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QSqlQuery::exec() freezes over OpenVPN

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  • P petar

    @JonB
    Maybe you are right. I don't know if it is doable.

    But the whole point is that if QSqlQuery::exec() would work ok, then there would be no need for another thread, because query (for 25-50,000 records) takes only few seconds over OpenVPN, and that is acceptable, and if query returns false, i can retrieve fewer records (LIMIT, or segment it,...) in another try or do whatever I want, but don't freeze the whole app for God's sake.

    For example, what if the network (over OpenVPN) is gone right before I start QSqlQuery::exec(), should it really freeze?

    Or just return false.

    JonBJ Offline
    JonBJ Offline
    JonB
    wrote on last edited by
    #10

    @petar
    The Qt SQL stuff uses drivers to do the low-level stuff. It may not be a "fault" in QSqlQuery::exec(), it may be the underlying protocol talking to SQL over OpenVPN which is what "freezes", and there is nothing the Qt level can do about that.

    P 1 Reply Last reply
    1
    • JonBJ JonB

      @petar
      The Qt SQL stuff uses drivers to do the low-level stuff. It may not be a "fault" in QSqlQuery::exec(), it may be the underlying protocol talking to SQL over OpenVPN which is what "freezes", and there is nothing the Qt level can do about that.

      P Offline
      P Offline
      petar
      wrote on last edited by
      #11

      @JonB
      Aren't those drivers a part of Qt (eg. QPSQL for Postgres), it's not external driver.

      JonBJ jsulmJ 2 Replies Last reply
      0
      • P petar

        @JonB
        Aren't those drivers a part of Qt (eg. QPSQL for Postgres), it's not external driver.

        JonBJ Offline
        JonBJ Offline
        JonB
        wrote on last edited by
        #12

        @petar
        Yes, but ultimately they must leverage some lower-level protocol code (e.g. ODBC, or whatever for MySQL or Postgres), and that is where it may be "freezing" over OpenVPN? I was just saying it may not be QSqlQuery::exec() code itself, which may be built atop other levels, which freezes, and it may not be able to do anything about it, which you are asking it to do.

        1 Reply Last reply
        0
        • P petar

          Ok, and then what. Suppose the query in another thread succeeds. How can I use that query in main thread and attach it to a QSqlQueryModel model and display data in QTableView?

          If the query freezes in another thread, I suppose I can handle that with some timeout. Is there a way to kill the thread that left frozen?

          A.A.SEZENA Offline
          A.A.SEZENA Offline
          A.A.SEZEN
          wrote on last edited by
          #13

          @petar said in QSqlQuery::exec() freezes over OpenVPN:

          Ok, and then what. Suppose the query in another thread succeeds. How can I use that query in main thread and attach it to a QSqlQueryModel model and display data in QTableView?
          If the query freezes in another thread, I suppose I can handle that with some timeout. Is there a way to kill the thread that left frozen?

          Yes.

          void MainWindow::threadStart()
          {
              threadQuery * tQuery = new threadQuery;
              connect(tQuery, &threadQuery::record, this, &MainWindow::record);
              tQuery->start();
          }
          
          void MainWindow::record(QSqlQueryModel *model)
          {
              ui->tableView->setModel(model);
          }
          
          void threadQuery::run()
          {
              QSqlQuery q(conn);
              if(q.exec("SELECT * FROM table")) {
                  model->setQuery(q);
                  emit this->record(model);
              }
          
          }
          
          
          P JonBJ 2 Replies Last reply
          0
          • A.A.SEZENA A.A.SEZEN

            @petar said in QSqlQuery::exec() freezes over OpenVPN:

            Ok, and then what. Suppose the query in another thread succeeds. How can I use that query in main thread and attach it to a QSqlQueryModel model and display data in QTableView?
            If the query freezes in another thread, I suppose I can handle that with some timeout. Is there a way to kill the thread that left frozen?

            Yes.

            void MainWindow::threadStart()
            {
                threadQuery * tQuery = new threadQuery;
                connect(tQuery, &threadQuery::record, this, &MainWindow::record);
                tQuery->start();
            }
            
            void MainWindow::record(QSqlQueryModel *model)
            {
                ui->tableView->setModel(model);
            }
            
            void threadQuery::run()
            {
                QSqlQuery q(conn);
                if(q.exec("SELECT * FROM table")) {
                    model->setQuery(q);
                    emit this->record(model);
                }
            
            }
            
            
            P Offline
            P Offline
            petar
            wrote on last edited by
            #14

            @A-A-SEZEN
            Ok, thanks, but what about this limit from the documentation?

            It says: "A connection can only be used from within the thread that created it. Moving connections between threads or creating queries from a different thread is not supported."

