Using QNetworkAccessManager->Post() causes SEGV on closing the application - c++

Question

****UPDATED: I noticed that I get the segfault only on Windows, on Linux it's fine. On Windows I use QT 5.5 and MinGW32. I still want to know why.
**** Initial Question:
Nothing tricky here, I create a basic Console Application. I have a QNetworkAccessManager sending a Post() request. When I close the console, there is a segfault.
Note that the request is sent and received successfully, my question is only about that segfault.
If no Post() request are sent, no crash on closing the console. There is not much help from the stack.
Stack
0 ntdll!RtlFreeHeap 0x77b5e041
1 ucrtbase!free 0x5e4c5eab
2 LIBEAY32!CRYPTO_free 0x5e5a123e
Main.cpp
#include <QCoreApplication>
#include "CNetworkHandleTest.h"
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
CNetworkHandleTest net;
net.start();
return a.exec();
}
CNetworkHandleTest.cpp
#include "CNetworkHandleTest.h"
CNetworkHandleTest::CNetworkHandleTest()
{
m_Manager = new QNetworkAccessManager(this);
// Connect the network manager so we can handle the reply
connect(m_Manager, SIGNAL(finished(QNetworkReply*)), this, SLOT(onFinished(QNetworkReply*)));
m_nTotalBytes = 0;
}
CNetworkHandleTest::~CNetworkHandleTest()
{
disconnect();
m_Manager->deleteLater();
}
void CNetworkHandleTest::onFinished(QNetworkReply* reply)
{
// Look at reply error
// Called when all the data is receivedqDebug() << "Error code:" << reply->error();
qDebug() << "Error string:" << reply->errorString();
reply->close();
reply->deleteLater();
}
void CNetworkHandleTest::start()
{
// Configure the URL string and then set the URL
QString sUrl(BASE_URL);
sUrl.append("/console/5555/upload");
m_Url.setUrl(sUrl);
// Make the request object based on our URL
QNetworkRequest request(m_Url);
// Set request header (not sure how or why this works, but it works)
// \todo investigate
request.setHeader(QNetworkRequest::ContentTypeHeader, "application/x-www-form-urlenconded");
// Make file object associated with our DB file
QFile file("/tx_database.db");
if(!file.open(QIODevice::ReadOnly))
{
qDebug() << "Failed to open file";
}
// Read the entire file as a binary blob
QByteArray data(file.readAll());
// Set our request to our request object
// Note: there should probably be a flag so that when start is called it does not do
// any processing in case we are already in the middle of processing a request
m_Request = request;
// Send it
m_Reply = m_Manager->post(m_Request, data);
// Need to connect the signals and slots to the new reply object (manager makes a new
// reply object every post; may need to investigate if memory should be freed when
// done processing a response)
connect(m_Reply, SIGNAL(readyRead()), this, SLOT(onReadyRead()));
connect(m_Manager, SIGNAL(authenticationRequired(QNetworkReply*, QAuthenticator*)),
this, SLOT(onAuthenticationRequired(QNetworkReply*, QAuthenticator*)));
}
void CNetworkHandleTest::onReadyRead()
{
// Whenever data becomes available, this slot is called. It is called every time data
// is available, not when all the data has been received. It is our responsibility to
// keep track of how much we have received if we want to show progress or whatever
// but we do not need to keep track if we have received all the data. The slot
// OnFinished will be called when the all the data has been received.
qDebug() << "Bytes available:" << m_Reply->bytesAvailable();
m_nTotalBytes += m_Reply->bytesAvailable();
qDebug() << "Bytes thus far:" << m_nTotalBytes;
QByteArray responseData = m_Reply->readAll();
qDebug() << "Response" << responseData;
m_Reply->size();
}
void CNetworkHandleTest::onAuthenticationRequired(QNetworkReply* reply, QAuthenticator* authenticator)
{
}
CNetworkHandleTest.h
#ifndef CNETWORKHANDLETEST_H
#define CNETWORKHANDLETEST_H
// Required packages
#include <QNetworkAccessManager>
#include <QNetworkRequest>
#include <QNetworkReply>
#include <QByteArray>
#include <QUrlQuery>
#include <QHostInfo>
#include <QObject>
#include <QUrl>
// Helper packages
#include <QCoreApplication>
#include <QFile>
#include <QDir>
// Our packages
#include <iostream>
#include <fstream>
#include <cstdlib>
#define BASE_URL "localhost:5000"
#define BOUNDARY "123456787654321"
class CNetworkHandleTest : public QObject
{
Q_OBJECT
public:
CNetworkHandleTest();
~CNetworkHandleTest();
void start();
protected Q_SLOTS:
void onFinished(QNetworkReply* reply);
void onReadyRead();
void onAuthenticationRequired(QNetworkReply* reply, QAuthenticator* authenticator);
private:
QNetworkAccessManager* m_Manager;
QNetworkRequest m_Request;
QNetworkReply* m_Reply;
QUrl m_Url;
int m_nTotalBytes;
};
#endif // CNETWORKHANDLETEST_H

Answer 1

When you close the console, your program dies in a most ungraceful manner. You need to write some code to make it graceful instead:
The below is a complete test case:
// https://github.com/KubaO/stackoverflown/tree/master/questions/network-cleanup-40695076
#include <QtNetwork>
#include <windows.h>
extern "C" BOOL WINAPI handler(DWORD)
{
qDebug() << "bye world";
qApp->quit();
return TRUE;
}
int main(int argc, char *argv[])
{
SetConsoleCtrlHandler(&handler, TRUE);
QCoreApplication a(argc, argv);
QNetworkAccessManager mgr;
int totalBytes = 0;
QObject::connect(&mgr, &QNetworkAccessManager::finished, [](QNetworkReply *reply){
qDebug() << "Error string:" << reply->errorString();
});
QNetworkRequest request(QUrl{"http://www.google.com"});
request.setHeader(QNetworkRequest::ContentTypeHeader, "application/x-www-form-urlenconded");
auto reply = mgr.post(request, QByteArray{"abcdefgh"});
QObject::connect(reply, &QIODevice::readyRead, [&]{
qDebug() << "Bytes available:" << reply->bytesAvailable();
totalBytes += reply->bytesAvailable();
qDebug() << "Bytes thus far:" << totalBytes;
reply->readAll();
});
QObject::connect(reply, &QObject::destroyed, []{
qDebug() << "reply gone";
});
QObject::connect(&mgr, &QObject::destroyed, []{
qDebug() << "manager gone";
});
return a.exec();
}
If you press Ctrl-C or click [x] on the console window, the shutdown is orderly, the output being:
[...]
bye world
reply gone
manager gone