diff -r 33532939c667 -r 36a0a1951d64 mercurial/revlogutils/flagutil.py
--- a/mercurial/revlogutils/flagutil.py	Tue Sep 03 23:51:34 2019 +0200
+++ b/mercurial/revlogutils/flagutil.py	Tue Sep 03 23:51:17 2019 +0200
@@ -120,7 +120,7 @@
         """
         return self._processflagsfunc(text, flags, 'read')
 
-    def _processflagswrite(self, text, flags):
+    def _processflagswrite(self, text, flags, sidedata):
         """Inspect revision data flags and applies write transformations defined
         by registered flag processors.
 
@@ -136,6 +136,7 @@
         processed text and ``validatehash`` is a bool indicating whether the
         returned text should be checked for hash integrity.
         """
+        assert not sidedata # XXX until it is actually processed
         return self._processflagsfunc(text, flags, 'write')[:2]
 
     def _processflagsraw(self, text, flags):