[Qt-interest] QtScript segfault on C++ signal (dis)connection/firing

[Kent Hansen]

Hi Sean,
Any chance that the same script engine is being accessed from multiple
threads? QtScript is not thread-safe, so be sure that there's only a
single thread evaluating code (unless you've explicitly added a
synchronization mechanism), and make sure that all connections are
queued. If QtScript intercepts a signal and starts to handle it, and
then another thread starts evaluating a script while this is going on,
one likely result is that the QtScript call stack will get corrupted,
which the backtrace seems to indicate.

Regards,
Kent


Sean M. Pappalardo wrote:
> Hello again.
>
> Just wondering if you've been able to find anything out? This issue is
> quite literally a show-stopper, considering this is a DJ app. :)
>
> After a bunch more testing, the crashing doesn't happen if I don't
> connect any of the C++ signals. I can call MidiScriptEngine::setValue()
> all day from the script with no problems. It's crashing when signals are
> connected and setValue() is called many times in a row, affecting
> objects that emit signals. And it's worse on faster systems (more
> setValue()s per second, I assume.) The signals only fire once per
> latency period though. I did also find that increasing the latency alot
> (400ms or more) makes it less likely to happen, depending on processor
> speed.
>
> I ran valgrind with Mixxx compiled against Qt 4.5 which seems to give
> more detail in the QtScript area, incase it helps:
>
> =20125== Process terminating with default action of signal 11 (SIGSEGV)
> ==20125==  Access not within mapped region at address 0x8
> ==20125==    at 0x530F078:
> QScriptContextPrivate::execute(QScript::Code*) (qscriptcontext_p.cpp:2020)
> ==20125==    by 0x5320B15:
> QScript::ScriptFunction::execute(QScriptContextPrivate*)
> (qscriptcontext_p.cpp:313)
> ==20125==    by 0x5335B7A: QScriptEnginePrivate::call(QScriptValueImpl
> const&, QScriptValueImpl const&, QList<QScriptValueImpl> const&, bool)
> (qscriptengine_p.cpp:1252)
> ==20125==    by 0x5371496: QScriptValue::call(QScriptValue const&,
> QList<QScriptValue> const&) (qscriptvalueimpl_p.h:719)
> ==20125==    by 0x80FA4C1: MidiObject::receive(MidiCategory, char, char,
> char, QString) (midiobject.cpp:273)
> ==20125==    by 0x81BA89F: MidiObjectALSASeq::run()
> (midiobjectalsaseq.cpp:286)
> ==20125==    by 0x502861D: QThreadPrivate::start(void*)
> (qthread_unix.cpp:184)
> ==20125==    by 0x42CB4BF: start_thread (in
> /lib/i686/cmov/libpthread-2.7.so)
> ==20125==    by 0x55B461D: clone (in /lib/i686/cmov/libc-2.7.so)
>
> Thanks again for your time. Let me know your thoughts and if there's
> anything else I can do to help.
>
> Sincerely,
> Sean M. Pappalardo
>
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