[mythtv] mythbackend segfault in libQtCore release-0-22-fixes

[Roger James]

Daniel Kristjansson wrote:
> On Fri, 2009-12-18 at 14:22 +0000, Roger James wrote:
>   
>> I have upgraded to a recent (22974) checkout of release-0-22-fixes and I 
>> am experiencing occasional crashes of mythbackend. I have attached the 
>> syslog and backend log to the end of this message.
>>
>> Has anyone else seen anything like this?
>>
>> When the backend restarts it seems that the recording starts OK.
>>
>> Why are three ASK_RECORDING requests made, is this significant?
>>
>> Has anyone get any tips for debugging this?
>>     
>
> Those logs don't really indicate anything terribly suspicious.
> ASK_RECORDING messages are sent so that a frontend can ask the
> user what to do if the backend wants to make a recording using
> a recorder they are currently using to watch LiveTV, they are
> ignored if a LiveTV session is not using that recorder and are
> probably just a red herring.
>
> The way to fix crashes is to get a backtrace that a developer
> can look at. First you will need to either compile the debugging
> version of MythTV, or add the debugging symbols package if you
> use a pre-packaged MythTV; this will make the backtrace you
> generate much more useful to the developer. My favorite way to
> get backtraces for the backend is to dump core on crashes and
> then use gdb mythbackend name_of_core_file, this allows you to
> restart the backend on crashes and still get a backtrace at your
> leisure. To do this add these three lines to your mythbackend
> startup script:
>  ulimit -c unlimited
>  echo 1 > /proc/sys/kernel/core_uses_pid
>  echo "/tmp/core.%t.%u.%p.%e" > /proc/sys/kernel/core_pattern
> The first allows core files to be used, the second allows the %p
> in the third line, and the third line creates individual core
> dumps for each program crash in tmp so they are easy to locate,
> and have the program that crashed and the time it crashed along
> with other useful information right in the name.
>
> To finally produce the backtrace run:
> $  sudo gdb mythbackend /tmp/core.[rest_of_the_file_name]
>   
>>  thread apply all bt
>>  quit
>>     
>
> Unless there was memory overwriting going on or there was
> a serious hardware failure, the output will tell us exactly
> what line of code the backend crashed on, and what other
> threads were doing at the time.
>
> -- Daniel
>   
Thanks Daniel,

A clear and concise response. Thanks very much, it is just what I 
needed. The core_uses_pid stuff is new to me and will be very useful.

Cheers,

Roger
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