[Interest] Annoying Bug - Qt 4.8.1 bug?

[Konrad Rosenbaum]

Hi,

On Thursday, Thursday 23 January 2014 at 10:15, Etienne Sandré-Chardonnal 
wrote:
> Here is the only non trivial code that could contain the bug if it's not a
> Qt bug. This is a QThread subclass worker doing the file saving (here it
> does nothing but this crashes anyway on the QProgressDialog destruction)

I don't think this code does what you expect it to do.

startWriting() is called in the main thread, this is probably intended

all signal-slot-connections made inside this function are direct connections 
and probably executed in the main thread - did you intend this or can I assume 
the timer was supposed to live in the new thread?

If some of those connections are handled outside the main thread, it could 
explain the crash: GUI events MUST NOT be executed outside the GUI thread. It 
also is usually a bad idea to  execute methods of the same object in multiple 
threads (unless they are specifically protected by locks).

Why is run() empty? As far as I understand QThread this means the thread lives 
for only a few microseconds and then self-destructs. I have not completely 
understood what that does to the timer and progress dialog yet.

I think I have a possible reason why the progress dialog may crash: the 
finished() signal is emitted from within the new thread, since the connection 
is direct (automatic means direct in this specific case) the close() method is 
called inside the wrong thread.

backgroundfileworker.cpp, line 35: try adding Qt::QueuedConnection

actually: I would add this to almost all connects in this scenario!


Somewhere else in this mailthread: it does not matter whether the program 
crashes on Linux, valgrind (helgrind, drd) detects race conditions no matter 
whether they are triggering a problem right now by detecting memory access 
that is not protected by locks/semaphores.



    Konrad
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