apt update blocked/locked, processes hung for days due to avahi-browse

[TJ]

Using 18.04 I hit a problem:

$ sudo apt -o=Debug::Acquire::gpgv=true update

WARNING: apt does not have a stable CLI interface. Use with caution in scripts.

Reading package lists...
E: Could not get lock /var/lib/apt/lists/lock - open (11: Resource temporarily unavailable)
E: Unable to lock directory /var/lib/apt/lists/

Investigating further I discovered a 3 day old cron job is hung:

$ date
Tue 6 Nov 08:50:20 GMT 2018

$ ps -efly | grep apt
S tj 31068 8492 0 80 0 1016 5384 pipe_w 08:48 pts/4 00:00:00 grep --color=auto apt
S root 31280 1 0 80 0 860 1157 - Nov03 ? 00:00:00 /bin/sh /usr/lib/apt/apt.systemd.daily update
S root 31285 31280 0 80 0 1748 1157 - Nov03 ? 00:00:00 /bin/sh /usr/lib/apt/apt.systemd.daily lock_is_held update
S root 31331 31285 0 80 0 8080 15493 - Nov03 ? 00:00:00 apt-get -qq -y update
S root 31336 31331 0 80 0 5556 19025 - Nov03 ? 00:00:00 /usr/lib/apt/methods/http
S root 32312 31331 0 80 0 5636 19025 - Nov03 ? 00:00:00 /usr/lib/apt/methods/http
S _apt 32314 31331 0 80 0 9548 10752 - Nov03 ? 00:00:00 /usr/bin/python /usr/share/squid-deb-proxy-client/apt-avahi-discover http://apt.postgresql.org/pub/repos/apt/dists/bionic-pgdg/InRelease
S _apt 32315 32314 0 80 0 1536 10458 - Nov03 ? 00:00:00 avahi-browse -kprtf _apt_proxy._tcp

I've successfully installed packages using "apt install" since the date these update processes started.

Checking the mDNS proxy command manually it returns with no result:

$ avahi-browse -kprtf _apt_proxy._tcp
$