[processpool] handle BoJo case

Summary:
Infer would crash when started in a context where it already had
children it didn't know about.

Reviewed By: martintrojer

Differential Revision: D19177589

fbshipit-source-id: 0c8831597

infer/src/base/ProcessPool.ml

@@ -189,11 +189,15 @@ let killall pool ~slot status =
 
 
 let has_dead_child pool =
+  let open Option.Monad_infix in
   Unix.wait_nohang `Any
-  |> Option.map ~f:(fun (dead_pid, status) ->
-         ( Array.find_mapi_exn pool.slots ~f:(fun slot {pid} ->
+  >>= fun (dead_pid, status) ->
+  (* Some joker can [exec] an infer binary from a process that already has children. When some of
+     these pre-existing children die they'll get detected here but won't appear in our list of
+     workers. Just return [None] in that case. *)
+  Array.find_mapi pool.slots ~f:(fun slot {pid} ->
       if Pid.equal pid dead_pid then Some slot else None )
-         , status ) )
+  >>| fun slot -> (slot, status)
 
 
 let child_is_idle = function Idle -> true | _ -> false