            How can I further work with this query/model in my main thread?

            What happens when your code in thread:

            if(q.exec("SELECT * FROM table")) {
            

            (precisely exec() function) freezes?

            1 Reply Last reply
            0
            • A.A.SEZENA Offline
              A.A.SEZENA Offline
              A.A.SEZEN
              wrote on last edited by
              #15

              @petar
              Your application will not freeze due to Thread.
              You know the integrity of your code.

              P 1 Reply Last reply
              0
              • A.A.SEZENA A.A.SEZEN

                @petar said in QSqlQuery::exec() freezes over OpenVPN:

                Ok, and then what. Suppose the query in another thread succeeds. How can I use that query in main thread and attach it to a QSqlQueryModel model and display data in QTableView?
                If the query freezes in another thread, I suppose I can handle that with some timeout. Is there a way to kill the thread that left frozen?

                Yes.

                void MainWindow::threadStart()
                {
                    threadQuery * tQuery = new threadQuery;
                    connect(tQuery, &threadQuery::record, this, &MainWindow::record);
                    tQuery->start();
                }
                
                void MainWindow::record(QSqlQueryModel *model)
                {
                    ui->tableView->setModel(model);
                }
                
                void threadQuery::run()
                {
                    QSqlQuery q(conn);
                    if(q.exec("SELECT * FROM table")) {
                        model->setQuery(q);
                        emit this->record(model);
                    }
                
                }
                
                
                JonBJ Offline
                JonBJ Offline
                JonB
                wrote on last edited by
                #16

                @A-A-SEZEN

                    QSqlQuery q(conn);
                    if(q.exec("SELECT * FROM table")) {
                        model->setQuery(q);
                

                I don't know, but have you verified whether this does not execute the query twice?

                P 1 Reply Last reply
                0
                • JonBJ JonB

                  @A-A-SEZEN

                      QSqlQuery q(conn);
                      if(q.exec("SELECT * FROM table")) {
                          model->setQuery(q);
                  

                  I don't know, but have you verified whether this does not execute the query twice?

                  P Offline
                  P Offline
                  petar
                  wrote on last edited by
                  #17

                  @JonB
                  No, only once.

                  1 Reply Last reply
                  1
                  • A.A.SEZENA A.A.SEZEN

                    @petar
                    Your application will not freeze due to Thread.
                    You know the integrity of your code.

                    P Offline
                    P Offline
                    petar
                    wrote on last edited by
                    #18

                    @A-A-SEZEN
                    Can I use thread for queries working with QSqlQuery and QSqlQueryModel? I am not sure. What about sharing connections (that is not allowed) between threads, and sharing queries?

                    A.A.SEZENA 1 Reply Last reply
                    0
                    • P petar

                      @JonB
                      Aren't those drivers a part of Qt (eg. QPSQL for Postgres), it's not external driver.

                      jsulmJ Offline
                      jsulmJ Offline
                      jsulm
                      Lifetime Qt Champion
                      wrote on last edited by
                      #19

                      @petar said in QSqlQuery::exec() freezes over OpenVPN:

                      Aren't those drivers a part of Qt (eg. QPSQL for Postgres), it's not external driver.

                      Those use client libraries provided by the SQL database system like libmysqlclient for MySQL. To see whether this is an issue in Qt or database system you can try to execute exactly the same query over VPN with the client application of your database system.

                      https://forum.qt.io/topic/113070/qt-code-of-conduct

                      P 1 Reply Last reply
                      2
                      • jsulmJ jsulm

                        @petar said in QSqlQuery::exec() freezes over OpenVPN:

                        Aren't those drivers a part of Qt (eg. QPSQL for Postgres), it's not external driver.

                        Those use client libraries provided by the SQL database system like libmysqlclient for MySQL. To see whether this is an issue in Qt or database system you can try to execute exactly the same query over VPN with the client application of your database system.

                        P Offline
                        P Offline
                        petar
                        wrote on last edited by petar
                        #20

                        @jsulm
                        Good point. I checked the same query over OpenVPN with LIMIT 25,000 - 200,000 records in Qt and in plain psql. In Qt QSqlQuery::exec() almost always freezes, while psql never.

                        1 Reply Last reply
                        0
                        • P Offline
                          P Offline
                          petar
                          wrote on last edited by
                          #21

                          I tested it and it turns out it freezes even in non GUI applications, so it doesn't have to do anything with the GUI thing.

                          I just started QSqlQuery::exec() in non GUI app and it freezes forever over OpenVPN, while the same code works ok over LAN or on localhost. On LAN sometimes QSqlQuery::exec() returns false which is ok, but never freezes forever.

