--- a/mercurial/revlog.py	Wed Sep 26 21:41:52 2018 +0900
+++ b/mercurial/revlog.py	Sun Jan 13 14:56:26 2019 +0900
@@ -440,6 +440,8 @@
             with self._indexfp() as f:
                 if (mmapindexthreshold is not None and
                     self.opener.fstat(f).st_size >= mmapindexthreshold):
+                    # TODO: should .close() to release resources without
+                    # relying on Python GC
                     indexdata = util.buffer(util.mmapread(f))
                 else:
                     indexdata = f.read()