fix: Fork_join.for_ 0 must not hang

src/fork_join.ml

@@ -92,6 +92,7 @@ let both f g : _ * _ =
 let both_ignore f g = ignore (both f g : _ * _)
 
 let for_ ?chunk_size n (f : int -> int -> unit) : unit =
+  if n > 0 then (
     let has_failed = A.make false in
     let missing = A.make n in
 
@@ -136,8 +137,8 @@ let for_ ?chunk_size n (f : int -> int -> unit) : unit =
           (fun ~run suspension ->
             (* run tasks, then we'll resume [suspension] *)
             start_tasks ~run suspension);
-    };
-  ()
+      }
+  )
 
 let all_array ?chunk_size (fs : _ array) : _ array =
   let len = Array.length fs in

test/effect-based/t_fork_join.ml

@@ -42,6 +42,11 @@ let () =
   let exp_sum = List.init 42 (fun x -> x * x) |> List.fold_left ( + ) 0 in
   assert (par_sum = exp_sum)
 
+let () =
+  Pool.run_wait_block pool (fun () ->
+      Fork_join.for_ 0 (fun _ _ -> assert false));
+  ()
+
 let () =
   let total_sum = Atomic.make 0 in