--- a/contrib/perf.py	Fri Jun 22 11:02:42 2018 +0100
+++ b/contrib/perf.py	Sat Jul 14 02:09:47 2018 +0200
@@ -980,9 +980,10 @@
 def perffncachewrite(ui, repo, **opts):
     timer, fm = gettimer(ui, opts)
     s = repo.store
+    lock = repo.lock()
     s.fncache._load()
-    lock = repo.lock()
     tr = repo.transaction('perffncachewrite')
+    tr.addbackup('fncache')
     def d():
         s.fncache._dirty = True
         s.fncache.write(tr)