shared_ptr<connection> make_connection(socket_address server_addr, connected_socket&& fd, socket_address addr, named_semaphore& sem, semaphore_units<named_semaphore_exception_factory> initial_sem_units) = 0;

    future<> for_each_gently(noncopyable_function<void(connection&)>);
};

}