postinst is in percona-xtradb-cluster-server-5.6 5.6.21-25.8-0ubuntu3.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
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set -e
. /usr/share/debconf/confmodule
if [ -n "$DEBIAN_SCRIPT_DEBUG" ]; then set -v -x; DEBIAN_SCRIPT_TRACE=1; fi
${DEBIAN_SCRIPT_TRACE:+ echo "#42#DEBUG# RUNNING $0 $*" 1>&2 }
 
export PATH=$PATH:/sbin:/usr/sbin:/bin:/usr/bin
# This command can be used as pipe to syslog. With "-s" it also logs to stderr.
ERR_LOGGER="logger -p daemon.err -t mysqld_safe -i"
invoke() {
  if [ -x /usr/sbin/invoke-rc.d ]; then
    invoke-rc.d mysql $1
  else
    /etc/init.d/mysql $1
  fi
}
MYSQL_BOOTSTRAP="/usr/sbin/mysqld --bootstrap --user=mysql --skip-grant-tables"
test_mysql_access() {
       mysql --no-defaults -u root -h localhost </dev/null >/dev/null 2>&1
}
# call with $1 = "online" to connect to the server, otherwise it bootstraps
set_mysql_rootpw() {
       # forget we ever saw the password.  don't use reset to keep the seen status
       db_set mysql-server/root_password ""
       db_set mysql-server/root_password_again ""
       tfile=`mktemp`
       if [ ! -f "$tfile" ]; then
               return 1
       fi
       # this avoids us having to call "test" or "[" on $rootpw
       cat << EOF > $tfile
USE mysql;
UPDATE user SET password=PASSWORD("$rootpw") WHERE user='root';
FLUSH PRIVILEGES;
EOF
       if grep -q 'PASSWORD("")' $tfile; then
               retval=0
       elif [ "$1" = "online" ]; then
               mysql --no-defaults -u root -h localhost <$tfile >/dev/null
               retval=$?
       else
               $MYSQL_BOOTSTRAP <$tfile
               retval=$?
       fi
       rm -f $tfile
       return $retval
}
# This is necessary because mysql_install_db removes the pid file in /var/run
# and because changed configuration options should take effect immediately.
# In case the server wasn't running at all it should be ok if the stop
# script fails. I can't tell at this point because of the cleaned /var/run.
set +e; invoke stop; set -e
    
case "$1" in
  configure)
    mysql_datadir=/usr/share/mysql
    mysql_statedir=/var/lib/percona-xtradb-cluster
    mysql_rundir=/var/run/mysqld
    mysql_logdir=/var/log
    mysql_cfgdir=/etc/mysql
    mysql_newlogdir=/var/log/mysql
    mysql_upgradedir=/var/lib/mysql-upgrade
    # New packaging paradigm for my.cnf as of Dec-2014 for sharing mysql
    # variants in Ubuntu.
    /usr/share/mysql-common/configure-symlinks install percona-xtradb-cluster "$mysql_cfgdir/percona-xtradb-cluster.cnf"
    # first things first, if the following symlink exists, it is a preserved
    # copy the old data dir from a mysql upgrade that would have otherwise
    # been replaced by an empty mysql dir.  this should restore it.
    for dir in DATADIR LOGDIR; do
        if [ "$dir" = "DATADIR" ]; then targetdir=$mysql_statedir; else targetdir=$mysql_newlogdir; fi
        savelink="$mysql_upgradedir/$dir.link"
        if [ -L "$savelink" ]; then
            # If the targetdir was a symlink before we upgraded it is supposed
            # to be either still be present or not existing anymore now.
            if [ -L "$targetdir" ]; then
                rm "$savelink"
            elif [ ! -d "$targetdir" ]; then
                mv "$savelink" "$targetdir"
            else
                # this should never even happen, but just in case...
                mysql_tmp=`mktemp -d -t mysql-symlink-restore-XXXXXX`
                echo "this is very strange!  see $mysql_tmp/README..." >&2
                mv "$targetdir" "$mysql_tmp"
                cat << EOF > "$mysql_tmp/README"
if you're reading this, it's most likely because you had replaced /var/lib/mysql
with a symlink, then upgraded to a new version of mysql, and then dpkg
removed your symlink (see #182747 and others).  the mysql packages noticed
that this happened, and as a workaround have restored it.  however, because
/var/lib/mysql seems to have been re-created in the meantime, and because
we don't want to rm -rf something we don't know as much about, we're going
to leave this unexpected directory here.  if your database looks normal,
and this is not a symlink to your database, you should be able to blow
this all away.
