--- a/hgext/mozext/__init__.py
+++ b/hgext/mozext/__init__.py
@@ -1536,17 +1536,26 @@ def reposetup(ui, repo):
 
         def prune_relbranch_refs(self):
             todelete = [bm for bm in self._bookmarks.keys()
                         if bm.endswith('RELBRANCH')]
             for bm in todelete:
                 ui.warn('Removing bookmark %s\n' % bm)
                 del self._bookmarks[bm]
 
-            self._bookmarks.write()
+            lock = self.lock()
+            try:
+                tr = repo.transaction('prunerelbranch')
+                try:
+                    self._bookmarks.recordchange(tr)
+                    tr.close()
+                finally:
+                    tr.release()
+            finally:
+                lock.release()
 
             todelete = [ref for ref in self.remoterefs.keys()
                         if ref.endswith('RELBRANCH')]
             for ref in todelete:
                 del self.remoterefs[ref]
 
             self.remoterefs.write()