Add Time.with_timeout

lib_eio/eio.ml

@@ -171,6 +171,8 @@ module Domain_manager = struct
 end
 
 module Time = struct
+  exception Timeout
+
   class virtual clock = object
     method virtual now : float
     method virtual sleep_until : float -> unit
@@ -181,6 +183,14 @@ module Time = struct
   let sleep_until (t : #clock) time = t#sleep_until time
 
   let sleep t d = sleep_until t (now t +. d)
+
+  let with_timeout t d fn =
+    Fibre.first fn
+      (fun () -> sleep t d; Error `Timeout)
+
+  let with_timeout_exn t d fn =
+    Fibre.first fn
+      (fun () -> sleep t d; raise Timeout)
 end
 
 module Dir = struct

lib_eio/eio.mli

@@ -460,6 +460,15 @@ module Time : sig
 
   val sleep : #clock -> float -> unit
   (** [sleep t d] waits for [d] seconds. *)
+
+  val with_timeout : #clock -> float -> (unit -> ('a, 'e) result) -> ('a, [> `Timeout] as 'e) result
+  (** [with_timeout clock d fn] runs [fn ()] but cancels it after [d] seconds. *)
+
+  exception Timeout
+
+  val with_timeout_exn : #clock -> float -> (unit -> 'a) -> 'a
+  (** [with_timeout_exn clock d fn] runs [fn ()] but cancels it after [d] seconds,
+      raising exception [Timeout]. *)
 end
 
 module Dir : sig