EOF
            fi
        fi
	rmdir $mysql_upgradedir 2>/dev/null || true
    done
    
    # Ensure the existence and right permissions for the database and
    # log files.
    if [ ! -d "$mysql_statedir"       -a ! -L "$mysql_statedir"       ]; then mkdir "$mysql_statedir"; fi
    if [ ! -d "$mysql_statedir/mysql" -a ! -L "$mysql_statedir/mysql" ]; then mkdir "$mysql_statedir/mysql"; fi
    if [ ! -d "$mysql_newlogdir"      -a ! -L "$mysql_newlogdir"      ]; then mkdir "$mysql_newlogdir"; fi
    # When creating an ext3 jounal on an already mounted filesystem like e.g.
    # /var/lib/mysql, you get a .journal file that is not modifyable by chown.
    # The mysql_datadir must not be writable by the mysql user under any
    # circumstances as it contains scripts that are executed by root.
    set +e
    chown -R 0:0 $mysql_datadir
    chown -R mysql $mysql_statedir
    chmod 700 $mysql_statedir $mysql_statedir/mysql
    if [ ! -d "$mysql_rundir" ]; then mkdir "$mysql_rundir"; fi
    chown -R mysql $mysql_rundir
    chown -R mysql:adm $mysql_newlogdir;	chmod 2750 $mysql_newlogdir;
    for i in log err; do
      touch             $mysql_logdir/mysql.$i
      chown mysql:adm   $mysql_logdir/mysql.$i
      chmod 0640        $mysql_logdir/mysql.$i
    done
    set -e
    # This is important to avoid dataloss when there is a removed
    # percona-xtradb-cluster-server version from Woody lying around which used the same
    # data directory and then somewhen gets purged by the admin.
    db_set percona-xtradb-cluster-server/postrm_remove_database false || true
    # To avoid downgrades.
    touch $mysql_statedir/debian-5.6.flag
    # initiate databases. Output is not allowed by debconf :-(
    # Debian: beware of the bashisms... 
    # Debian: can safely run on upgrades with existing databases 
    set +e
    /usr//bin/mysql_install_db --user=mysql --rpm 2>&1 |
        $ERR_LOGGER
    if [ "$?" != "0" ]; then
      echo "ATTENTION: An error has occured. More info is in the syslog!"
    fi
    set -e
    
    ## On every reconfiguration the maintenance user is recreated.
    #
    # - It is easier to regenerate the password every time but as people
    #   use fancy rsync scripts and file alteration monitors, the existing
    #   password is used and existing files not touched.
    # - The mysqld statement is like that in mysql_install_db because the
    #   server is not already running. This has some implications:
    # 	- The amount of newlines and semicolons in the query is important!
    #   - GRANT is not possible with --skip-grant-tables and "INSERT
    #     (user,host..) VALUES" is not --ansi compliant
    # - The echo is just for readability. ash's buildin has no "-e" so use /bin/echo.
    # - The Super_priv, Show_db_priv, Create_tmp_table_priv and Lock_tables_priv
    #   may not be present as old Woody 3.23 databases did not have it and the
    #   admin might not already have run mysql_upgrade which adds them.
    #   As the binlog cron scripts to need at least the Super_priv, I do first
    #   the old query which always succeeds and then the new which may or may not.
    # recreate the credentials file if not present or without mysql_upgrade stanza
    dc=$mysql_cfgdir/debian.cnf; 
    if [ -e "$dc" -a -n "`fgrep mysql_upgrade $dc 2>/dev/null`" ]; then
        pass="`sed -n 's/^[     ]*password *= *// p' $dc | head -n 1`"
    else
	pass=`perl -e 'print map{("a".."z","A".."Z",0..9)[int(rand(62))]}(1..16)'`;
