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QSqlQuery::exec() freezes over OpenVPN

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  • P petar
    8 Oct 2019, 16:57

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

    J Offline
    J Offline
    JonB
    wrote on 8 Oct 2019, 17:05 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
      7 Oct 2019, 21:39

      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 Offline
      A Offline
      A.A.SEZEN
      wrote on 8 Oct 2019, 17:08 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 J 2 Replies Last reply 8 Oct 2019, 17:49
      0
      • A A.A.SEZEN
        8 Oct 2019, 17:08

        @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 8 Oct 2019, 17:49 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?

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        0
        • A Offline
          A Offline
          A.A.SEZEN
          wrote on 8 Oct 2019, 18:16 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 8 Oct 2019, 19:15
          0
          • A A.A.SEZEN
            8 Oct 2019, 17:08

            @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);
                }
            
            }
            
            
            J Offline
            J Offline
            JonB
            wrote on 8 Oct 2019, 18:39 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 8 Oct 2019, 18:59
            0
            • J JonB
              8 Oct 2019, 18:39

              @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 8 Oct 2019, 18:59 last edited by
              #17

              @JonB
              No, only once.

              1 Reply Last reply
              1
              • A A.A.SEZEN
                8 Oct 2019, 18:16

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

                P Offline
                P Offline
                petar
                wrote on 8 Oct 2019, 19:15 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 1 Reply Last reply 10 Oct 2019, 05:04
                0
                • P petar
                  8 Oct 2019, 16:57

                  @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 9 Oct 2019, 04:48 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 9 Oct 2019, 17:30
                  2
                  • jsulmJ jsulm
                    9 Oct 2019, 04:48

                    @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 9 Oct 2019, 17:30 last edited by petar 10 Sept 2019, 17:31
                    #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 9 Oct 2019, 21:07 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 10 Oct 2019, 05:18
                      0
                      • P petar
                        8 Oct 2019, 19:15

                        @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 Offline
                        A Offline
                        A.A.SEZEN
                        wrote on 10 Oct 2019, 05:04 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 10 Oct 2019, 07:55
                        0
                        • P petar
                          9 Oct 2019, 21:07

                          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 10 Oct 2019, 05:18 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 10 Oct 2019, 07:46
                          0
                          • jsulmJ jsulm
                            10 Oct 2019, 05:18

                            @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 10 Oct 2019, 07:46 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 10 Oct 2019, 07:48
                            0
                            • P petar
                              10 Oct 2019, 07:46

                              @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 10 Oct 2019, 07:48 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 10 Oct 2019, 08:10
                              0
                              • A A.A.SEZEN
                                10 Oct 2019, 05:04

                                @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 10 Oct 2019, 07:55 last edited by
                                #26

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

                                1 Reply Last reply
                                0
                                • jsulmJ jsulm
                                  10 Oct 2019, 07:48

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

                                  P Offline
                                  P Offline
                                  petar
                                  wrote on 10 Oct 2019, 08:10 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 11 Nov 2020, 10:43 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
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