nixos/mysql: use systemd StateDirectory to provision the data directory

nixos/modules/services/databases/mysql.nix

@@ -68,7 +68,14 @@ in
       dataDir = mkOption {
         type = types.path;
         example = "/var/lib/mysql";
-        description = "Location where MySQL stores its table files.";
+        description = ''
+          The data directory for MySQL.
+
+          <note><para>
+          If left as the default value of <literal>/var/lib/mysql</literal> this directory will automatically be created before the MySQL
+          server starts, otherwise you are responsible for ensuring the directory exists with appropriate ownership and permissions.
+          </para></note>
+        '';
       };
 
       configFile = mkOption {
@@ -341,11 +348,6 @@ in
 
     environment.etc."my.cnf".source = cfg.configFile;
 
-    systemd.tmpfiles.rules = [
-      "d '${cfg.dataDir}' 0700 '${cfg.user}' '${cfg.group}' - -"
-      "z '${cfg.dataDir}' 0700 '${cfg.user}' '${cfg.group}' - -"
-    ];
-
     systemd.services.mysql = let
       hasNotify = isMariaDB;
     in {
@@ -489,7 +491,8 @@ in
             '') cfg.ensureUsers}
         '';
 
-        serviceConfig = {
+        serviceConfig = mkMerge [
+          {
             Type = if hasNotify then "notify" else "simple";
             Restart = "on-abort";
             RestartSec = "5s";
@@ -523,7 +526,12 @@ in
             PrivateMounts = true;
             # System Call Filtering
             SystemCallArchitectures = "native";
-        };
+          }
+          (mkIf (cfg.dataDir == "/var/lib/mysql") {
+            StateDirectory = "mysql";
+            StateDirectoryMode = "0700";
+          })
+        ];
       };
 
   };