Do not reuse fiber scheduler between tests

Signed-off-by: Rudi Grinberg <me@rgrinberg.com>

Author: rgrinberg

fiber-test/fiber_test.ml

@@ -7,24 +7,33 @@ let print pp = Format.printf "%a@." Pp.render_ignore_tags pp
 let print_dyn dyn = print (Dyn.pp dyn)
 
 module Scheduler : sig
+  type t
+
   exception Never
 
   val yield : unit -> unit Fiber.t
 
-  val run : 'a Fiber.t -> 'a
+  val create : unit -> t
+
+  val run : t -> 'a Fiber.t -> 'a
 end = struct
-  let suspended = Queue.create ()
+  type t = unit Fiber.Ivar.t Queue.t
+
+  let t_var = Fiber.Var.create ()
+
+  let create () = Queue.create ()
 
   let yield () =
     let ivar = Fiber.Ivar.create () in
-    Queue.push suspended ivar;
+    let t = Fiber.Var.get_exn t_var in
+    Queue.push t ivar;
     Fiber.Ivar.read ivar
 
   exception Never
 
-  let run t =
-    Fiber.run t ~iter:(fun () ->
-        match Queue.pop suspended with
+  let run t fiber =
+    Fiber.run fiber ~iter:(fun () ->
+        match Queue.pop t with
         | None -> raise Never
         | Some e -> Fiber.Fill (e, ()))
 end
@@ -37,7 +46,7 @@ let test ?(expect_never = false) to_dyn f =
     in
     Fiber.with_error_handler (fun () -> f) ~on_error
   in
-  ( try Scheduler.run f |> to_dyn |> print_dyn
+  ( try Scheduler.run (Scheduler.create ()) f |> to_dyn |> print_dyn
     with Scheduler.Never -> never_raised := true );
   match (!never_raised, expect_never) with
   | false, false ->