[project] fix a rare but critical bug of Project.on leading to an incorrect...

[project] fix a rare but critical bug of Project.on leading to an incorrect restoration of the former project

src/libraries/project/project.ml

@@ -379,12 +379,12 @@ let on ?selection p f x =
   if old_current == p then f x
   else
     let set p = set_current ~on:true ?selection p in
-    let set_to_old () =
+    let rec set_to_old old =
       if not !keep_current then
         try
           (* if someone asks for keeping [p] as current during the execution of
              [f], do not restore [old_current] at the end. *)
-          set old_current
+          set old
         with Invalid_argument _ ->
           (* the old current project has been remove: replace it by the previous
              one, if any *)
@@ -393,16 +393,16 @@ let on ?selection p f x =
               old_current.unique_name
               (current ()).unique_name
           else
-            Q.move_at_top (Q.nth 1 projects) projects
+            set_to_old (Q.nth 1 projects)
     in
     let go () =
       set p;
       let r = f x in
-      set_to_old ();
+      set_to_old old_current;
       r
     in
     if debug_atleast 1 then go ()
-    else begin try go () with e -> set_to_old (); raise e end
+    else begin try go () with e -> set_to_old old_current; raise e end
 
 (* [set_current] must never be called internally. *)
 module Hide_set_current = struct let set_current () = assert false end