Unmask the systemd mysql.service if left behind by mysql-server-5.6

Without this a simple 'apt-get install mariadb-server' would end
up with a system where mysqld is not running despite it was
running OK with mysql-server-5.6 installed, and users might wrongly
think mariadb-server package is broken when the real cause was
the removal of mysql-server-5.6 that left a /etc/systemd/system/
file behind pointing to /dev/null.

debian/changelog

@@ -20,6 +20,8 @@ mariadb-10.0 (10.0.23-2) UNRELEASED; urgency=low
   * Remove Taocrypt workaround fixed upstream long since #627208
   * Removed CFLAGS and CXXFLAGS as suggested by Lars Tangvald and also done
     in mysql-5.6 packaging commit id 16a64e810e28f1d0b66ede274cd4c2b1a425fecb
+  * Unmask the systemd mysql.service if left behind by a mysql-server-5.6
+    installation, otherwise the MariaDB service would remain masked too.
 
  -- Otto Kekäläinen <otto@seravo.fi>  Mon, 28 Dec 2015 09:51:36 +0200
 

debian/mariadb-server-10.0.postinst

@@ -171,6 +171,11 @@ esac
 
 db_stop # in case invoke failes
 
+# If we upgrade from MySQL mysql.service may be masked, which also
+# means init.d script is disabled. Unmask mysql service explicitely.
+# Ignore exit code as command is not available everywhere.
+deb-systemd-helper unmask mysql.service > /dev/null || true
+
 #DEBHELPER#
 
 exit 0