Qt DBus unable to detect an exit status of zero

josephdp

Hi Qt community,

I would like to ask for your help.

I am developing a system using Qt Dbus.

It was able to launch dbus-daemon's session bus and launch the specific service on demand.

Problem:
The problem is when the service exits as zero, it will wait until the QDBusPendingCall timeout is exhausted and will report a false positive error:
"Failed to activate service 'com.example.foo': timed out.

Other non-zero exit codes will be returned immediately (not waiting for the pending call timeout) and will be reported as:
"Process com.example.foo exited with status 1"

Goal:
I wanted to trap a success (zero) exit code for the service activated without waiting for the timeout.

Any suggestions is really appreciated.

Set-up:
OS: Widows 7 64 bit.
Qt toolkit: Qt 5.5.1 MSVC2013 64 bit

I look forward to your response.

Sincerely,
JosephDP

SGaist

Hi and welcome to devnet,

I'm not sure I'm following you. Do you mean that you have this problem if the dbus-daemon exits normally ?

josephdp

Hi @SGaist,

Thank you for your response and for the warm welcome.

There's no problem with dbus-daemon since it will never exit and is always running.

The problem is with the services launched and when it exits normally (exit code: 0).

P.S. all services are running on the session bus

SGaist

To be sure I understand correctly: you are doing a call to your service that results in it exiting normally but then you get a timeout error, right ?

josephdp

Hi @SGaist,

Yes you got it right.

Do you happen to know if this is a bug in QT 5.5.1 DBus?

kshegunov

@josephdp
How does your service exit? Can you provide a very minimal snippet illustrating the function you're calling?

josephdp

Hi @kshegunov, @SGaist,

My service exits with a exit code of 0 if successful and non-zero for error.

The problem is QT DBus can't understand perhaps an exit code of 0 which means success?

Please see minimal snippet:

QDBusConnection session = QDBusConnection::sessionBus ();
QDBusMessage msg = QDBusMessage::createMethodCall ("com.myservice.foo",
                                                    "/com/myservice/foo",
                                                    "com.myservice.foo",
                                                    "mymethod");

QDBusPendingCall call = session.asyncCall (msg,  120000);

QDBusPendingCallWatcher * watcher = new QDBusPendingCallWatcher (call);

QObject::connect(watcher,
                 SIGNAL (finished(QDBusPendingCallWatcher*)),
                 this,
                 SLOT (onMethodFinished(QDBusPendingCallWatcher*)));

and for the onMethodFinished ():

QDBusPendingCall reply = * call;

if (reply.isError()) {
    QString text = reply.reply().errorMessage();
    qDebug () << __FUNCTION__ << "Error: " << text;
} else {
    qDebug () << __FUNCTION__ << "OK";
}
call->deleteLater();

kshegunov

@josephdp
I meant the service's method you're calling - mymethod from /com/myservice/foo.

josephdp

Hi @kshegunov

Basically it will just return 0 on success and 1 on failure.

mymethod ()
{
    bool rc = do_something ();

    if (rc) {
        // success
        return 0;
    } else {
        // failure
        return 1;
    }
}

kshegunov

What I was concerned about is if your application gives up before it's able to close the DBus connection cleanly. Meaning you calling ::exit() somewhere or having an unexpected SIGSEGV signal raised somewhere.

josephdp

Hi @kshegunov

Basically this is the final sequence of the app.

It is able to return immediately if the app returns a non-zero value (which in this case is 1).
However, if it will return a zero value (success in this case), the caller will wait for the timeout set (120000) and will inform a false negative result (that "Failed to activate service 'com.myservice.foo': timed out.)

kshegunov

Is it possible you don't have an event loop running at this point? I don't see a good reason for that behavior, so I'm starting to guess now ...