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

[Stuart Auchterlonie]

On 29/01/17 12:07, Roger James wrote:
> On 28 January 2017 11:46:50 pm Roger James <roger at beardandsandals.co.uk>
> wrote:
> 
>> 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
> 
> Hi Roger
> 
> I was a bit loose with my description of my original findings. The
> problem is definitely related to loading and unloading of Qt image
> handler plugins. The crash is caused by a QImage::Load calling a pointer
> to a plugin that has already unloaded. I am currently testing with a
> build directly from 28 fixes (rather than mythbuntu debs) and that
> crashes at a different place in the startup process. It crashes in the
> logging thread when it access a null pointer in the incoming log
> messages queue.
> 
> I will look at this later today. The weird thing with all these crashes
> is that when the wrapper script restarts the front-end it works perfectly.
> 

Do you use Qt-5.6.1 ???

This seems to bite us on a regular basis, and it seems to have been
fixed in Qt-5.7


Regards
Stuart