nixos/mpd: support passwords in separate files

This allows to use files containing only the mpd password without the
permissions, making it easier for other programs connecting to mpd to read the
password from the same password file.

nixos/modules/services/audio/mpd.nix

@@ -10,6 +10,14 @@ let
   gid = config.ids.gids.mpd;
   cfg = config.services.mpd;
 
+  credentialsPlaceholder = (creds:
+    let
+      placeholders = (imap0
+        (i: c: ''password "{{password-${toString i}}}@${concatStringsSep "," c.permissions}"'')
+        creds);
+    in
+      concatStringsSep "\n" placeholders);
+
   mpdConf = pkgs.writeText "mpd.conf" ''
     # This file was automatically generated by NixOS. Edit mpd's configuration
     # via NixOS' configuration.nix, as this file will be rewritten upon mpd's
@@ -32,6 +40,8 @@ let
       }
     ''}
 
+    ${credentialsPlaceholder cfg.credentials}
+
     ${cfg.extraConfig}
   '';
 
@@ -152,6 +162,37 @@ in {
         '';
       };
 
+      credentials = mkOption {
+        type = types.listOf (types.submodule {
+          options = {
+            passwordFile = mkOption {
+              type = types.path;
+              description = ''
+                Path to file containing the password.
+              '';
+            };
+            permissions = let
+              perms = ["read" "add" "control" "admin"];
+            in mkOption {
+              type = types.listOf (types.enum perms);
+              default = [ "read" ];
+              description = ''
+                List of permissions that are granted with this password.
+                Permissions can be "${concatStringsSep "\", \"" perms}".
+              '';
+            };
+          };
+        });
+        description = ''
+          Credentials and permissions for accessing the mpd server.
+        '';
+        default = [];
+        example = [
+          {passwordFile = "/var/lib/secrets/mpd_readonly_password"; permissions = [ "read" ];}
+          {passwordFile = "/var/lib/secrets/mpd_admin_password"; permissions = ["read" "add" "control" "admin"];}
+        ];
+      };
+
       credentialsFile = mkOption {
         type = types.path;
         description = ''
@@ -201,12 +242,16 @@ in {
       serviceConfig = mkMerge [
         {
           User = "${cfg.user}";
-          ExecStart = "${pkgs.mpd}/bin/mpd --no-daemon /etc/mpd.conf";
-          ExecStartPre = pkgs.writeScript "mpd-start-pre" ''
-            #!${pkgs.runtimeShell}
-            set -euo pipefail
-            cat ${mpdConf} ${cfg.credentialsFile} > /etc/mpd.conf
+          ExecStart = "${pkgs.mpd}/bin/mpd --no-daemon /run/mpd/mpd.conf";
+          ExecStartPre = pkgs.writeShellScript "mpd-start-pre" ''
+            set -xeuo pipefail
+            umask 077
+            cat ${mpdConf} ${cfg.credentialsFile} > /run/mpd/mpd.conf
+            ${pkgs.replace}/bin/replace-literal -fe ${
+              concatStringsSep " -a " (imap0 (i: c: "\"{{password-${toString i}}}\" \"$(cat ${c.passwordFile})\"") cfg.credentials)
+            } /run/mpd/mpd.conf
           '';
+          RuntimeDirectory = "mpd";
           Type = "notify";
           LimitRTPRIO = 50;
           LimitRTTIME = "infinity";
@@ -230,14 +275,6 @@ in {
         })
       ];
     };
-    environment.etc."mpd.conf" = {
-      mode = "0640";
-      group = cfg.group;
-      user = cfg.user;
-      # To be modified by the service' ExecStartPre
-      text = ''
-      '';
-    };
 
     users.users = optionalAttrs (cfg.user == name) {
       ${name} = {