nixos/kavita: improve credential management

* allow specifying arbitrary number of secrets to replace in
  `appsettings.json`
* update option documentation
* move `services.kavita.tokenKeyFile` to
  `services.kavita.settings.TokenKey` to sync with updated credential
  management
* fix nixosTests.kavita

Resolves: #558959

nixos/modules/services/web-apps/kavita.nix

@@ -8,9 +8,7 @@
 let
   cfg = config.services.kavita;
   settingsFormat = pkgs.formats.json { };
-  appsettings = settingsFormat.generate "appsettings.json" (
-    { TokenKey = "@TOKEN@"; } // cfg.settings
-  );
+  appsettings = settingsFormat.generate "appsettings.json" cfg.settings;
 in
 {
   imports = [
@@ -26,6 +24,10 @@ in
       )
     )
     (lib.mkRenamedOptionModule [ "services" "kavita" "port" ] [ "services" "kavita" "settings" "Port" ])
+    (lib.mkRenamedOptionModule
+      [ "services" "kavita" "tokenKeyFile" ]
+      [ "services" "kavita" "settings" "TokenKey" ]
+    )
   ];
 
   options.services.kavita = {
@@ -45,18 +47,20 @@ in
       description = "The directory where Kavita stores its state.";
     };
 
-    tokenKeyFile = lib.mkOption {
-      type = lib.types.path;
-      description = ''
-        A file containing the TokenKey, a secret with at 512+ bits.
-        It can be generated with `head -c 64 /dev/urandom | base64 --wrap=0`.
-      '';
-    };
-
     settings = lib.mkOption {
       default = { };
       description = ''
         Kavita configuration options, as configured in {file}`appsettings.json`.
+
+        Any configuration option that contains sensitive data should use a placeholder
+        value (ie, `oidc_client_secret`) enclosed with an `@` character:
+
+        `services.kavita.settings.OpenIdConnectSettings.Secret = "@oidc_client_secret@";`
+
+        Then, use the placeholder as unique key and hydrate the secret using your
+        preferred secrets management (ie, `sops-nix`):
+
+        `services.kavita.credentials."oidc_client_secret" = config.sops.secrets."client_secret".path;`
       '';
       type = lib.types.submodule {
         freeformType = settingsFormat.type;
@@ -75,8 +79,46 @@ in
               IP Addresses to bind to. The default is to bind to all IPv4 and IPv6 addresses.
             '';
           };
+
+          TokenKey = lib.mkOption {
+            default = "@token_key@";
+            type = lib.types.str;
+            description = ''
+              A secret with at least 512 bits. It may be generated with `head -c 64 /dev/urandom | base64 --wrap=0`.
+
+              Suggest using a placeholder value (eg, `token_key`) enclosed by `@` character (eg, `@tokey_key@`) to avoid writing
+              sensitive data to world readable nix store. Placeholder value will be defined as key in `services.kavita.credentials`
+              and the associated path containing the secret will be replaced at service startup.
+            '';
+          };
         };
       };
+      example = {
+        IpAddresses = "::2,127.0.0.2";
+        Port = 5001;
+        TokenKey = "@token_key@";
+        OpenIdConnectSettings = {
+          Enabled = true;
+          Authority = "https://idp.example.com";
+          ClientId = "kavita";
+          Secret = "@oidc_client_secret@";
+        };
+      };
+    };
+
+    credentials = lib.mkOption {
+      type = lib.types.attrsOf lib.types.str;
+      default = { };
+      example = {
+        token_key = "/run/secrets/kavita/token_key";
+        oidc_client_secret = "/run/secrets/kavita/client_secret";
+      };
+      description = ''
+        Secrets that will be passed to systemd service and subsequently all placeholders in {file}`appsettings.json` will be replaced
+        prior to service startup.
+
+        It is not necessary to enclose the keys/placeholder values with `@` characters here.
+      '';
     };
   };
 
@@ -87,13 +129,16 @@ in
       after = [ "network.target" ];
       preStart = ''
         install -m600 ${appsettings} ${lib.escapeShellArg cfg.dataDir}/config/appsettings.json
-        ${pkgs.replace-secret}/bin/replace-secret '@TOKEN@' \
-          ''${CREDENTIALS_DIRECTORY}/token \
-          '${cfg.dataDir}/config/appsettings.json'
-      '';
+      ''
+      + lib.concatStringsSep "\n" (
+        lib.mapAttrsToList (key: val: ''
+          ${lib.getExe pkgs.replace-secret} '@${key}@' \
+            ''${CREDENTIALS_DIRECTORY}/${key} \
+            ${cfg.dataDir}/config/appsettings.json'') cfg.credentials
+      );
       serviceConfig = {
         WorkingDirectory = cfg.dataDir;
-        LoadCredential = [ "token:${cfg.tokenKeyFile}" ];
+        LoadCredential = lib.mapAttrsToList (key: value: "${key}:${value}") cfg.credentials;
         ExecStart = lib.getExe cfg.package;
         Restart = "always";
         User = cfg.user;

nixos/tests/kavita.nix

@@ -16,7 +16,7 @@ in
       kavita = {
         services.kavita = {
           enable = true;
-          tokenKeyFile = builtins.toFile "kavita.key" key;
+          settings.TokenKey = key;
         };
       };
       has_template =
@@ -27,7 +27,7 @@ in
         {
           services.kavita = {
             enable = true;
-            tokenKeyFile = builtins.toFile "kavita.key" key;
+            settings.TokenKey = key;
           };
           systemd.tmpfiles.rules = [
             "d ${emailTemplDir} 0750 ${u} ${u} - -"