                          jsulmJ 1 Reply Last reply
                          0
                          • P petar

                            @A-A-SEZEN
                            Can I use thread for queries working with QSqlQuery and QSqlQueryModel? I am not sure. What about sharing connections (that is not allowed) between threads, and sharing queries?

                            A.A.SEZENA Offline
                            A.A.SEZENA Offline
                            A.A.SEZEN
                            wrote on last edited by
                            #22

                            @petar said in QSqlQuery::exec() freezes over OpenVPN:

                            Can I use thread for queries working with QSqlQuery and QSqlQueryModel? I am not sure. What about sharing connections (that is not allowed) between threads, and sharing queries?

                            The codes above are thread codes I've tried for you.
                            I've already shared critical parts. The rest are standard thread class and database connections. It is up to you how much you can use within the integrity of your code.
                            SQLite does not allow concurrent operations to the same table. Other than that, you will have no problems. You can create a new SQLite connection in a thread, or you can include your connection class in a thread class. I have used both cases. I like to manage it myself instead of the model. I am using QTableWidget instead of QTableView. So I can't comment much on your situation.
                            Regards.

                            P 1 Reply Last reply
                            0
                            • P petar

                              I tested it and it turns out it freezes even in non GUI applications, so it doesn't have to do anything with the GUI thing.

                              I just started QSqlQuery::exec() in non GUI app and it freezes forever over OpenVPN, while the same code works ok over LAN or on localhost. On LAN sometimes QSqlQuery::exec() returns false which is ok, but never freezes forever.

                              jsulmJ Offline
                              jsulmJ Offline
                              jsulm
                              Lifetime Qt Champion
                              wrote on last edited by
                              #23

                              @petar You can check Qt bug tracker and file a bug if there is nothing yet for this issue

                              https://forum.qt.io/topic/113070/qt-code-of-conduct

                              P 1 Reply Last reply
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                              • jsulmJ jsulm

                                @petar You can check Qt bug tracker and file a bug if there is nothing yet for this issue

                                P Offline
                                P Offline
                                petar
                                wrote on last edited by
                                #24

                                @jsulm
                                I've already done this but got the answer from Christian Ehrlicher that this is probably a bug inside the libpq Postgres lib and that there is nothing they can do about it (concerning Qt).

                                jsulmJ 1 Reply Last reply
                                0
                                • P petar

                                  @jsulm
                                  I've already done this but got the answer from Christian Ehrlicher that this is probably a bug inside the libpq Postgres lib and that there is nothing they can do about it (concerning Qt).

                                  jsulmJ Offline
                                  jsulmJ Offline
                                  jsulm
                                  Lifetime Qt Champion
                                  wrote on last edited by
                                  #25

                                  @petar You can comment in the bug and write that you tested without Qt and it worked.

                                  https://forum.qt.io/topic/113070/qt-code-of-conduct

                                  P 1 Reply Last reply
                                  0
                                  • A.A.SEZENA A.A.SEZEN

                                    @petar said in QSqlQuery::exec() freezes over OpenVPN:

                                    Can I use thread for queries working with QSqlQuery and QSqlQueryModel? I am not sure. What about sharing connections (that is not allowed) between threads, and sharing queries?

                                    The codes above are thread codes I've tried for you.
                                    I've already shared critical parts. The rest are standard thread class and database connections. It is up to you how much you can use within the integrity of your code.
                                    SQLite does not allow concurrent operations to the same table. Other than that, you will have no problems. You can create a new SQLite connection in a thread, or you can include your connection class in a thread class. I have used both cases. I like to manage it myself instead of the model. I am using QTableWidget instead of QTableView. So I can't comment much on your situation.
                                    Regards.

                                    P Offline
                                    P Offline
                                    petar
                                    wrote on last edited by
                                    #26

                                    @A-A-SEZEN
                                    Thanks for your reply. Interesting approach.

                                    1 Reply Last reply
                                    0
                                    • jsulmJ jsulm

                                      @petar You can comment in the bug and write that you tested without Qt and it worked.

                                      P Offline
                                      P Offline
                                      petar
                                      wrote on last edited by
                                      #27

                                      @jsulm
                                      Thanks, I just did that, but Christian is a tough guy, hehe.

                                      1 Reply Last reply
                                      0
                                      • I Offline
                                        I Offline
                                        imahgin
                                        wrote on last edited by
                                        #28

                                        Although it's been a while, here is my solution.
                                        I experienced a similar issue when connection was lost while a query was running.
                                        For PostgreSQL, there is an option to set a timeout in TCP connection: tcp_user_timeout

                                        1 Reply Last reply
                                        0

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