        if [ ! -d "$mysql_cfgdir" ]; then install -o 0 -g 0 -m 0755 -d $mysql_cfgdir; fi
        umask 066
        cat /dev/null > $dc
        umask 022
        echo "# Automatically generated for Debian scripts. DO NOT TOUCH!" >>$dc
        echo "[client]"                                                    >>$dc
        echo "host     = localhost"                                        >>$dc
        echo "user     = debian-sys-maint"                                 >>$dc
        echo "password = $pass"                                            >>$dc
        echo "socket   = $mysql_rundir/mysqld.sock"                        >>$dc
        echo "[mysql_upgrade]"                                             >>$dc
        echo "host     = localhost"                                        >>$dc
        echo "user     = debian-sys-maint"                                 >>$dc
        echo "password = $pass"                                            >>$dc
        echo "socket   = $mysql_rundir/mysqld.sock"                        >>$dc
        echo "basedir  = /usr"                                             >>$dc
    fi
    # If this dir chmod go+w then the admin did it. But this file should not.
    chown 0:0 $dc
    chmod 0600 $dc
    # update privilege tables
    password_column_fix_query=`echo -e \
        "USE mysql\n" \
        "ALTER TABLE user CHANGE Password Password char(41) character set latin1 collate latin1_bin DEFAULT '' NOT NULL"`;
    replace_query=`echo -e \
        "USE mysql;\n" \
        "REPLACE INTO user SET " \
        "  host='localhost', user='debian-sys-maint', password=password('$pass'), " \
        "  Select_priv='Y', Insert_priv='Y', Update_priv='Y', Delete_priv='Y', " \
        "  Create_priv='Y', Drop_priv='Y', Reload_priv='Y', Shutdown_priv='Y', " \
        "  Process_priv='Y',  File_priv='Y', Grant_priv='Y', References_priv='Y', " \
        "  Index_priv='Y', Alter_priv='Y', Super_priv='Y', Show_db_priv='Y', " \
        "  Create_tmp_table_priv='Y', Lock_tables_priv='Y', Execute_priv='Y', " \
        "  Repl_slave_priv='Y', Repl_client_priv='Y', Create_view_priv='Y', " \
        "  Show_view_priv='Y', Create_routine_priv='Y', Alter_routine_priv='Y', " \
        "  Create_user_priv='Y', Event_priv='Y', Trigger_priv='Y', Create_tablespace_priv='Y', " \
        "  ssl_cipher='', x509_issuer='', x509_subject=''; " `
    fix_privs=`echo -e \
        "USE mysql;\n" \
        "ALTER TABLE user ADD column Create_view_priv enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N'; " \
        "ALTER TABLE user ADD column Show_view_priv enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N'; " \
        "ALTER TABLE user ADD column Create_routine_priv enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N'; " \
        "ALTER TABLE user ADD column Alter_routine_priv enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N'; " \
        "ALTER TABLE user ADD column Create_user_priv enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N'; " \
        "ALTER TABLE user ADD column Event_priv enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N'; " \
        "ALTER TABLE user ADD column Trigger_priv enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N'; " `
    db_get mysql-server/root_password && rootpw="$RET"
    if ! set_mysql_rootpw; then
        password_error="yes"
    fi
    echo "$replace_query"                                    | $MYSQL_BOOTSTRAP 2>&1 | $ERR_LOGGER
  ;;
  abort-upgrade|abort-remove|abort-configure)
  ;;
  *)
    echo "postinst called with unknown argument $1" 1>&2
    exit 1
  ;;
esac
# here we check to see if we can connect as root without a password
# this should catch upgrades from previous versions where the root
# password was not set.  if there is a password, or if the connection
# fails for any other reason, nothing happens.
if [ "$1" = "configure" ]; then
       if test_mysql_access; then
               db_input medium mysql-server/root_password || true
               db_go
               db_get mysql-server/root_password && rootpw="$RET"
               if ! set_mysql_rootpw "online"; then
                       password_error="yes"
               fi
       fi
       if [ "$password_error" = "yes" ]; then
               db_input high mysql-server/error_setting_password || true
               db_go
       fi
fi
db_stop # in case invoke failes
# Automatically added by dh_installinit
if [ "$1" = "configure" ] || [ "$1" = "abort-upgrade" ]; then
	if [ -x "/etc/init.d/mysql" ]; then
		update-rc.d mysql defaults 19 21 >/dev/null
	fi
	if [ -x "/etc/init.d/mysql" ] || [ -e "/etc/init/mysql.conf" ]; then
		invoke-rc.d mysql start || exit $?
	fi
fi
# End automatically added section
exit 0
 |