nickva commented on code in PR #4828:
URL: https://github.com/apache/couchdb/pull/4828#discussion_r1380652572


##########
src/mango/src/mango_idx_view.erl:
##########
@@ -417,7 +417,7 @@ range(_, _, LCmp, Low, HCmp, High) ->
 % beginsWith requires both a high and low bound
 range({[{<<"$beginsWith">>, Arg}]}, LCmp, Low, HCmp, High) ->
     {LCmp0, Low0, HCmp0, High0} = range({[{<<"$gte">>, Arg}]}, LCmp, Low, 
HCmp, High),
-    range({[{<<"$lte">>, <<Arg/binary, 16#10FFFF>>}]}, LCmp0, Low0, HCmp0, 
High0);
+    range({[{<<"$lte">>, <<Arg/binary, 16#10FFFF/utf8>>}]}, LCmp0, Low0, 
HCmp0, High0);

Review Comment:
   > "Erlang itself supports U+10FFFF and it is numerically greater than 
U+FFFF" -- while I accept that U+FFFF might be the Unicode-official marker, 
Erlang's support for a higher range worries me a bit.
   
   Perhaps confusingly unicode collation rules don't sort by code point values. 
Higher code-points may sort lower. Besides just `U+10FFFF` and `U+FFFF` there 
are other such example:
   
   ```erlang
   mango_json:cmp(<<16#1D175/utf8>>, <<16#41/utf8>>).
   -1
   ```
   
   Now, there may not be any other characters sorting higher than U+10FFFF and 
lower than U+FFFF, so either one is probably ok in practice, but it's a bit 
neater if we can use the canonical max sorting character. Jiffy / erlang / 
couch_util:validate_utf8/1 should pass through both of those characters, so 
technically users could insert keys with those crazy values in them:
   
   ```erlang
   <<16#FFFF/utf8>> = jiffy:decode(<<"\"\\uFFFF\"">>).
   ```



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