[mythtv] Help diagnosing a lockup in mythbackend

[Dan Wilga]

I have experienced a lockupin 0.28which seems to be happening even more 
frequently now that I've upgraded to 29.1 (self-compiled). I have three 
types of tuner:an HD-PVR, a HDHomerun (original, dual-tuner), and a 
Blackmagic DeckLink Mini card that is accessed as an external recorder. 
This uses a modified version of mythfilerecorderto run ffmpeg, as 
documented here:

https://code.mythtv.org/trac/ticket/13251

I believe the lockup begins to happen whena recording completes on the 
DeckLink. For some reason, the mythfilerecorder process is not always 
told to terminate; in some cases the backend seems to just reuse this 
process to record subsequent programs. But on other occasions, the next 
time an attempt is made to use that capture card there is a lockup where 
the backend still outputs occasional messages to the log but its CPU 
usage is pegged at 100%, it no longer responds to any requests from 
frontends, and it has to be terminated withSIGKILL.

A thread backtrace can be found here:

   https://pastebin.com/yhSHXuq5

Any suggestions about what to try?
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