nixos/mpd: use pkgs.writeTextFile, with a checkPhase

nixos/modules/services/audio/mpd.nix

@@ -24,7 +24,9 @@ let
     lib.concatStringsSep "\n" placeholders
   );
 
-  mpdConf = pkgs.writeText "mpd.conf" ''
+  mpdConf = pkgs.writeTextFile {
+    name = "mpd.conf";
+    text = ''
     # This file was automatically generated by NixOS. Edit mpd's configuration
     # via NixOS' configuration.nix, as this file will be rewritten upon mpd's
     # restart.
@@ -48,6 +50,27 @@ let
 
     ${cfg.extraConfig}
   '';
+    derivationArgs = {
+      expectScript = ''
+        spawn ${lib.getExe pkgs.buildPackages.mpd} --no-daemon "$env(out)"
+        expect {
+          "exception: Error in \"$env(out)\"" {
+            puts "Config file invalid\n"
+            exit 1
+          }
+          "exception:" {
+            exit 0
+          }
+        }
+      '';
+      passAsFile = [
+        "expectScript"
+      ];
+    };
+    checkPhase = ''
+      ${lib.getExe pkgs.buildPackages.expect} -f "$expectScriptPath"
+    '';
+  };
 
 in
 {