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

[Roger James]

On 28 January 2017 11:06:20 pm Roger Siddons <dizygotheca at ntlworld.com> wrote:

> 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.
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It is a Shuttle XS35GT, hardly a supercomputer😀. It only started 
exhibiting these problems when I stuck a SSD in it.

Roger