nixos mysql: make start-up more robust (#41010)

1) Change start-type to ```notify``` when running MariaDB so that we don't have to busy-wait for the
socket to appear.

2) Do not manually create the directory under /run as we can get systemd to do
that for us. This opens up the possibility later for not having to launch as root.

nixos/modules/services/databases/mysql.nix

@@ -231,8 +231,10 @@ in
 
     environment.systemPackages = [mysql];
 
-    systemd.services.mysql =
-      { description = "MySQL Server";
+    systemd.services.mysql = let
+      hasNotify = (cfg.package == pkgs.mariadb);
+    in {
+        description = "MySQL Server";
 
         after = [ "network.target" ];
         wantedBy = [ "multi-user.target" ];
@@ -256,17 +258,16 @@ in
 
             mkdir -m 0755 -p ${cfg.pidDir}
             chown -R ${cfg.user} ${cfg.pidDir}
-
-            # Make the socket directory
-            mkdir -p /run/mysqld
-            chmod 0755 /run/mysqld
-            chown -R ${cfg.user} /run/mysqld
           '';
 
-        serviceConfig.ExecStart = "${mysql}/bin/mysqld --defaults-extra-file=${myCnf} ${mysqldOptions}";
+        serviceConfig = {
+          Type = if hasNotify then "notify" else "simple";
+          RuntimeDirectory = "mysqld";
+          ExecStart = "${mysql}/bin/mysqld --defaults-extra-file=${myCnf} ${mysqldOptions}";
+        };
 
-        postStart =
-          ''
+        postStart = ''
+          ${lib.optionalString (!hasNotify) ''
             # Wait until the MySQL server is available for use
             count=0
             while [ ! -e /run/mysqld/mysqld.sock ]
@@ -281,6 +282,7 @@ in
                 count=$((count++))
                 sleep 1
             done
+          ''}
 
             if [ -f /tmp/mysql_init ]
             then