val shutdown : ?force:unit Import.Deferred.t -> int -> unit
shutdown ?force status initiates shutdown, which runs all the at_shutdown functions, waits for them to finish, and then exits with the supplied status. The at_shutdown functions can block -- one can use ~force to forcibly exit (with status 1) if the at_shutdown functions do not finish in a reasonable amount of time.

By default, force is after (sec 10.).

Repeated calls to shutdown with the same status will have no effect. Any call to shutdown with nonzero status will cause that to be the status that is exited with. A call to shutdown with different nonzero status from the original call will raise.

val exit : ?force:unit Import.Deferred.t -> int -> 'a Import.Deferred.t
exit ?force status is shutdown ?force status; Deferred.never ().

We do not have an exit function that returns a non-deferred:

      val exit : ?force:unit Deferred.t -> int -> _

Such a function should not exist, for the same reason that we do not have:

      val block : 'a Deferred.t -> 'a

The semantics of such an exit function would allow one to block a running Async job, and to switch to another one (to run the at_shutdown handlers), without expressing that switch in the type system via a Deferred.t. That would eliminate all the nice reasoning guarantees that Async gives about concurrent jobs.

val shutting_down : unit -> [ `No | `Yes of int ]
shutting_down () reports whether we are currently shutting down, and if so, with what status.
val at_shutdown : (unit -> unit Import.Deferred.t) -> unit
at_shutdown f causes f () to be run when shutdown is called, and for shutdown to wait until the returned deferred finishes.

If shutdown has already been called, then calling at_shutdown f does nothing.

val don't_finish_before : unit Import.Deferred.t -> unit
don't_finish_before d causes shutdown to wait until d becomes determined before finishing. It is like at_shutdown (fun _ -> d), except it is more efficient, and will not take any space once d is determined. There is a a single at_shutdown shared among all deferreds supplied to don't_finish_before.