diff -r 349482d726ee -r 399ddd3227a4 mercurial/exchangev2.py
--- a/mercurial/exchangev2.py	Wed Aug 29 17:03:19 2018 -0700
+++ b/mercurial/exchangev2.py	Wed Aug 22 14:51:11 2018 -0700
@@ -147,6 +147,9 @@
             for mark in cset.get(b'bookmarks', []):
                 remotebookmarks[mark] = node
 
+            # TODO add mechanism for extensions to examine records so they
+            # can siphon off custom data fields.
+
             # Some entries might only be metadata only updates.
             if b'revisionsize' not in cset:
                 continue