feat: setup email notification on systemd failures

modules/emailNotify.nix

@@ -0,0 +1,67 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+  cfg = config.systemd.emailNotify;
+  sendmail = pkgs.writeScript "sendmail" ''
+    #!${pkgs.runtimeShell}
+
+    ${pkgs.system-sendmail}/bin/sendmail -t <<ERRMAIL
+    To: ${cfg.mailTo}
+    From: ${cfg.mailFrom}
+    Subject: Status of service $1
+    Content-Transfer-Encoding: 8bit
+    Content-Type: text/plain; charset=UTF-8
+    $(systemctl status --full "$1")
+    ERRMAIL
+  '';
+in {
+  options = {
+    systemd.emailNotify = {
+      enable = mkOption {
+        type = types.bool;
+        default = false;
+        description =
+          "Whether to enable email notification for failed services.";
+      };
+
+      mailTo = mkOption {
+        type = types.str;
+        default = null;
+        description =
+          "Email address to which the service status will be mailed.";
+      };
+
+      mailFrom = mkOption {
+        type = types.str;
+        default = null;
+        description =
+          "Email address from which the service status will be mailed.";
+      };
+    };
+
+    systemd.services = mkOption {
+      type = with types;
+        attrsOf (submodule {
+          config.onFailure = optional cfg.enable "email@%n.service";
+        });
+    };
+  };
+
+  config = mkIf cfg.enable {
+    assertions = singleton {
+      assertion = cfg.mailTo != null && cfg.mailFrom != null;
+      message = "You need to specify a sender and a receiver";
+    };
+
+    systemd.services."email@" = {
+      description = "Sends a status mail via sendmail on service failures.";
+      onFailure = lib.mkForce [ ];
+      serviceConfig = {
+        ExecStart = "${sendmail} %i";
+        Type = "oneshot";
+      };
+    };
+  };
+}