Extend mariadb-server to purge gracefully if datadir is a mountpoint …

…(Closes: #829491)

This will also
* not use the existence of lost+found as indicator for /var/lib/mysql
  being a mount point.
* to also keep erroneously renamed lost+found directories as they
  might continue to exist after a switch from MySQL to MariaDB.

debian/mariadb-server-10.0.postrm

@@ -56,7 +56,17 @@ if [ "$1" = "purge" -a ! \( -x /usr/sbin/mysqld -o -L /usr/sbin/mysqld \) ]; the
     # never remove the debian.cnf when the databases are still existing
     # else we ran into big trouble on the next install!
     rm -f /etc/mysql/debian.cnf
-    rm -rf /var/lib/mysql
+    # Remove all contents from /var/lib/mysql except if it's a
+    # directory with file system data. See #829491 for details and
+    # #608938 for potential mysql-server leftovers which erroneously
+    # had been renamed.
+    find /var/lib/mysql -mindepth 1 \
+         -not -path '*/lost+found/*'     -not -name 'lost+found' \
+         -not -path '*/lost@002bfound/*' -not -name 'lost@002bfound' \
+         -delete
+    # "|| true" still needed as rmdir still exits with non-zero if
+    # /var/lib/mysql is a mount point
+    rmdir --ignore-fail-on-non-empty /var/lib/mysql || true
     rm -rf /var/run/mysqld # this directory is created by the init script, don't leave behind
     userdel mysql || true
   fi