Source file lwt_utils.ml

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open Lwt.Syntax

exception None_successful of string

let never_ending () = fst (Lwt.wait ())

(* A worker launcher, takes a cancel callback to call upon *)
let worker name ~on_event ~run ~cancel =
  let stop, stopper = Lwt.wait () in
  let fail e =
    Lwt.finalize
      (fun () ->
        on_event
          name
          (`Failed (Printf.sprintf "Exception: %s" (Printexc.to_string e))))
      cancel
  in
  let* () = on_event name `Started in
  let p = Lwt.catch run fail in
  Lwt.on_termination p (Lwt.wakeup stopper) ;
  let* () = stop in
  Lwt.catch (fun () -> on_event name `Ended) (fun _ -> Lwt.return_unit)

let worker name ~on_event ~run ~cancel =
  Lwt.no_cancel (worker name ~on_event ~run ~cancel)

let pick_successful = function
  | [] ->
      raise
        (Invalid_argument
           "Lwt_utils.pick_successful [] would return a promise that is \
            pending forever")
  | promises ->
      let promise, resolver = Lwt.task () in
      let pending_count = ref (List.length promises) in
      let on_success value =
        try Lwt.wakeup_later resolver value
        with Stdlib.Invalid_argument _ ->
          (* If the promise is already resolved, the raised
               exception [Stdlib.Invalid_argument _] is caught
               and ignored. *)
          ()
      in
      let on_failure _exn =
        decr pending_count ;
        if !pending_count = 0 then
          Lwt.wakeup_later_exn
            resolver
            (None_successful
               "All pending tasks were rejected, canceled or did not pass \n\
                the success criteria.")
        else ()
      in
      List.iter
        (fun promise ->
          Lwt.on_success promise on_success ;
          Lwt.on_failure promise on_failure)
        promises ;
      let cancel_all promises = List.iter Lwt.cancel promises in
      Lwt.on_success promise (fun _ -> cancel_all promises) ;
      Lwt.on_cancel promise (fun () -> cancel_all promises) ;
      promise