Solution to recent "Waiting for database schema upgrade lock…

Are you getting "Waiting for database schema upgrade lock" repeated errors
after a recent upgrade?

The TLDR is this appears to indicate a locking problem but its actually
resolving both a "AAAA" and a "A" record for the backend hostname, and
recently myth will only attempt to connect via the ipv6 AAAA record... so
if mysql is configured to only accept connections via ipv4, it'll fail and
give you a rather mysterious lock error, rather than an error explaining
whats actually happening (like, say, "failed connection to 123::456 or
whatever your ipv6 addrs is). One way to fix it is to modify mysql to
accept ipv6 connections...

Here's the detailed long story:

server.mulhollon.com has both A and AAAA dns records and all my frontends
point to it by hostname.

mythtv-backend and -setup will now ONLY will try ipv6 connections to the
mysql server. Doesn't matter if ipv4 is open, it won't be tried anymore
(used to, in the recent past)

if /etc/mysql/my.cnf has a line like

bind-address = 0.0.0.0 (aka listen only to all ipv4 interfaces NOT ipv6)

you'll get the error quoted above.

Simply change the bind-address for mysql to

bind-address = :: (aka listen to all interfaces ipv4 and ipv6)

restart mysql, then mysql will allow connections to both ipv4 AND ipv6, so
when mythtv demands to only connect via ipv6, it works perfectly.

Or at least this worked for me....