Possible memory leak on QDnsLookup?

Comandillos

Hi there, i'v been developing a small DNS server for caching some request in local (dnsmasq / BIND like). For that, I use a QMap for caching request and a QDnsLookup for domains I can't resolve locally.

Everything works fine, until QDnsLookup gets in. I'v been profiling the application for memory leaks and i'v discovered everytime I call the method 'lookup' from a QDnsLookup instance, a memory leak of ~9kb appears. The slot where I connect the lookup instance is totally empty, and I free the instance correctly.

If I don't call the 'lookup' method, no memory leaks appears, so I was thinking about QDnsLookup class having a memory leak.

This is the class that I use as the DNS resolver

#include "dnsresolver.h"

quint32 DNSResolver::resolveDomain(QString domain)
{
    if (cacheResolved){
        return cache->value(domain).toIPv4Address();
    }else{
        lookup->lookup();
        return 0;
    }
}

void DNSResolver::handleExternalResolve()
{
    if (!lookup->hostAddressRecords().isEmpty()){
        auto record = lookup->hostAddressRecords().first();
        auto host = record.value();

        dnspack->addArecord(host.toIPv4Address());
        cache->insert(dnspack->domain, host);
    }

    delete lookup;
    emit sendResponse(*dnspack->datagram, senderHost.toIPv4Address(), senderPort);
}

DNSResolver::DNSResolver(t_resolve t, QObject *parent)
    : QThread(parent)
{
    this->socket = t.socket;
    this->datagram = t.datagram;
    this->senderHost = t.senderHost;
    this->cache = t.cache;
    this->senderPort = t.senderPort;

    // Alloc a DNS packet for parsing
    dnspack = new DNS;
    dnspack->parse(&datagram);
    dnspack->makeAnswer();

    // Check if we can resolve by cache
    cacheResolved = cache->contains(dnspack->domain);

    if (!cacheResolved){
        // Alloc a external lookup object
        this->lookup = new QDnsLookup(QDnsLookup::A, dnspack->domain, QHostAddress(FORWARDING_DNS_SERVER));
        connect(lookup, SIGNAL(finished()), this, SLOT(handleExternalResolve()));
    }

}

DNSResolver::~DNSResolver()
{
    delete dnspack;
}

void DNSResolver::run()
{
    auto ip = resolveDomain(dnspack->domain);
    if (ip != 0){
        dnspack->addArecord(ip);
        emit sendResponse(*dnspack->datagram, senderHost.toIPv4Address(), senderPort);
    }
}

Thanks!!!!

SGaist

Hi,

What version of Qt are you using ? On what platform ?

Can you reproduce that with a smaller example ?

Comandillos

Qt 5.8, macOS 10.12.4.
That's only a simple use case, but as I said, the problem can be reproduced just instancing a QDnsLookup instance and calling the lookup method. This is an smaller example.

#include "dnsresolver.h"

DnsResolver::DnsResolver(QObject *parent) : QObject(parent)
{
    lookup = new QDnsLookup(QDnsLookup::A, "qt.io",QHostAddress("8.8.8.8"));
    connect(lookup, SIGNAL(finished()), this, SLOT(handleLookup()));
    lookup->lookup();
}

void DnsResolver::handleLookup()
{
    // I use sleep for letting the profiler inspect the memory trace with enough time
    QThread::sleep(10);

    // I won't use lookup anymore
    delete lookup;

    // Let's close the app
    QCoreApplication::quit();
}

This code gives me a memory leak of 944 bytes on 'libresolv'. I don't know if it's Qt problem or 'libresolv' problem (I don't know what's even libresolv).

Here's the profiler message.

Profiler message image

SGaist

Which version of libresolv is it exactly ?

AFAIK, that's the library that currently implements the name resolution and related stuff on Linux style OSs.