diff -r 07e479ef7c96 mercurial/obsolete.py
--- a/mercurial/obsolete.py	Thu Apr 25 19:17:02 2019 +0200
+++ b/mercurial/obsolete.py	Thu Jul 04 05:19:36 2019 +0000
@@ -635,6 +635,8 @@
         if new:
             f = self.svfs('obsstore', 'ab')
             try:
+                import os
+                f.seek(0, os.SEEK_END)
                 offset = f.tell()
                 transaction.add('obsstore', offset)
                 # offset == 0: new file - add the version header