[GitHub] [couchdb] janl commented on a diff in pull request #4394: Mango fields pushdown

[GitBox <gi...@apache.org>]

janl commented on code in PR #4394:
URL: https://github.com/apache/couchdb/pull/4394#discussion_r1082422633


##########
src/mango/src/mango_cursor_view.erl:
##########
@@ -248,6 +266,19 @@ view_cb({row, Row}, #mrargs{extra = Options} = Acc) ->
         key = couch_util:get_value(key, Row),
         doc = couch_util:get_value(doc, Row)
     },
+    % This supports receiving our "arguments" either as just the `selector`
+    % or in the new record in `callback_args`. This is to support mid-upgrade
+    % clusters where the non-upgraded coordinator nodes will send the older style.
+    % TODO remove this in a couple of couchdb versions.

Review Comment:
   might be worth codifying this a little. I know we have other places, but maybe e can start here:
   
   maybe we can make a comment like `% x-couch-remove: 4.0.0` that we can then grep for prior to releasing 4.0.0.
   
   The reasoning here that we can say: upgrade everything to the latest 3.x version and THEN go to 4.0.0 will be smooth, rather than supporting any 3.x-> 4.0.0



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