QSqlDatabase open() crashes when db cannot be accessed ?

[R-P-H]

Hi, my application segfaults when trying to open a remote PostgreSQL database that is not accessible. When the database is accessible it works fine.

#include <QSqlDatabase>
QSqlDatabase db = QSqlDatabase::addDatabase("QPSQL");
db.setHostName("my_host");
db.setPort(5432);
db.setDatabaseName("my_db");
db.setUserName("my_user");
db.setPassword("my_password");
bool ok = db.open(); //-> crashes here after hanging for about 10 to 20 seconds
//...error checking if ok

I did see other posts about similar issues with mysql but they are nearly 5 years old.

[R-P-H]

So through much debugging I finally found the issue. Just 1 line of code:

QCoreApplication::processEvents();

I have a thread that runs at application startup and emits an image to a label for displaying. The above line of code was at the end of this loop. Removing the line of code doesn't appear to cause any other issues and solves the segfault when trying to open the database.

Why this causes the segfault in the first place is still bizarre to me...

[JonB]

@R-P-H
Would it help others to state the Qt version, operating system etc.?
Were you able to say whether it happens on any other versions?

[R-P-H]

@JonB said in QSqlDatabase open() crashes when db cannot be accessed ?:

@R-P-H
Would it help others to state the Qt version, operating system etc.?
Were you able to say whether it happens on any other versions?

@JonB Qt 5.14.0. MinGW 64-bit. Windows 10. No I have not tested with any other versions yet.

[Cobra91151]

@R-P-H

Hello!

I am curious, while it hanging for about 10 to 20 seconds, can you check this code: qDebug() << "isOpenError: " << db.isOpenError() << "\n" << db.lastError().nativeErrorCode() << "\n" << db.lastError().text();. What will it print?

[R-P-H]

@Cobra91151 said in QSqlDatabase open() crashes when db cannot be accessed ?:

@R-P-H

Hello!

I am curious, while it hanging for about 10 to 20 seconds, can you check this code: qDebug() << "isOpenError: " << db.isOpenError() << "\n" << db.lastError().nativeErrorCode() << "\n" << db.lastError().text();. What will it print?

Hi !

isOpenError:  true 
 "" 
 "could not connect to server: Connection timed out (0x0000274C/10060)\n\tIs the server running on host \"REDACTED\" and accepting\n\tTCP/IP connections on port 5432?\nQPSQL: Unable to connect"
22:09:22: The program has unexpectedly finished.

[Christian Ehrlicher]

So did you actually check if the server is running there? A simple telnet from the host to the server at port 5432 should show you if it's reachable.

[SGaist]

@Christian-Ehrlicher said in QSqlDatabase open() crashes when db cannot be accessed ?:

So did you actually check if the server is running there? A simple telnet from the host to the server at port 5432 should show you if it's reachable.

I think the point here is that open should cleanly fail and not trigger an application crash.

[R-P-H]

@Christian-Ehrlicher said in QSqlDatabase open() crashes when db cannot be accessed ?:

So did you actually check if the server is running there? A simple telnet from the host to the server at port 5432 should show you if it's reachable.

No, it's purposely not reachable.

[R-P-H]

@SGaist said in QSqlDatabase open() crashes when db cannot be accessed ?:

@Christian-Ehrlicher said in QSqlDatabase open() crashes when db cannot be accessed ?:

So did you actually check if the server is running there? A simple telnet from the host to the server at port 5432 should show you if it's reachable.

I think the point here is that open should cleanly fail and not trigger an application crash.

Exactly, yes.

[Christian Ehrlicher]

@SGaist said in QSqlDatabase open() crashes when db cannot be accessed ?:

I think the point here is that open should cleanly fail and not trigger an application crash.

It does not crash here with psql - the backtrace would be interesting.

[R-P-H]

@Christian-Ehrlicher said in QSqlDatabase open() crashes when db cannot be accessed ?:

@SGaist said in QSqlDatabase open() crashes when db cannot be accessed ?:

I think the point here is that open should cleanly fail and not trigger an application crash.

It does not crash here with psql - the backtrace would be interesting.

Hi, how can I get that ?

[SGaist]

Start your application in debug mode and you'll have it when it crashes through the debugger.

[R-P-H]

@SGaist said in QSqlDatabase open() crashes when db cannot be accessed ?:

Start your application in debug mode and you'll have it when it crashes through the debugger.

Ok, so very interesting, when I run the application in Debug mode it works as expected and does not crash...

[Cobra91151]

@R-P-H

Add the code below to the main.cpp file and try to run this code in both Debug and Release modes:

   qputenv("QT_FATAL_WARNINGS", "1");
   qSetMessagePattern("Type: %{type}\nProduct Name: %{appname}\nFile: %{file}\nLine: %{line}\nMethod: %{function}\nThreadID: %{threadid}\nThreadPtr: %{qthreadptr}\nMessage: %{message}");

It should print some info about this crash issue.

[R-P-H]

@Cobra91151 said in QSqlDatabase open() crashes when db cannot be accessed ?:

@R-P-H

Add the code below to the main.cpp file and try to run this code in both Debug and Release modes:

   qputenv("QT_FATAL_WARNINGS", "1");
   qSetMessagePattern("Type: %{type}\nProduct Name: %{appname}\nFile: %{file}\nLine: %{line}\nMethod: %{function}\nThreadID: %{threadid}\nThreadPtr: %{qthreadptr}\nMessage: %{message}");

It should print some info about this crash issue.

Hi, nothing is printed. It is a widget application, not a console app if that's why ?

[Cobra91151]

@R-P-H

You need to clean, run qmake and rebuild your project. It should display the debug info when the crash occurs in the Application output window.

[R-P-H]

@Cobra91151 said in QSqlDatabase open() crashes when db cannot be accessed ?:

@R-P-H

You need to clean, run qmake and rebuild your project. It should display the debug info when the crash occurs in the Application output window.

Yes, I did as you said and there is no additional output. I also created a new console project with the same database code and it runs fine. No idea what's wrong with my widget application in release mode...

[SGaist]

One other thing you can do is set the QT_DEBUG_PLUGINS environment to 1 in the Run part of the Project panel to see if there's something happening there.

Since you talked about a console app working fine, did you try to create a dummy widget project and just do the connection there ?

[R-P-H]

@SGaist said in QSqlDatabase open() crashes when db cannot be accessed ?:

One other thing you can do is set the QT_DEBUG_PLUGINS environment to 1 in the Run part of the Project panel to see if there's something happening there.

Since you talked about a console app working fine, did you try to create a dummy widget project and just do the connection there ?

I set the environment variable. I get the output from all the plugins on startup but nothing extra when calling open().

I created a fresh widget project and just added the connection part of the code and it works fine. I figure that this means there's some other library included in my original application that may be causing this issue ?

[R-P-H]

So through much debugging I finally found the issue. Just 1 line of code:

QCoreApplication::processEvents();

I have a thread that runs at application startup and emits an image to a label for displaying. The above line of code was at the end of this loop. Removing the line of code doesn't appear to cause any other issues and solves the segfault when trying to open the database.

Why this causes the segfault in the first place is still bizarre to me...