TrackableSignal.hpp

//------------------------------------------------------------------------------
/*
    This file is part of clio: https://github.com/XRPLF/clio
    Copyright (c) 2024, the clio developers.
 
    Permission to use, copy, modify, and distribute this software for any
    purpose with or without fee is hereby granted, provided that the above
    copyright notice and this permission notice appear in all copies.
 
    THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
    ANY  SPECIAL,  DIRECT,  INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
    WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
    ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
*/
//==============================================================================
 
#pragma once
 
#include "util/Mutex.hpp"
 
#include <boost/signals2.hpp>
#include <boost/signals2/connection.hpp>
#include <boost/signals2/variadic_signal.hpp>
 
#include <cstddef>
#include <functional>
#include <memory>
#include <mutex>
#include <unordered_map>
 
namespace feed::impl {

template <typename Session, typename... Args>
class TrackableSignal {
    using ConnectionPtr = Session*;
    using ConnectionSharedPtr = std::shared_ptr<Session>;
 
    // map of connection and signal connection, key is the pointer of the connection object
    // allow disconnect to be called in the destructor of the connection
    using ConnectionsMap = std::unordered_map<ConnectionPtr, boost::signals2::connection>;
    util::Mutex<ConnectionsMap> connections_;
 
    using SignalType = boost::signals2::signal<void(Args...)>;
    SignalType signal_;
 
public:
    bool
    connectTrackableSlot(ConnectionSharedPtr const& trackable, std::function<void(Args...)> slot)
    {
        auto connections = connections_.template lock<std::scoped_lock>();
        if (connections->contains(trackable.get())) {
            return false;
        }
 
        // This class can't hold the trackable's shared_ptr, because disconnect should be able to be
        // called in the the trackable's destructor. However, the trackable can not be destroyed
        // when the slot is being called either. `track_foreign` is racey when one shared_ptr is
        // tracked by multiple signals. Therefore we are storing a weak_ptr of the trackable and
        // using weak_ptr::lock() to atomically check existence and acquire a shared_ptr during slot
        // invocation. This guarantees to keep the trackable alive for the duration of the slot call
        // and avoids potential race conditions.
        connections->emplace(
            trackable.get(),
            signal_.connect([slot, weakTrackable = std::weak_ptr(trackable)](Args&&... args) {
                if (auto lifeExtender = weakTrackable.lock(); lifeExtender)
                    std::invoke(slot, std::forward<Args...>(args)...);
            })
        );
        return true;
    }

    bool
    disconnect(ConnectionPtr trackablePtr)
    {
        if (auto connections = connections_.template lock<std::scoped_lock>();
            connections->contains(trackablePtr)) {
            connections->operator[](trackablePtr).disconnect();
            connections->erase(trackablePtr);
            return true;
        }
        return false;
    }

    void
    emit(Args const&... args) const
    {
        signal_(args...);
    }

    std::size_t
    count() const
    {
        return connections_.template lock<std::scoped_lock>()->size();
    }
};
}  // namespace feed::impl