nixos/invidious: generate hmac_key automatically

This change also generates the invidious config by putting JSON
snippets into a bash array and then using jq to merge them all into
a single configuration where later elements override previous
elements.

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

@@ -7,6 +7,9 @@ let
 
   settingsFile = settingsFormat.generate "invidious-settings" cfg.settings;
 
+  generatedHmacKeyFile = "/var/lib/invidious/hmac_key";
+  generateHmac = cfg.hmacKeyFile == null;
+
   serviceConfig = {
     systemd.services.invidious = {
       description = "Invidious (An alternative YouTube front-end)";
@@ -14,22 +17,47 @@ let
       after = [ "network-online.target" ];
       wantedBy = [ "multi-user.target" ];
 
-      script =
+      preStart = lib.optionalString generateHmac ''
-        let
+        if [[ ! -e "${generatedHmacKeyFile}" ]]; then
-          jqFilter = "."
+          ${pkgs.pwgen}/bin/pwgen 20 1 > "${generatedHmacKeyFile}"
-            + lib.optionalString (cfg.database.host != null) "[0].db.password = \"'\"'\"$(cat ${lib.escapeShellArg cfg.database.passwordFile})\"'\"'\""
+          chmod 0600 "${generatedHmacKeyFile}"
-            + " | .[0]"
+        fi
-            + lib.optionalString (cfg.extraSettingsFile != null) " * .[1]";
+      '';
-          jqFiles = [ settingsFile ] ++ lib.optional (cfg.extraSettingsFile != null) cfg.extraSettingsFile;
+
-        in
+      script = ''
+        configParts=()
       ''
-          export INVIDIOUS_CONFIG="$(${pkgs.jq}/bin/jq -s "${jqFilter}" ${lib.escapeShellArgs jqFiles})"
+      # autogenerated hmac_key
+      + lib.optionalString generateHmac ''
+        configParts+=("$(${pkgs.jq}/bin/jq -R '{"hmac_key":.}' <"${generatedHmacKeyFile}")")
+      ''
+      # generated settings file
+      + ''
+        configParts+=("$(< ${lib.escapeShellArg settingsFile})")
+      ''
+      # optional database password file
+      + lib.optionalString (cfg.database.host != null) ''
+        configParts+=("$(${pkgs.jq}/bin/jq -R '{"db":{"password":.}}' ${lib.escapeShellArg cfg.database.passwordFile})")
+      ''
+      # optional extra settings file
+      + lib.optionalString (cfg.extraSettingsFile != null) ''
+        configParts+=("$(< ${lib.escapeShellArg cfg.extraSettingsFile})")
+      ''
+      # explicitly specified hmac key file
+      + lib.optionalString (cfg.hmacKeyFile != null) ''
+        configParts+=("$(< ${lib.escapeShellArg cfg.hmacKeyFile})")
+      ''
+      # merge all parts into a single configuration with later elements overriding previous elements
+      + ''
+        export INVIDIOUS_CONFIG="$(${pkgs.jq}/bin/jq -s 'reduce .[] as $item ({}; . * $item)' <<<"''${configParts[*]}")"
         exec ${cfg.package}/bin/invidious
       '';
 
       serviceConfig = {
         RestartSec = "2s";
         DynamicUser = true;
+        StateDirectory = "invidious";
+        StateDirectoryMode = "0750";
 
         CapabilityBoundingSet = "";
         PrivateDevices = true;
@@ -171,6 +199,18 @@ in
       '';
     };
 
+    hmacKeyFile = lib.mkOption {
+      type = types.nullOr types.path;
+      default = null;
+      description = lib.mdDoc ''
+        A path to a file containing the `hmac_key`. If `null`, a key will be generated automatically on first
+        start.
+
+        If non-`null`, this option overrides any `hmac_key` specified in {option}`services.invidious.settings` or
+        via {option}`services.invidious.extraSettingsFile`.
+      '';
+    };
+
     extraSettingsFile = lib.mkOption {
       type = types.nullOr types.str;
       default = null;