Comment 6 for bug 1760128

Analysis here appears related:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=768620

Seems the interactions here are quite complex

To reproduce the problem, you can do on xenial:
(install old version)
# apt install avahi-daemon=0.6.32~rc+dfsg-1ubuntu2 avahi-dnsconfd=0.6.32~rc+dfsg-1ubuntu2 avahi-utils=0.6.32~rc+dfsg-1ubuntu2

(in a second terminal, leaves this running)
while true; do avahi-browse -a; done

(back in the first terminal)
apt upgrade # will upgrade from the xenial version 1ubuntu2 to xenial-updates version 1ubuntu2.1

Looks like the --restart-after-upgrade change may be what we need except I can't see where that's actually used in the current Ubuntu or Debian package. Seems this shipped patch may have "fixed" the issue instead. This patch is in bionic, but not in xenial where are are seeing this issue reported.

lathiat@optane:~/src/debian/avahi-0.7$ cat debian/patches/no-systemd-also.patch
Description: Don't use 'Also=' in dnsconfd systemd unit
 'Also=avahi-daemon.socket' means that 'systemctl disable avahi-dnsconfd'
 will also disable avahi-daemon.socket, which is definitely not what we
 want, and it also causes debhelper to throw an error. Just drop this entry
 from the configuration.
Author: Steve Langasek <email address hidden>
Last-Modified: 2018-01-02 20:30:00 -0800
Bug-Debian: https://bugs.debian.org/878911

Index: avahi-0.7-3ubuntu1/avahi-dnsconfd/avahi-dnsconfd.service.in
===================================================================
--- avahi-0.7-3ubuntu1.orig/avahi-dnsconfd/avahi-dnsconfd.service.in
+++ avahi-0.7-3ubuntu1/avahi-dnsconfd/avahi-dnsconfd.service.in
@@ -26,4 +26,3 @@

 [Install]
 WantedBy=multi-user.target
-Also=avahi-daemon.socket