MediaWiki backup failures due to mysqldump

Hi Everyone,

A quick heads up for others in this situation... We attempted to
migrate to MW 1.38.6 tonight. Our first step is a manual backup.

We found our manual backup (and backups for the last couple of months)
have been failing (yuk!):

# /sbin/mw-backup
Repair wiki database ok
mysqldump: Couldn't execute 'FLUSH TABLES': Access denied; you need
(at least one of) the RELOAD or FLUSH_TABLES privilege(s) for this
operation (1227)
Failed to dump wiki database

It appears this is a known issue on Ubuntu:
https://bugs.launchpad.net/ubuntu/+source/mysql-5.7/+bug/2003866

In our case, workarounds like adding RELOAD or FLUSH_TABLES to the
backup user also failed:

mysql> GRANT FLUSH_TABLES ON my_wiki.* TO 'mwuser'@'%';
ERROR 3619 (HY000): Illegal privilege level specified for FLUSH_TABLES
mysql> GRANT RELOAD ON my_wiki.* TO 'mwuser'@'%';
ERROR 1410 (42000): You are not allowed to create a user with GRANT

We reached our limit of knowledge for MySQL administration. I think we
will have to wait until the Ubuntu 2003866 bug is fixed.

Jeff