add unitstatus

with some TODOs

modules/base/unitstatus/default.nix

@@ -0,0 +1,64 @@
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}: {
+  # TODO:
+  # - accept unit names:
+  #   - assert they exist
+  #   - add 'systemd.<unit>.unitConfig.OnFailure' to point to this one.
+  # - assert postfix is configured
+  options.mj.base.unitstatus = with lib.types; {
+    enable = lib.mkEnableOption "alert by email on unit failure";
+    email = lib.mkOption {type = str;};
+    #units = lib.mkOption {type = lisOf str;};
+  };
+
+  config =
+    lib.mkIf config.mj.base.unitstatus.enable {
+      systemd.services."unit-status-mail@" = let
+        # https://northernlightlabs.se/2014-07-05/systemd-status-mail-on-unit-failure.html
+        script = pkgs.writeShellScript "unit-status-mail" ''
+          set -e
+          MAILTO="${config.mj.base.unitstatus.email}"
+          UNIT=$1
+          EXTRA=""
+          for e in "''${@:2}"; do
+            EXTRA+="$e"$'\n'
+          done
+          UNITSTATUS=$(${pkgs.systemd}/bin/systemctl status "$UNIT")
+          ${pkgs.postfix}/bin/sendmail $MAILTO <<EOF
+          Subject:Status mail for unit: $UNIT
+
+          Status report for unit: $UNIT
+          $EXTRA
+
+          $UNITSTATUS
+          EOF
+
+          echo -e "Status mail sent to: $MAILTO for unit: $UNIT"
+        '';
+      in {
+        description = "Send an email on unit failure";
+        serviceConfig = {
+          Type = "simple";
+          ExecStart = ''${script} "%I" "Hostname: %H" "Machine ID: %m" "Boot ID: %b" '';
+        };
+      };
+    #};
+    # See TODO above.
+    #// {
+    #  systemd.services =
+    #    lib.listToAttrs
+    #    (map (
+    #        unit: {
+    #          name = unit;
+    #          value = {
+    #            unitConfig = {OnFailure = "unit-status-mail@${unit}.service";};
+    #          };
+    #        }
+    #      )
+    #      config.mj.base.unitstatus.units);
+    };
+}