IXWebSocket/ixwebsocket/IXDNSLookup.cpp

/*
 *  IXDNSLookup.cpp
 *  Author: Benjamin Sergeant
 *  Copyright (c) 2018 Machine Zone, Inc. All rights reserved.
 */

#include "IXDNSLookup.h"
#include "IXNetSystem.h"

#include <string.h>
#include <chrono>
#include <thread>

namespace ix
{
    const int64_t DNSLookup::kDefaultWait = 1; // ms

    DNSLookup::DNSLookup(const std::string& hostname, int port, int64_t wait) :
        _hostname(hostname),
        _port(port),
        _wait(wait),
        _res(nullptr),
        _done(false)
    {
        ;
    }

    struct addrinfo* DNSLookup::getAddrInfo(const std::string& hostname,
                                            int port,
                                            std::string& errMsg)
    {
        struct addrinfo hints;
        memset(&hints, 0, sizeof(hints));
        hints.ai_flags = AI_ADDRCONFIG | AI_NUMERICSERV;
        hints.ai_family = AF_UNSPEC;
        hints.ai_socktype = SOCK_STREAM;

        std::string sport = std::to_string(port);

        struct addrinfo* res;
        int getaddrinfo_result = getaddrinfo(hostname.c_str(), sport.c_str(),
                                             &hints, &res);
        if (getaddrinfo_result)
        {
            errMsg = gai_strerror(getaddrinfo_result);
            res = nullptr;
        }
        return res;
    }

    struct addrinfo* DNSLookup::resolve(std::string& errMsg,
                                        const CancellationRequest& isCancellationRequested,
                                        bool cancellable)
    {
        return cancellable ? resolveCancellable(errMsg, isCancellationRequested)
                           : resolveUnCancellable(errMsg, isCancellationRequested);
    }

    struct addrinfo* DNSLookup::resolveUnCancellable(std::string& errMsg,
                                                     const CancellationRequest& isCancellationRequested)
    {
        errMsg = "no error";

        // Maybe a cancellation request got in before the background thread terminated ?
        if (isCancellationRequested && isCancellationRequested())
        {
            errMsg = "cancellation requested";
            return nullptr;
        }

        return getAddrInfo(_hostname, _port, errMsg);
    }

    struct addrinfo* DNSLookup::resolveCancellable(std::string& errMsg,
                                                   const CancellationRequest& isCancellationRequested)
    {
        errMsg = "no error";

        // Can only be called once, otherwise we would have to manage a pool
        // of background thread which is overkill for our usage.
        if (_done)
        {
            return nullptr; // programming error, create a second DNSLookup instance
                            // if you need a second lookup.
        }

        //
        // Good resource on thread forced termination
        // https://www.bo-yang.net/2017/11/19/cpp-kill-detached-thread
        //
        auto ptr = shared_from_this();
        std::weak_ptr<DNSLookup> self(ptr);

        int port = _port;
        std::string hostname(_hostname);

        // We make the background thread doing the work a shared pointer
        // instead of a member variable, because it can keep running when
        // this object goes out of scope, in case of cancellation
        auto t = std::make_shared<std::thread>(&DNSLookup::run, this, self, hostname, port);
        t->detach();

        while (!_done)
        {
            // Wait for 1 milliseconds, to see if the bg thread has terminated.
            // We do not use a condition variable to wait, as destroying this one
            // if the bg thread is alive can cause undefined behavior.
            std::this_thread::sleep_for(std::chrono::milliseconds(_wait));

            // Were we cancelled ?
            if (isCancellationRequested && isCancellationRequested())
            {
                errMsg = "cancellation requested";
                return nullptr;
            }
        }

        // Maybe a cancellation request got in before the bg terminated ?
        if (isCancellationRequested && isCancellationRequested())
        {
            errMsg = "cancellation requested";
            return nullptr;
        }

        errMsg = getErrMsg();
        return getRes();
    }

    void DNSLookup::run(std::weak_ptr<DNSLookup> self, std::string hostname, int port) // thread runner
    {
        // We don't want to read or write into members variables of an object that could be
        // gone, so we use temporary variables (res) or we pass in by copy everything that
        // getAddrInfo needs to work.
        std::string errMsg;
        struct addrinfo* res = getAddrInfo(hostname, port, errMsg);

        if (self.lock())
        {
            // Copy result into the member variables
            setRes(res);
            setErrMsg(errMsg);

            _done = true;
        }
    }

    void DNSLookup::setErrMsg(const std::string& errMsg)
    {
        std::lock_guard<std::mutex> lock(_errMsgMutex);
        _errMsg = errMsg;
    }

    const std::string& DNSLookup::getErrMsg()
    {
        std::lock_guard<std::mutex> lock(_errMsgMutex);
        return _errMsg;
    }

    void DNSLookup::setRes(struct addrinfo* addr)
    {
        std::lock_guard<std::mutex> lock(_resMutex);
        _res = addr;
    }

    struct addrinfo* DNSLookup::getRes()
    {
        std::lock_guard<std::mutex> lock(_resMutex);
        return _res;
    }
}