[mythtv] Startup crash (Was DVB SI rework - Help needed)

[Roger Siddons]

On 22/01/17 16:26, Roger James wrote:
>
> I did raise an issue on this a while back here
>
> https://code.mythtv.org/trac/ticket/12909
>
> And here
>
> https://bugs.launchpad.net/mythbuntu/+bug/1635685
>
> I coded up a fix for the crash in QImage but that just moved the crash a
> bit further down the mythfrontend initialisation sequence into a number
> of random other crashes all of which seem to be Qt signalling related. I
> can send you stack traces for these if you wish. I think I still have
> the cores.
>
> This all seems to relate to Qt not being thread safe or reentrant in a
> lot of areas. This exacerbated by running on a fast multi core 64bit
> processor with a fast SSD drive.
>

I doubt there's an issue in the Myth image loading. It's old code and 
heavily exercised when navigating the UI so any issue would have 
surfaced before.
Each thread loads a single image so re-entrancy is sufficient. There's 
explicit code to prevent multiple threads from loading the same image so 
thread-safety isn't required.

Qt seems to have recurring issues with loading/unloading plugins at 
startup/shutdown that involve QFactoryLoader. It crops up in many bug 
reports.

Qt5 bundles 20+ image plugins (QT_DIR/plugins/imageformats) that seem to 
get loaded when the first screen (main menu) is displayed. Usually that 
completes before MythUi starts but I wonder whether, on your 
super-whizzy machine, Myth starts loading its image cache whilst Qt is 
still loading its plugins.

To see the plugin action, try:
QT_DEBUG_PLUGINS=1 mythfrontend -v gui,file

Qt logging isn't date-stamped and will interleave with Myth logs but it 
may give you a feel for any interference that's occurring.

AFAIK Myth only uses png & jpg images, which are supposedly supported by 
core Qt. So a quick test is to remove all the image plugins to see if 
that mitigates the crash.

If I'm right then Peter's work may well solve it. His splash-screen 
(presumably a Qt widget) should initialise Qt gui and load the plugins 
long before the themed UI starts loading images.