Multithreading with QtDBus

[Ogden, Nick]

Thanks for your help Thiago.

I've filed a bug at:  https://bugreports.qt-project.org/browse/QTBUG-27809

Regards,
Nick Ogden

-----Original Message-----
From: dbus-bounces+nick.ogden=usa.g4s.com at lists.freedesktop.org [mailto:dbus-bounces+nick.ogden=usa.g4s.com at lists.freedesktop.org] On Behalf Of Thiago Macieira
Sent: 01 November 2012 15:04
To: dbus at lists.freedesktop.org
Subject: Re: Multithreading with QtDBus

On quinta-feira, 1 de novembro de 2012 14.14.11, Ogden, Nick wrote:
> Greetings,
>
> I have a program that does the following:
> 1) Creates a QCoreApplication
> 2) Creates a third party component (read: black box) which internally
> spawns multiple threads 3) Creates a QDBusConnection and connects to
> some DBus signals
> 4) Enters the main Qt event loop
>
> From here onwards, the third party black box makes calls, on whatever
> thread it feels is appropriate, to some handler functions in the
> application, that are required to retrieve some data via DBus. Each
> thread creates its own proxy object and makes a blocking call over
> DBus by means of QDBusPendingReply::waitForFinished(). These proxy
> objects share the same underlying QDBusConnection object.
>
> My first question is whether or not this usage scenario is supported /
> expected to work with QtDBus?

I don't see anything that shouldn't work.

> I am experiencing frequent crashes from deep in libdbus-1 when running
> this application on Windows (I have not tested on Linux etc). I have
> attached a small test case which is representative of the usage
> scenario and reproduces this crash 100% of the time. Could someone
> please confirm whether this is a bug, or an unsupported use case for
> QtDBus and I will file a bug report as appropriate.

I can reproduce the crash here too, on Linux.

process 10929: arguments to dbus_pending_call_block() were incorrect, assertion "pending != NULL" failed in file dbus-pending-call.c line 704.

Backtrace:

#2  0x0000003a3c630035 in _dbus_abort () at dbus-sysdeps.c:94
#3  0x0000003a3c6270c5 in _dbus_warn_check_failed (format=
    0x3a3c636290 "arguments to %s() were incorrect, assertion \"%s\" failed in file %s line %d.\nThis is normally a bug in some application using the D-Bus
library.\n") at dbus-internals.c:289
#4  0x000000397182144a in q_dbus_pending_call_block (pending=0x0)
    at /home/thiago/src/qt/qt-4.8/src/dbus/qdbus_symbols_p.h:313
#5  QDBusConnectionPrivate::waitForFinished(QDBusPendingCallPrivate*)
(this=0x621220, pcall=0x7fffcc002fc0)
    at /home/thiago/src/qt/qt-4.8/src/dbus/qdbusintegrator.cpp:1776

Clearly a QtDBus bug because it passed a null where it shouldn't. Please file your bug report.
--
Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org
   Software Architect - Intel Open Source Technology Center
      PGP/GPG: 0x6EF45358; fingerprint:
      E067 918B B660 DBD1 105C  966C 33F5 F005 6EF4 5358

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