Starting second wsl QProcess kills the first one

[mjs225]

I want to use a Windows Qt program to manage processes in wsl. Each process should be running in the background for some time.
After some trials, I find starting second QProcess will kill the first one. This only happens when running wsl processes.

#include <QApplication>
#include <QBoxLayout>
#include <QLabel>
#include <QMetaEnum>
#include <QProcess>
#include <QPushButton>
#include <QWidget>

using namespace Qt::StringLiterals;
template <typename E> const char *EnumToString(E enumValue) {
  return QMetaEnum::fromType<E>().valueToKey(int(enumValue));
}
struct ProcessCtrl : QWidget {
  QHBoxLayout lout{this};
  QLabel lbState;
  QPushButton btnKill{u"Kill"_s};
  QPushButton btnStart{u"Start"_s};

  QProcess process;
  ProcessCtrl() {
    lout.addWidget(&lbState);
    lout.addWidget(&btnKill);
    lout.addWidget(&btnStart);
    connect(&btnKill, &QPushButton::clicked, &process, &QProcess::kill);
    connect(&btnStart, &QPushButton::clicked, [this] {
      process.start("wsl", QStringList{"-e", "sleep", "5s"});
      // `wsl -e` execute the following cmd in wsl without entering wsl shell
      // `sleep 5s` sleeps for 5 seconds
    });
    connect(&process, &QProcess::stateChanged,
            [this](QProcess::ProcessState state) {
              lbState.setText(QString{EnumToString(state)});
            });
    lbState.setText(QString{EnumToString(process.state())});
  }
};
struct Widget : QWidget {
  QVBoxLayout lout{this};
  ProcessCtrl ctrl1;
  ProcessCtrl ctrl2;

  Widget() {
    lout.addWidget(&ctrl1);
    lout.addWidget(&ctrl2);
  }
};
int main(int argc, char **argv) {
  QApplication a{argc, argv};
  Widget w;
  w.show();
  return a.exec();
}

In this demo application, if the first Start button is clicked, the first process will run for 5 seconds. If the second Start button is clicked during the period, both the first and the second process will exit immediately.

I'm using Qt 6.8.2 in Windows 10.

[mjs225]

@JonB You're right. I always launch one wsl process in one cmd window, and wsl processes in different cmd windows can live with each other. Now I tried launching multiple wsl processes in one cmd using start /b, the result is the same as this demo application: all wsl processes will exit immediately.
So the problem is multiple wsl processes cannot be launched by a single parent. It has nothing to do with QProcess.

[Christian Ehrlicher]

I don't have wsl here but it is working with a simple cmd.exe
Did you try it from the command line if it works?

[JonB]

@mjs225
My thought is same as @Christian-Ehrlicher's. QProcess does nothing special other than launching processes. Is this wsl behaviour to only allow one instance?

On another note. I don't think this will affect your current issue, but think twice before ever using a connect() with 3 arguments and a lambda for the slot. There are problems about when the slot should disappear and be disconnected (there was a topic here a few weeks ago where this reared its head). Consider changing to the standard 4 arguments and specify a context object for the slot, probably this in your case.

[mjs225]

@JonB You're right. I always launch one wsl process in one cmd window, and wsl processes in different cmd windows can live with each other. Now I tried launching multiple wsl processes in one cmd using start /b, the result is the same as this demo application: all wsl processes will exit immediately.
So the problem is multiple wsl processes cannot be launched by a single parent. It has nothing to do with QProcess.