nixos/systemd: add options for hardware watchdog management

nixos/modules/system/boot/systemd.nix

@@ -818,6 +818,45 @@ in
       '';
     };
 
+    systemd.watchdog.device = mkOption {
+      type = types.path;
+      example = "/dev/watchdog";
+      description = ''
+        The path to a hardware watchdog device which will be managed by systemd.
+        If not specified, systemd will default to /dev/watchdog.
+      '';
+    };
+
+    systemd.watchdog.runtimeTime = mkOption {
+      type = types.str;
+      example = "30s";
+      description = ''
+        The amount of time which can elapse before a watchdog hardware device
+        will automatically reboot the system. Valid time units include "ms",
+        "s", "min", "h", "d", and "w".
+      '';
+    };
+
+    systemd.watchdog.rebootTime = mkOption {
+      type = types.str;
+      example = "10m";
+      description = ''
+        The amount of time which can elapse after a reboot has been triggered
+        before a watchdog hardware device will automatically reboot the system.
+        Valid time units include "ms", "s", "min", "h", "d", and "w".
+      '';
+    };
+
+    systemd.watchdog.kexecTime = mkOption {
+      type = types.str;
+      example = "10m";
+      description = ''
+        The amount of time which can elapse when kexec is being executed before
+        a watchdog hardware device will automatically reboot the system. This
+        option should only be enabled if reloadTime is also enabled. Valid
+        time units include "ms", "s", "min", "h", "d", and "w".
+      '';
+    };
   };
 
 
@@ -889,6 +928,19 @@ in
           DefaultIPAccounting=yes
         ''}
         DefaultLimitCORE=infinity
+        ${optionalString (config.systemd.watchdog.device != "") ''
+          WatchdogDevice=${config.systemd.watchdog.device}
+        ''}
+        ${optionalString (config.systemd.watchdog.runtimeTime != "") ''
+          RuntimeWatchdogSec=${config.systemd.watchdog.runtimeTime}
+        ''}
+        ${optionalString (config.systemd.watchdog.rebootTime != "") ''
+          RebootWatchdogSec=${config.systemd.watchdog.rebootTime}
+        ''}
+        ${optionalString (config.systemd.watchdog.kexecTime != "") ''
+          KExecWatchdogSec=${config.systemd.watchdog.kexecTime}
+        ''}
+
         ${config.systemd.extraConfig}
       '';
 

nixos/tests/systemd.nix

@@ -50,6 +50,13 @@ import ./make-test-python.nix ({ pkgs, ... }: {
         fi
       '';
     };
+
+    systemd.watchdog = {
+      device = "/dev/watchdog";
+      runtimeTime = "30s";
+      rebootTime = "10min";
+      kexecTime = "5min";
+    };
   };
 
   testScript = ''
@@ -117,5 +124,20 @@ import ./make-test-python.nix ({ pkgs, ... }: {
         retcode, output = machine.execute("systemctl status testservice1.service")
         assert retcode in [0, 3]  # https://bugs.freedesktop.org/show_bug.cgi?id=77507
         assert "CPU:" in output
+
+    # Test systemd is configured to manage a watchdog
+    with subtest("systemd manages hardware watchdog"):
+        machine.wait_for_unit("multi-user.target")
+
+        # It seems that the device's path doesn't appear in 'systemctl show' so
+        # check it separately.
+        assert "WatchdogDevice=/dev/watchdog" in machine.succeed(
+            "cat /etc/systemd/system.conf"
+        )
+
+        output = machine.succeed("systemctl show | grep Watchdog")
+        assert "RuntimeWatchdogUSec=30s" in output
+        assert "RebootWatchdogUSec=10m" in output
+        assert "KExecWatchdogUSec=5m" in output
   '';
 })