fix(Lwt_utils.mkdir_p): Surround the Lwt_unix.mkdir call with a Lwt.catch guard

Aim: mitigate a TOC/TOU race condition which occurs when
Learnocaml_process_exercise_repository.n_processes > 1,
triggered by a command such as "learn-ocaml build -j 2".

This isn't the only possible fix/workaround, just a concise one
relying on error handling.

Author: erikmd

src/utils/lwt_utils.ml

@@ -18,7 +18,14 @@ let rec mkdir_p ?(perm=0o755) dir =
           (Printf.sprintf "Can't create dir: file %s is in the way" dir)
   | false ->
       mkdir_p (Filename.dirname dir) >>= fun () ->
-      Lwt_unix.mkdir dir perm
+      (* This exn handler mitigates a TOC/TOU race condition which
+         occurs when [mkdir_p] is called from parallel build jobs
+         (namely, for some "learn-ocaml build -j 2" command) *)
+      Lwt.catch (fun () -> Lwt_unix.mkdir dir perm)
+        (function
+         | Unix.Unix_error(Unix.EEXIST, "mkdir", _path)
+              when Sys.is_directory dir -> Lwt.return ()
+         | e -> Lwt.fail e)
 
 let copy_file src dst =
   Lwt.catch (fun () ->