Improve emitted change feed sequence after a split

 When we get sequence before the split, we'll fill in the missing (now split)
ranges with a special `{split, OldNodeUUid}` marker. However, when sequences
are emitted in the changes API, that will make the N- prefix (SeqSum) bounce
around from higher to lower numbers, while users expect those to be mostly
incrementing. So take a conservative approach and assume it will be full
rewind for that ramge, and use 0 for that range. This is a purely cosmetic
thing, when we decode the sequence that prefix gets thrown away anyway.

Fixes a part of issue: #4640

src/fabric/src/fabric_view_changes.erl

@@ -425,6 +425,15 @@ pack_seqs(Workers) ->
  Opaque = couch_util:encodeBase64Url(?term_to_bin(SeqList, [compressed])),
  ?l2b([integer_to_list(SeqSum), $-, Opaque]).
 
+% When we get sequence before the split, we'll fill in the missing (now split)
+% ranges with a special {split, OldNodeUUid} marker. However, when sequences
+% are emitted in the changes API, that will make the N- prefix (SeqSum) bounce
+% around from higher to lower numbers, while users expect those to be mostly
+% incrementing. So take a conservative approach and assume it will be full
+% rewind for that ramge, and use 0 for that range. This is a purely cosmetic
+% thing, when we decode the sequence that prefix gets thrown away anyway.
+%
+seq({Seq, {split, <<_/binary>>}, _Node}) when is_integer(Seq) -> 0;
 seq({Seq, _Uuid, _Node}) -> Seq;
 seq({Seq, _Uuid}) -> Seq;
 seq(Seq) -> Seq.
@@ -1065,4 +1074,18 @@ find_replacement_sequence_test() ->
  ?assertEqual(0, find_replacement_sequence(Dead4, [0, 10])),
  ?assertEqual(0, find_replacement_sequence(Dead4, [0, 5])).
 
+pack_split_seq_test() ->
+ Shards = [{"n1", 0, 10}, {"n2", 11, ?RING_END}],
+ Workers = mk_workers(Shards, {42, {split, <<"abc1234">>}, '[email protected]'}),
+ PackedSeq = pack_seqs(Workers),
+ ?assertMatch(<<"0-", _/binary>>, PackedSeq),
+ DecodedSeq = decode_seq(PackedSeq),
+ ?assertEqual(
+ [
+ {n1, [0, 10], {42, {split, <<"abc1234">>}, '[email protected]'}},
+ {n2, [11, 4294967295], {42, {split, <<"abc1234">>}, '[email protected]'}}
+ ],
+ DecodedSeq
+ ).
+
 -endif.