Only run codesign if there have been substitutions

This ensures that we're not running `codesign` in cases we don't
strictly need it. This in turn prevents a regression in macos+nix, where
the codesign binary is not in PATH.

Closes ocaml#6226

Signed-off-by: Etienne Millon <me@emillon.org>

Author: emillon

CHANGES.md

@@ -79,8 +79,8 @@
   Coq's standard library by including `(stdlib no)`.
   (#6165 #6164, fixes #6163, @ejgallego @Alizter @LasseBlaauwbroek)
 
-- on macOS, sign executables produced by artifact substitution (#6137, fixes
-  #5650, @emillon)
+- on macOS, sign executables produced by artifact substitution (#6137, #6231,
+  fixes #5650, fixes #6226, @emillon)
 
 - Added an (aliases ...) field to the (rules ...) stanza which allows the
   specification of multiple aliases per rule (#6194, @Alizter)

src/dune_rules/artifact_substitution.ml

@@ -568,10 +568,10 @@ let copy_file_non_atomic ~conf ?chmod ~src ~dst () =
       Fiber.return ())
     (fun () -> copy ~conf ~input_file:src ~input:(input ic) ~output:(output oc))
 
-let run_sign_hook conf file =
-  match conf.sign_hook with
-  | Some hook -> hook file
-  | None -> Fiber.return ()
+let run_sign_hook conf ~has_subst file =
+  match (conf.sign_hook, has_subst) with
+  | Some hook, true -> hook file
+  | _ -> Fiber.return ()
 
 (** This is just an optimisation: skip the renaming if the destination exists
     and has the right digest. The optimisation is useful to avoid unnecessary
@@ -611,10 +611,10 @@ let copy_file ~conf ?chmod ?(delete_dst_if_it_is_a_directory = false) ~src ~dst
   Fiber.finalize
     (fun () ->
       let open Fiber.O in
-      let* (_ : bool) =
+      let* has_subst =
         copy_file_non_atomic ~conf ?chmod ~src ~dst:temp_file ()
       in
-      let+ () = run_sign_hook conf temp_file in
+      let+ () = run_sign_hook conf ~has_subst temp_file in
       replace_if_different ~delete_dst_if_it_is_a_directory ~src:temp_file ~dst)
     ~finally:(fun () ->
       Path.unlink_no_err temp_file;