daemon.ml

(** daemon utilities *)

module U = ExtUnix.Specific

let log = Log.from "daemon"

let logfile = ref None
let pidfile = ref None
let runas = ref None
let foreground = ref false

let managed = ref false

(** global flag indicating that process should exit,
    [manage] will automatically set this flag on SIGTERM unless default signal handling is overriden
*)
let should_exit_ = ref false

(** [should_exit_lwt] usage is discouraged.
    Use [wait_exit] instead, which makes it harder to ignore "should exit" state and loop infinitely
*)
let (should_exit_lwt,signal_exit_lwt) = Lwt.wait ()
let should_exit () = !should_exit_
let should_run () = not !should_exit_

(** exception to be raised by functions that wish to signal premature termination due to [!should_exit = true] *)
exception ShouldExit

let signal_exit =
  let do_lwt = lazy (Lwt.wakeup_later signal_exit_lwt ()) in
  (* invariant: should_exit_ = (Lwt.state should_exit_lwt = Lwt.Return) *)
  fun () -> should_exit_ := true; Lazy.force do_lwt

(** @raise ShouldExit if [should_exit] condition is set, otherwise do nothing *)
let break () = if !should_exit_ then raise ShouldExit

(** wait until [should_exit] is set and raise [ShouldExit] *)
let wait_exit =
  (* NOTE
    Bind to should_exit_lwt only once, because every bind will create an immutable waiter on
    should_exit_lwt's sleeper, that is only removed after should_exit_lwt thread terminates.
  *)
  let thread = lazy (Lwt.bind should_exit_lwt (fun () -> raise ShouldExit)) in
  fun () -> Lazy.force thread

(** [break_lwt = Lwt.wrap break] *)
let break_lwt () = Lwt.wrap break

(** [unless_exit x] resolves promise [x] or raises [ShouldExit] *)
let unless_exit x = Lwt.pick [wait_exit (); x]

let get_args () =
  [
    ("-loglevel", Arg.String Log.set_loglevels, " ([<facil|prefix*>=]debug|info|warn|error[,])+");
    ExtArg.may_str "logfile" logfile "<file> Log file";
    ExtArg.may_str "pidfile" pidfile "<file> PID file";
    "-runas",
      Arg.String (fun name -> try runas := Some (Unix.getpwnam name) with exn -> Exn.fail ~exn "runas: unknown user %s" name),
      "<user> run as specified user";
    "-fg", Arg.Set foreground, " Stay in foreground";
  ]

let args = get_args ()

let install_signal_handlers () =
  let unix_stderr s =
    let s = Log.State.format_simple `Info log#facility s in
    try
      let (_:int) = Unix.write_substring Unix.stderr s 0 (String.length s) in ()
    with _ ->
      () (* do not fail, can be ENOSPC *)
  in
  Signal.set [Sys.sigpipe] ignore;
  Signal.set_verbose [Sys.sigusr1] "reopen log" (fun () -> Log.reopen !logfile);
  Signal.set_verbose [Sys.sigusr2] "memory reclaim and stats" begin fun () ->
    match Signal.is_safe_output () with
    | true -> Memory.log_stats (); Memory.reclaim ()
    | false ->
      (* output directly to fd to prevent deadlock, but breaks buffering *)
      Memory.get_stats () |> List.iter unix_stderr;
      Memory.reclaim_s () |> unix_stderr
  end;
  Signal.set_exit signal_exit

let manage () =
  match !managed with
  | true -> () (* be smart *)
  | false ->
(*
  this will fail if files don't exists :(
  (* fail before fork if something is wrong *)
  Option.may (fun path -> Unix.(access path [R_OK;W_OK])) !logfile;
  Option.may (fun path -> Unix.(access path [R_OK;W_OK])) !pidfile;
*)
  Option.may Nix.check_pidfile !pidfile; (* check pidfile before fork to fail early *)
  if not !foreground then Nix.daemonize ();
  begin match !runas with
  | None -> ()
  | Some pw ->
    let uid = pw.Unix.pw_uid and gid = pw.Unix.pw_gid in
    U.setreuid uid uid;
    U.setregid gid gid;
  end;
  Log.reopen !logfile; (* immediately after fork *)
  Log.read_env_config ();
  Option.may Nix.manage_pidfile !pidfile; (* write pidfile after fork! *)
  if Option.is_some !logfile then
  begin
    log #info "run: %s" Nix.cmdline;
    log #info "GC settings: %s" (Action.gc_settings ());
  end;
  install_signal_handlers ();
  Nix.raise_limits ();
  managed := true;
  ()