--- a/hgext/evolve.py	Fri Jun 26 00:59:22 2015 -0700
+++ b/hgext/evolve.py	Mon Jun 29 02:30:27 2015 -0700
@@ -3336,7 +3336,7 @@
     if new_format == repo.obsstore._version:
         msg = _('New format is the same as the old format, not upgrading!')
         raise util.Abort(msg)
-    f = repo.sopener('obsstore', 'wb', atomictemp=True)
+    f = repo.svfs('obsstore', 'wb', atomictemp=True)
     origmarkers = repo.obsstore._all
     known = set()
     markers = []