[mythtv-users] delaying backend startup until the network is up

James Abernathy jfabernathy at gmail.com
Mon Dec 17 21:31:47 UTC 2018


On 12/11/18 8:24 AM, Stephen Worthington wrote:
> On Tue, 11 Dec 2018 08:17:33 -0500, you wrote:
>
>> On 12/11/18 2:04 AM, Stephen Worthington wrote:
>>> On Mon, 10 Dec 2018 17:03:02 -0500, you wrote:
>>>
>>>> on my production system I use without issue the systemd override provided by.
>>>>
>>>> sudo systemctl edit mythtv-backend.service
>>>>
>>>> [Unit]
>>>> After=NetworkManager-wait-online.service
>>>>
>>>> When I built a test system using the same HDHR tuners but with the mythhdhrrecorder external recorder I used the same overide setup. However, if I boot my test system I get a lot of errors on 2-4 tuners and the errors are related to opening the tuner. This is solved by doing the following after boot:
>>>>
>>>> sudo systemctl stop mythtv-backend
>>>> sudo systemctl start mythtv-backend
>>>>
>>>> What is special about mythhdhrrecorder and external recorder blackbox that makes this systemd override not work???
>>>>
>>>> Jim A
>>> Looking at the source code for mythhdhrrecorder, I can see that as it
>>> starts up, it tries to access https://my.hdhomerun.com/discover as
>>> part of its discovery of the HDHR devices.  So it looks like you need
>>> full Internet access up (including the name servers working) before
>>> that will work.  So I think what is needed is a systemd unit that
>>> first does a DNS lookup of my.hdhomerun.com (say once a second), and
>>> when that works, does a ping of my.hdhomerun.com and when that works,
>>> the unit startup succeeds.  Then you can have mythbackend wait for
>>> that unit to be started before it starts.  However, there should be a
>>> timeout (say 30 seconds) after which mythbackend gets to start anyway,
>>> otherwise if your Internet access is down, you will be unable to start
>>> mythbackend.
>>>
>>> I have never built a systemd unit like that, but I think it would work
>>> if you wrote a script (or some Python) to do the DNS lookup and ping,
>>> and put it in the ExecStartPre line to be run before the main
>>> executable of the unit gets run from the ExecStart line.  The
>>> ExecStart line would then need to do nothing.  I am not sure how you
>>> are supposed to do nothing in systemd, but running /bin/true should
>>> work.
>>>
>>> So a unit "full-internet.service" like this might work:
>>>
>>> [Unit]
>>> Description=Wait for full Internet service to be available including
>>> DNS.
>>> After=NetworkManager-wait-online.service
>>>
>>> [Service]
>>> Type=simple
>>> ExecStartPre=/usr/bin/python3 /usr/bin/full-internet.py
>>> ExecStart=/bin/true
>>>
>>> [Install]
>>> WantedBy=multi-user.target
>>>
>>> and in your mythtv-backend override file, use:
>>>
>>> After=full-internet.service
>>>
>>> The full-internet.py script would handle the DNS lookup, ping, and
>>> timeout after 30 seconds.  It might be a good idea if it took the DNS
>>> name and timeout values as command line parameters, to make it more
>>> generally useful.  I have a script I use for reloading the drivers of
>>> an Ethernet card when the connection drops that I could use for a
>>> framework to make a full-internet.py script.  It does regular pings to
>>> see if the connection is up so it has most of the code I would need.
>>> Let me know if you would like me to do this.  I think this is a
>>> problem that has cropped up in various forms for years, so it would be
>>> worth my writing this if it was going to be useful.
>>>
>> ---------------------
>>
>
I created a delaying mechanism so mythtv backend would not run until 
mythhdhrrecorder could "discover" the HDHR tuners requiring full 
internet access.  As Stephen mentioned I added a full-internet.service 
and a script. This is not elegant but with my poor programming skills 
it's the best I could do for now.

I put the following service file in /etc/systemd/system:

"full-internet.service"

Description=Wait for full internet service to be available including DNS
After=NetworkManager-wait-online.service

[Service]
Type=simple
ExecStartPre=/home/jim/delay-backend-till-network-up.py
ExecStart=/bin/true

[Install]
WantedBy=multi-user.target

------------------------------------

Then I put this script in my home directory:

"delay-backend-till-network-up.py"

#!/usr/bin/env python3
# -*- coding: UTF-8 -*

import os, sys, socket, struct, select, time, signal
import syslog

syslog.syslog("delaying mythtv-backend - starting")

def check_ping():
     hostname = "my.hdhomerun.com"
     response = os.system("ping -c 1 " + hostname)

     return response


time.sleep(30)
if check_ping() == 0:
         syslog.syslog("ping worked after 30 sec sleep")
syslog.syslog("delaying mythtv-backend - ending")

------------------------------------

I changed the override.conf file in 
/etc/systemd/system/mythtv-backend.service.d to :

[Unit]
After=full-internet.service

---------------------

Don't forget the enable the full-internet.service: sudo systemctl enable 
full-internet

Make delay-backend-till-network-up.py executable.

This basically sleep for 30 seconds and then pings my.hdhomerun.com.

works for me so I can continue testing.

Jim A






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