package mysql-server-5.7 5.7.17-0ubuntu0.16.04.1 failed to install/upgrade: subprocess installed post-installation script returned error exit status 1

[Richard Rega]

Simply used the 'Software Updater' to update the system after a few weeks, no other applications where open except the Chrome web browser.

ProblemType: Package
DistroRelease: Ubuntu 16.04
Package: mysql-server-5.7 5.7.17-0ubuntu0.16.04.1
ProcVersionSignature: Ubuntu 4.4.0-53.74-generic 4.4.30
Uname: Linux 4.4.0-53-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.5
Architecture: amd64
Date: Sat Jan 28 10:47:00 2017
ErrorMessage: subprocess installed post-installation script returned error exit status 1
InstallationDate: Installed on 2016-11-03 (86 days ago)
InstallationMedia: Ubuntu 16.04 LTS "Xenial Xerus" - Release amd64 (20160420.1)
Logs.var.log.daemon.log:

MySQLConf.etc.mysql.conf.d.mysql.cnf: [mysql]
MySQLConf.etc.mysql.conf.d.mysqldump.cnf:
 [mysqldump]
 quick
 quote-names
 max_allowed_packet = 16M
MySQLConf.etc.mysql.mysql.conf.d.mysqld_safe_syslog.cnf:
 [mysqld_safe]
 syslog
MySQLVarLibDirListing: ['performance_schema', 'debian-5.7.flag', 'ib_logfile1', 'ib_buffer_pool', 'mysql', 'bi', 'ibtmp1', 'brand_aff', 'sys', 'npr', 'insight', 'tomcat_authentication', 'auto.cnf', 'ib_logfile0', 'analyst', 'customer', 'mysql_upgrade_info', 'revolveclothing_com_@002d_db', 'ibdata1']
ProcCmdline: BOOT_IMAGE=/boot/vmlinuz-4.4.0-53-generic root=UUID=e57af6a9-f03b-4ec2-ade7-16b2ef7c843e ro quiet splash vt.handoff=7
RelatedPackageVersions:
 dpkg 1.18.4ubuntu1.1
 apt 1.2.19
SourcePackage: mysql-5.7
Title: package mysql-server-5.7 5.7.17-0ubuntu0.16.04.1 failed to install/upgrade: subprocess installed post-installation script returned error exit status 1
UpgradeStatus: No upgrade log present (probably fresh install)
modified.conffile..etc.mysql.mysql.cnf: [modified]
mtime.conffile..etc.mysql.mysql.cnf: 2016-11-10T19:04:39.890995

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Christian Ehrhardt  (paelzer)]

From the log:
Setting up mysql-server-5.7 (5.7.17-0ubuntu0.16.04.1) ...
mysql_upgrade: Got error: 1045: Access denied for user 'debian-sys-maint'@'localhost' (using password: YES) while connecting to the MySQL server
Upgrade process encountered error and will not continue.
mysql_upgrade failed with exit status 11
dpkg: error processing package mysql-server-5.7 (--configure):

That matched the mysql error log:
2017-01-28T18:48:37.511488Z 0 [Note] /usr/sbin/mysqld: ready for connections.
Version: '5.7.17-0ubuntu0.16.04.1' socket: '/var/run/mysqld/mysqld.sock' port: 3306 (Ubuntu)
2017-01-28T18:48:38.201169Z 3 [Note] Access denied for user 'root'@'localhost' (using password: NO)
2017-01-28T18:48:38.204196Z 4 [Note] Access denied for user 'debian-sys-maint'@'localhost' (using password: YES)

It needs to perform some actions on upgrade.
IIRC it uses the root socket (or prompts you) to create a temp access and uses that for the upgrades.

Was any modifications to the user setup done that might cause this?