export flake information into monitoring

flake.nix

@@ -47,6 +47,7 @@
           inherit (self.inputs.nixpkgs.lib) nixosSystem;
           common = [
             self.inputs.sops-nix.nixosModules.sops
+            { _module.args.inputs = self.inputs; }
           ];
         in {
           "build01.nix-community.org" = nixosSystem {

roles/nix-daemon.nix

@@ -1,4 +1,4 @@
-{ config, lib, pkgs, ... }:
+{ config, lib, pkgs, inputs, ... }:
 
 let
   asGB = size: toString (size * 1024 * 1024);
@@ -42,5 +42,24 @@ in
     Restart = "on-failure";
   };
 
+  # inputs == flake inputs in configurations.nix
+  environment.etc = let
+    inputsWithDate = lib.filterAttrs (_: input: input ? lastModified) inputs;
+    flakeAttrs = input: (lib.mapAttrsToList (n: v: ''${n}="${v}"'')
+      (lib.filterAttrs (n: v: (builtins.typeOf v) == "string") input));
+    lastModified = name: input: ''
+      flake_input_last_modified{input="${name}",${lib.concatStringsSep "," (flakeAttrs input)}} ${toString input.lastModified}
+    '';
+  in {
+    "flake-inputs.prom" = {
+      mode = "0555";
+      text = ''
+        # HELP flake_registry_last_modified Last modification date of flake input in unixtime
+        # TYPE flake_input_last_modified gauge
+        ${lib.concatStringsSep "\n" (lib.mapAttrsToList lastModified inputsWithDate)}
+      '';
+    };
+  };
+
   users.groups.trusted = { };
 }