package mysql-server-5.7 5.7.15-0ubuntu0.16.04.1 failed to install/upgrade: subprocess installed post-installation script returned error exit status 1

[Francesco Facconi]

Upgrading from 14.04 to 16.04.

Errors were encountered while processing:
 mysql-server-5.7
 mysql-server
Error in function:

*** Send problem report to the developers?

ProblemType: Package
DistroRelease: Ubuntu 16.04
Package: mysql-server-5.7 5.7.15-0ubuntu0.16.04.1
ProcVersionSignature: Ubuntu 3.13.0-79.123-generic 3.13.11-ckt33
Uname: Linux 3.13.0-79-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.1
Architecture: amd64
Date: Tue Oct 4 13:06:42 2016
ErrorMessage: subprocess installed post-installation script returned error exit status 1
InstallationDate: Installed on 2014-04-18 (899 days ago)
InstallationMedia:

Logs.var.log.daemon.log:

MySQLConf.etc.mysql.conf.d.mysql.cnf: [mysql]
MySQLConf.etc.mysql.conf.d.mysqld_safe_syslog.cnf:
 [mysqld_safe]
 syslog
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: ['anticamurrina', 'medicivimodrone', 'debian-5.5.flag', 'mlnsfastcasual', 'ib_logfile1', 'colleonibocce', 'alfaromeotheredtable', 'fan', 'anticamurrinamurano', 'cementoitaliano', 'ib_logfile0', 'victimsandcorporations', 'blog', 'performance_schema', 'ibdata1', 'trkr', 'spazioformamentis', 'workitect', 'mysql_upgrade_info', 'ateliervoltolina', 'diegocostantini', 'mysql']
ProcCmdline: root=LABEL=DOROOT ro
RelatedPackageVersions:
 dpkg 1.18.4ubuntu1.1
 apt 1.2.12~ubuntu16.04.1
SourcePackage: mysql-5.7
Title: package mysql-server-5.7 5.7.15-0ubuntu0.16.04.1 failed to install/upgrade: subprocess installed post-installation script returned error exit status 1
UpgradeStatus: Upgraded to xenial on 2016-10-04 (0 days ago)

[Robie Basak (racb)]

From log:

ERROR: Unable to start MySQL server:
2016-10-04T12:06:42.745709Z 0 [ERROR] unknown variable 'thread_concurrency=8'
2016-10-04T12:06:42.749777Z 0 [ERROR] Aborting
Please take a look at https://wiki.debian.org/Teams/MySQL/FAQ for tips on fixing common upgrade issues.
Once the problem is resolved, run apt-get --fix-broken install to retry.
dpkg: error processing package mysql-server-5.7 (--configure):
 subprocess installed post-installation script returned error exit status 1