diff -r f0cf560475a3 -r f030c7d22032 mercurial/upgrade_utils/engine.py
--- a/mercurial/upgrade_utils/engine.py	Tue Jul 13 14:54:09 2021 +0530
+++ b/mercurial/upgrade_utils/engine.py	Mon Jul 19 22:39:08 2021 +0200
@@ -200,6 +200,17 @@
         if not rl_type & store.FILEFLAGS_REVLOG_MAIN:
             continue
 
+        # the store.walk function will wrongly pickup transaction backup and
+        # get confused. As a quick fix for 5.9 release, we ignore those.
+        # (this is not a module constants because it seems better to keep the
+        # hack together)
+        skip_undo = (
+            b'undo.backup.00changelog.i',
+            b'undo.backup.00manifest.i',
+        )
+        if unencoded in skip_undo:
+            continue
+
         rl = _revlogfrompath(srcrepo, rl_type, unencoded)
 
         info = rl.storageinfo(