[mythtv-users] What does restarting mythbackend actually do?

[Stephen Worthington]

On Thu, 23 Jan 2020 17:24:24 -0700, you wrote:

>[ 0.29.1 on Fedora 26 in Canada ]
>
>I proactively reset my Firewire connection to my HD STB and restart 
>mythbackend every morning, because I've learned that CNN and sometimes CTV 
>really mess things up - like yellow or red Watch Recordings entries messes. 
>Sometimes even that's not enough, so I have to do the reset+restart again 
>after a failure.
>
>I've gotten used to it as the price to pay for it to work at all, and I'm well 
>behind the latest release - I'll be fixing that.
>
>But, sometimes   systemctl restart mythbackend   takes a couple seconds, and 
>sometimes it can take a couple minutes, especially after a failure.
>
>So I'm wondering what's going on with a restart. I'm assuming it has to wait 
>for various things to be signalled to stop or be killed and whatever is broken 
>is probably not responding.
>
>How could I find out what the stop/start sequence is?

Slow restarting of mythbackend is usually caused by a bug that all the
MythTV programs seem to suffer from, where they react to an external
shutdown request (kill -15 aks kill -SIGTERM) by shutting down all
their threads then waiting for that to complete.  Unfortunately, the
bug prevents this from working properly and one thread remains
running, so mythbackend keeps running until systemd does a timeout on
the shutdown process (configurable, 60 or 90 seconds usually).  On
timeout, systemd does a kill -9 aka kill -SIGKILL, which kills off the
last mythbackend thread.  Since all the other threads have actually
shut down properly and the last thread seems to be just waiting
forever for it to see that has happened, there is no damage done by
this problem, except for the slow shutdown.  This slow mythbackend
shutdown is also why MythTV boxes frequently are slow to shutdown or
restart - systemd is waiting for the mythbackend timeout before it can
complete the shutdown process.

If you want mythbackend to shut down rapidly, you need to issue a
second kill -15 shortly after the first one.  I have a script that
does this for Ubuntu:

root at mypvr:~# cat /usr/local/bin/mythbackendstop.sh
#!/bin/bash
PID=$(systemctl show -p MainPID mythtv-backend.service 2>/dev/null |
cut -d= -f2)
if [ "$PID" != "0" ]; then
    kill -15 $PID
    sleep 1
    kill -15 $PID
fi

(Note that my email client wraps long lines - the PID= line is all one
long line)

And I have modified my systemd settings for mythbackend so that it
uses mythbackendstop.sh to shut down mythbackend.  See this mailing
list thread for details and the download URL:

https://lists.gt.net/mythtv/users/628020

As for your recording problems, if they still continue after upgrading
your MythTV version, then you should post details and see if we can
help you fix the actual problem, rather than keep on with your nasty
workaround of having to restart mythbackend all the time.