nickva commented on pull request #3488:
URL: https://github.com/apache/couchdb/pull/3488#issuecomment-812699068
It was interesting that we didn't hit this issue on CentOS 7 on 3.x so I was
curious and traced the `less` call when running that mango test:
```
10:9:04.429561 <0.11140.1> couch_ejson_compare:less_nif([{<<"ÿÿÿÿ">>}],
[{[{<<"forename">>,<<"Eddie">>}]}])
10:9:04.429783 <0.11140.1> couch_ejson_compare:less_nif/2 error badarg
10:9:04.429984 <0.11140.1> couch_ejson_compare:less_erl([{<<"ÿÿÿÿ">>}],
[{[{<<"forename">>,<<"Eddie">>}]}])
10:9:04.430254 <0.11140.1> couch_ejson_compare:less_list([{<<"ÿÿÿÿ">>}],
[{[{<<"forename">>,<<"Eddie">>}]}])
10:9:04.430506 <0.11140.1> couch_ejson_compare:less_erl({<<"ÿÿÿÿ">>},
{[{<<"forename">>,<<"Eddie">>}]})
10:9:04.430745 <0.11140.1> couch_ejson_compare:less_erl/2 --> 1
10:9:04.430903 <0.11140.1> couch_ejson_compare:less_list/2 --> 1
10:9:04.431088 <0.11140.1> couch_ejson_compare:less_erl/2 --> 1
```
Apparently in `3.x` we compare an object `{[{<<"forename">>,<<"Eddie">>}]}`
against a (highest?) marker, but that marker is not an object but a tuple with
one element `{<<"ÿÿÿÿ">>}`. `less_nif/2` then notices the object is not valid
ejson and throws a `badarg` error. The the code then falls back to Erlang
using `less_erl/2` and at least in this case return the correct result (highest
marker is greater than (`1`) than the object).
In `main` we compare against an actual object `{[{<<"ÿÿÿÿ">>, <<>>}]}` so
everything gets unpacked nicely then the old unicode library at least older
than 59 (and possible 53) didn't error out, but also didn't sort the
`<<255,255,255,255>>` as the highest element as intended. Here is centos 7 with
libicu-50.2-4.el7_7.x86_64, for example
```
couch_ejson_compare:less(<<"forename">>, <<255,255,255,255>>).
1
> couch_ejson_compare:less(<<"/">>, <<255,255,255,255>>).
-1
> couch_ejson_compare:less(<<"0">>, <<255,255,255,255>>).
1
```
and on mac os with icu4c 59:
```
> couch_ejson_compare:less(<<"forename">>, <<255,255,255,255>>).
-1
> couch_ejson_compare:less(<<"/">>, <<255,255,255,255>>).
-1
> couch_ejson_compare:less(<<"0">>, <<255,255,255,255>>).
-1
```
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