Use proper pid namespace

    Problem:
    ==============
    By testing `pgrep` with `--ns` option,
    introduced with MDEV-21331, commit fb7c1b94,
    I noted that:
    a) `--ns`  cannot use more than single PID.
    b) `--ns` is returning the processes of the namespace to which supplied PID belongs to.
    So by that sense command `pgrep -x --ns $$ mysqld` will always return an error and skip
    checking of the existing PID of the server.

    Solution:
    ==============
    Suggested solution is to add `--nslist pid`, since `--ns` needs to know in which namespace type it should look for.
    See `pgrep --help` for different namespace types.
    Note also that this works *only* if script is run as a `root` (we have that case here).

    Current PR is a part of:
    1. MDEV-21331: sync preinst and postrm script
    2. MDEV-15718: check for exact mysqld process

    This commit:
    a) fixes fb7c1b94
    b) Closes PR #2068 (obsolete)
    c) Closes PR #2069 (obsolete)

    Thanks Faustin Lammler <faustin@mariadb.org> for testing and verifying
    Reviewed by <>

debian/mariadb-server-10.2.postrm

@@ -11,6 +11,11 @@ MYADMIN="/usr/bin/mysqladmin --defaults-file=/etc/mysql/debian.cnf"
 # do it himself. No database directories should be removed while the server
 # is running!
 stop_server() {
+  # Return immediately if there are no mysql processes running
+  # as there is no point in trying to shutdown in that case.
+  # Compatibility with versions that ran 'mariadbd'
+  if ! pgrep -x --nslist pid --ns $$ "mysqld|mariadbd" > /dev/null; then return; fi
+
   set +e
   if [ -x /usr/sbin/invoke-rc.d ]; then
     invoke-rc.d mysql stop

debian/mariadb-server-10.2.preinst

@@ -22,10 +22,11 @@ mysql_upgradedir=/var/lib/mysql-upgrade
 # is running! Another mysqld in e.g. a different chroot is fine for us.
 stop_server() {
     if [ ! -x /etc/init.d/mysql ]; then return; fi
-
-    # Return immediately if there are no mysql processes running
+    # Return immediately if there are no mysql processes running on a host
+    # (leave containerized processes with the same name in other namespaces)
     # as there is no point in trying to shutdown in that case.
-    if ! pgrep --ns $$ mysqld > /dev/null; then return; fi
+    # Compatibility with versions that ran 'mariadbd'
+    if ! pgrep -x --nslist pid --ns $$ "mysqld|mariadbd" > /dev/null; then return; fi
 
     set +e
     if [ -x /usr/sbin/invoke-rc.d ]; then