mysql-server update fails if systemd mysql.service is disabled

[Ben Landin]

If the user has disabled the mysql service in systemd, attempting to update the mysql-server metapackage (or its dependencies) causes the update process to fail.

ProblemType: Package
DistroRelease: Ubuntu 16.04
Package: mysql-server-5.7 5.7.17-0ubuntu0.16.04.2
ProcVersionSignature: Ubuntu 4.4.0-72.93-generic 4.4.49
Uname: Linux 4.4.0-72-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.5
Architecture: amd64
Date: Fri Apr 7 19:25:08 2017
ErrorMessage: subprocess installed post-installation script returned error exit status 1
InstallationDate: Installed on 2016-11-04 (154 days ago)
InstallationMedia: Ubuntu-GNOME 16.04.1 LTS "Xenial Xerus" - Release amd64 (20160720)
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: ['ib_logfile1', 'debian-5.7.flag', 'mysql', 'mysql_upgrade_info', 'performance_schema', 'ib_buffer_pool', 'ibdata1', 'auto.cnf', 'sys', 'ib_logfile0']
ProcCmdline: BOOT_IMAGE=/boot/vmlinuz-4.4.0-72-generic.efi.signed root=UUID=2552125a-47f3-4208-82ec-ba6ce904c304 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 5.7.17-0ubuntu0.16.04.2 failed to install/upgrade: subprocess installed post-installation script returned error exit status 1
UpgradeStatus: No upgrade log present (probably fresh install)
upstart.mysql.override: manual

[Apport retracing service (apport)]

Thank you for taking the time to report this crash and helping to make this software better. This particular crash has already been reported and is a duplicate of bug #1680747, so is being marked as such. Please look at the other bug report to see if there is any missing information that you can provide, or to see if there is a workaround for the bug. Additionally, any further discussion regarding the bug should occur in the other report. Please continue to report any other bugs you may find.