diff --git a/mercurial/store.py b/mercurial/store.py
--- a/mercurial/store.py
+++ b/mercurial/store.py
@@ -730,6 +730,11 @@
         else:
             return self.vfs.join(path)
 
+    def register_file(self, path):
+        """generic hook point to lets fncache steer its stew"""
+        if path.startswith(b'data/') or path.startswith(b'meta/'):
+            self.fncache.add(path)
+
 
 class fncachestore(basicstore):
     def __init__(self, path, vfstype, dotencode):
diff --git a/mercurial/vfs.py b/mercurial/vfs.py
--- a/mercurial/vfs.py
+++ b/mercurial/vfs.py
@@ -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