Comment 22 for bug 1505670

Testing in Impish without extra network interfaces:

# works
$ sudo apt install avahi-utils squid-deb-proxy-client
$ avahi-browse -kprtf _apt_proxy._tcp
+;enp1s0;IPv4;apt-cacher-ng\032proxy\032on\032Keschdeichel;_apt_proxy._tcp;local
=;enp1s0;IPv4;apt-cacher-ng\032proxy\032on\032Keschdeichel;_apt_proxy._tcp;local;Keschdeichel.local;192.168.122.1;3142;
$ /usr/share/squid-deb-proxy-client/apt-avahi-discover
http://192.168.122.1:3142/

# break it in the host by disabling cacher
$ systemctl stop apt-cacher-ng.service
# ensure it is gone
$ curl 192.168.122.1:3142

# check the output now
# redirect stderr to show it is not gone
$ /usr/share/squid-deb-proxy-client/apt-avahi-discover 2>/dev/null
error: uncaptured python exception, closing channel <AptAvahiClient> ('192.168.122.1', 3142): 9223372036854775807 (<class 'ConnectionRefusedError'>:[Errno 111] Connection refused [/usr/lib/python3.9/asyncore.py|read|83] [/usr/lib/python3.9/asyncore.py|handle_read_event|417] [/usr/lib/python3.9/asyncore.py|handle_connect_event|425])

Upgrade:
ubuntu@i-deb-proxy2:~$ sudo apt upgrade
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
Calculating upgrade... Done
The following packages were automatically installed and are no longer required:
  accountsservice language-selector-common libaccountsservice0 net-tools
Use 'sudo apt autoremove' to remove them.
The following packages have been kept back:
  libgirepository-1.0-1 libglib2.0-0 libglib2.0-bin libp11-kit0 libpython3.9
  libpython3.9-minimal libpython3.9-stdlib python3-cffi-backend python3-gi python3.9
  python3.9-minimal
The following packages will be upgraded:
  squid-deb-proxy-client
1 upgraded, 0 newly installed, 0 to remove and 11 not upgraded.
Need to get 9124 B of archives.
After this operation, 0 B of additional disk space will be used.
Do you want to continue? [Y/n] Y
Get:1 http://ppa.launchpad.net/mirespace/impish-squid-deb-proxy-lp1505670-apt-avahi-discover/ubuntu impish/main amd64 squid-deb-proxy-client all 0.8.15+nmu1ubuntu1.1~ppa-mirespace [9124 B]
Fetched 9124 B in 1s (7007 B/s)
(Reading database ... 95211 files and directories currently installed.)
Preparing to unpack .../squid-deb-proxy-client_0.8.15+nmu1ubuntu1.1~ppa-mirespace_all.deb ...
Unpacking squid-deb-proxy-client (0.8.15+nmu1ubuntu1.1~ppa-mirespace) over (0.8.15+nmu1) ...
Setting up squid-deb-proxy-client (0.8.15+nmu1ubuntu1.1~ppa-mirespace) ...
Scanning processes...
Scanning candidates...
Scanning linux images...

Restarting services...
Service restarts being deferred:
 /etc/needrestart/restart.d/dbus.service
 systemctl restart <email address hidden>
 systemctl restart networkd-dispatcher.service
 systemctl restart systemd-logind.service
 systemctl restart unattended-upgrades.service

No containers need to be restarted.

User sessions running outdated binaries:
 ubuntu @ session #1: sshd[1208,1305]
 ubuntu @ user manager service: systemd[1211]

# now no output bleeding through
$ /usr/share/squid-deb-proxy-client/apt-avahi-discover 2>/dev/null

# actually since there is no proxy left there is juts nothing now in this setup
/usr/share/squid-deb-proxy-client/apt-avahi-discover

# But as soon as the cacher is back up it works:
$ /usr/share/squid-deb-proxy-client/apt-avahi-discover
http://192.168.122.1:3142/