nixos/promtail: configFile for passing path as config (#322426)

nixos/modules/services/logging/promtail.nix

@@ -10,15 +10,29 @@ let
 
   allowPositionsFile = !lib.hasPrefix "/var/cache/promtail" positionsFile;
   positionsFile = cfg.configuration.positions.filename;
+
+  configFile = if cfg.configFile != null
+                   then cfg.configFile
+                   else prettyJSON cfg.configuration;
+
 in {
   options.services.promtail = with types; {
     enable = mkEnableOption "the Promtail ingresser";
 
-
     configuration = mkOption {
       type = (pkgs.formats.json {}).type;
       description = ''
         Specify the configuration for Promtail in Nix.
+        This option will be ignored if `services.promtail.configFile` is defined.
+      '';
+    };
+
+    configFile = mkOption {
+      type = nullOr path;
+      default = null;
+      description = ''
+        Config file path for Promtail.
+        If this option is defined, the value of `services.promtail.configuration` will be ignored.
       '';
     };
 
@@ -42,14 +56,14 @@ in {
       stopIfChanged = false;
 
       preStart = ''
-        ${lib.getExe pkgs.promtail} -config.file=${prettyJSON cfg.configuration} -check-syntax
+        ${lib.getExe pkgs.promtail} -config.file=${configFile} -check-syntax
       '';
 
       serviceConfig = {
         Restart = "on-failure";
         TimeoutStopSec = 10;
 
-        ExecStart = "${pkgs.promtail}/bin/promtail -config.file=${prettyJSON cfg.configuration} ${escapeShellArgs cfg.extraFlags}";
+        ExecStart = "${pkgs.promtail}/bin/promtail -config.file=${configFile} ${escapeShellArgs cfg.extraFlags}";
 
         ProtectSystem = "strict";
         ProtectHome = true;