add watchdog service

modules/watchdog/default.nix

@@ -0,0 +1,24 @@
+{ lib, pkgs, config, ...}:
+let
+  inherit (lib) mkOption types;
+  inherit (pkgs) liminix;
+in
+{
+  options = {
+    system.service.watchdog =  mkOption {
+      type = liminix.lib.types.serviceDefn;
+    };
+  };
+  config.system.service.watchdog = liminix.callService ./watchdog.nix {
+    watched = mkOption {
+      description = "services to watch";
+      type = types.listOf liminix.lib.types.service;
+    };
+    headStart = mkOption {
+      description = "delay in seconds before watchdog starts checking service health";
+      default = 60;
+      type = types.int;
+    };
+  };
+  config.kernel.config.WATCHDOG = "y";
+}

modules/watchdog/gaspode.sh

@@ -0,0 +1,23 @@
+#!/bin/sh
+deadline=$(expr $(date +%s) + ${HEADSTART})
+services=$@
+echo started feeding the dog
+exec 3> ${WATCHDOG-/dev/watchdog}
+
+healthy(){
+    test $(date +%s) -le $deadline && return 0
+
+    for i in $services; do
+	if test "$(s6-svstat -o up /run/service/$i)" != "true" ; then
+	   echo "service $i is down"
+	   return 1
+	fi
+    done
+}
+
+while healthy ;do
+    sleep 10
+    echo >&3
+done
+echo "stopped feeding the dog"
+sleep 6000  # don't want s6-rc to restart

modules/watchdog/watchdog.nix

@@ -0,0 +1,12 @@
+{
+  liminix
+, lib
+}:
+{ watched, headStart } :
+let
+  inherit (liminix.services) longrun;
+in longrun {
+  name = "watchdog";
+  run =
+    "HEADSTART=${toString headStart} ${./gaspode.sh} ${lib.concatStringsSep " " (builtins.map (s: s.name) watched)}";
+}