apcupsd-service: add services.apcupsd.hooks option

Each attribute in this option should name an apcupsd event and the
string value it contains will be executed in a shell in response to that
event. See "man apccontrol" for the list of events and what they
represent.

Now it is easy to hook into the apcupsd event system:

  services.apcupsd.hooks = {
    onbattery  = ''# shell commands to run when the onbattery event is emitted'';
    doshutdown = ''# shell commands to notify that the computer is shutting down'';
  };

modules/services/monitoring/apcupsd.nix

@@ -2,12 +2,66 @@
 
 with pkgs.lib;
 
-let cfg = config.services.apcupsd;
- configFile = pkgs.writeText "apcupsd.conf" ''
- ## apcupsd.conf v1.1 ##
- # apcupsd complains if the first line is not like above.
- ${cfg.configText}
- '';
+let
+ cfg = config.services.apcupsd;
+
+ configFile = pkgs.writeText "apcupsd.conf" ''
+ ## apcupsd.conf v1.1 ##
+ # apcupsd complains if the first line is not like above.
+ ${cfg.configText}
+ SCRIPTDIR ${toString scriptDir}
+ '';
+
+ # List of events from "man apccontrol"
+ eventList = [
+ "annoyme"
+ "battattach"
+ "battdetach"
+ "changeme"
+ "commfailure"
+ "commok"
+ "doreboot"
+ "doshutdown"
+ "emergency"
+ "failing"
+ "killpower"
+ "loadlimit"
+ "mainsback"
+ "onbattery"
+ "offbattery"
+ "powerout"
+ "remotedown"
+ "runlimit"
+ "timeout"
+ "startselftest"
+ "endselftest"
+ ];
+
+ shellCmdsForEventScript = eventname: commands: ''
+ echo "#!${pkgs.stdenv.shell}" > "$out/${eventname}"
+ echo "${commands}" >> "$out/${eventname}"
+ chmod a+x "$out/${eventname}"
+ '';
+
+ eventToShellCmds = event: if builtins.hasAttr event cfg.hooks then (shellCmdsForEventScript event (builtins.getAttr event cfg.hooks)) else "";
+
+ scriptDir = pkgs.runCommand "apcupsd-scriptdir" {} (''
+ mkdir "$out"
+ # Copy SCRIPTDIR from apcupsd package
+ cp -r ${pkgs.apcupsd}/etc/apcupsd/* "$out"/
+ # Make the files writeable (nix will unset the write bits afterwards)
+ chmod u+w "$out"/*
+ # Remove the sample event notification scripts, because they don't work
+ # anyways (they try to send mail to "root" with the "mail" command)
+ (cd "$out" && rm changeme commok commfailure onbattery offbattery)
+ # Remove the sample apcupsd.conf file (we're generating our own)
+ rm "$out/apcupsd.conf"
+ # Set the SCRIPTDIR= line in apccontrol to the dir we're creating now
+ sed -i -e "s|^SCRIPTDIR=.*|SCRIPTDIR=$out|" "$out/apccontrol"
+ '' + concatStringsSep "\n" (map eventToShellCmds eventList)
+
+ );
+
 in
 
 {
@@ -47,6 +101,24 @@ in
  '';
  };
 
+ hooks = mkOption {
+ default = {};
+ example = {
+ doshutdown = ''# shell commands to notify that the computer is shutting down'';
+ };
+ type = types.attrsOf types.string;
+ description = ''
+ Each attribute in this option names an apcupsd event and the string
+ value it contains will be executed in a shell, in response to that
+ event (prior to the default action). See "man apccontrol" for the
+ list of events and what they represent.
+
+ A hook script can stop apccontrol from doing its default action by
+ exiting with value 99. Do not do this unless you know what you're
+ doing.
+ '';
+ };
+
  };
 
  };
@@ -56,6 +128,15 @@ in
 
  config = mkIf cfg.enable {
 
+ assertions = [ {
+ assertion = let hooknames = builtins.attrNames cfg.hooks; in all (x: elem x eventList) hooknames;
+ message = ''
+ One (or more) attribute names in services.apcupsd.hooks are invalid.
+ Current attribute names: ${toString (builtins.attrNames cfg.hooks)}
+ Valid attribute names : ${toString eventList}
+ '';
+ } ];
+
  # Give users access to the "apcaccess" tool
  environment.systemPackages = [ pkgs.apcupsd ];
 
@@ -66,10 +147,6 @@ in
  # not connected to a tty (it is connected to the journal):
  # wall: cannot get tty name: Inappropriate ioctl for device
  # The message still gets through.
- #
- # TODO: apcupsd calls "mail" on powerfail etc. events, how should we
- # handle that? A configurable mail package or let the event logic be
- # configured from nix expressions?
  systemd.services.apcupsd = {
  description = "UPS daemon";
  wantedBy = [ "multi-user.target" ];