diff -r d1c1fd7ac46d -r 9ab54aa56982 mercurial/vfs.py
--- a/mercurial/vfs.py	Wed Jun 09 01:10:34 2021 +0200
+++ b/mercurial/vfs.py	Tue Jun 08 02:05:05 2021 +0200
@@ -329,6 +329,9 @@
                     None  # pytype: disable=attribute-error
                 )
 
+    def register_file(self, path):
+        """generic hook point to lets fncache steer its stew"""
+
 
 class vfs(abstractvfs):
     """Operate files relative to a base directory