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 a full
rewind for that range, and use 0 instead. This is a purely cosmetic thing, when
we decode the sequence that prefix gets thrown away anyway.

To emphasize that the sequence prefix is mostly a visual aid, rename the seq/1
function to fake_packed_seq/1 with a comment about what it does and a note
about the special split case.

Fixes a part of issue: #4640

src/fabric/src/fabric_view_changes.erl

@@ -421,13 +421,28 @@ pending_count(Dict) ->
 
 pack_seqs(Workers) ->
  SeqList = [{N, R, S} || {#shard{node = N, range = R}, S} <- Workers],
- SeqSum = lists:sum([seq(S) || {_, _, S} <- SeqList]),
+ SeqSum = lists:sum([fake_packed_seq(S) || {_, _, S} <- SeqList]),
  Opaque = couch_util:encodeBase64Url(?term_to_bin(SeqList, [compressed])),
  ?l2b([integer_to_list(SeqSum), $-, Opaque]).
 
-seq({Seq, _Uuid, _Node}) -> Seq;
-seq({Seq, _Uuid}) -> Seq;
-seq(Seq) -> Seq.
+% Generate the sequence number used to build the emitted N-... prefix.
+%
+% The prefix is discarded when the sequence is decoded in the current CouchDB
+% version. It's mostly there as a visual heuristic about all the packed
+% sequences and to keep backwards compatibility with previous CouchDB versions.
+%
+% We handle of few backwards compatibility clauses, and also the when we get a
+% sequence before a shard split. In that case we fill in the missing (now
+% split) ranges with a special {split, OldNodeUUid} marker. However, when
+% sequences are emitted, that will make the N- prefix (SeqSum) bounce around
+% from higher to lower numbers if we leave it as is. To fix that, use 0 for
+% that range, assuming it will be full rewind. That way the N-... prefix is
+% more likely to be incrementing, which is what users would generally expect.
+%
+fake_packed_seq({Seq, {split, <<_/binary>>}, _Node}) when is_integer(Seq) -> 0;
+fake_packed_seq({Seq, _Uuid, _Node}) -> Seq;
+fake_packed_seq({Seq, _Uuid}) -> Seq;
+fake_packed_seq(Seq) -> Seq.
 
 unpack_seq_regex_match(Packed) ->
  Pattern = "^\"?([0-9]+-)?(?<opaque>.*?)\"?$",
@@ -1065,4 +1080,18 @@ 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.