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New next/kernel/backfill.erl owns the §13.3 backfill mode
slicing. Given an outbox log + a mode, returns the activity
list to send to a new follower as backfill.
Public API:
slice/2(Mode, LogState) default Wrap=false
slice/3(Mode, LogState, Wrap) Wrap=true wraps entries
wrap_backfill/1 add {backfilled, true}
parse_mode/1 lift Follow :backfill field
Modes:
none new follower: forward-only content
full entire outbox
{last_n, N} last N activities (FIFO)
{last_t, T, NowFn} entries with :published in
(NowFn()-T .. NowFn()]
{since_cid, Cid} entries after the one with :id = Cid
(consumes the matched entry; returns
every entry after it)
wrap_backfill/1 marks each entry {backfilled, true}. Per §13.3
wrapped bodies preserve :id so the receiver's replay defence
still catches duplicates from the live stream.
parse_mode/1 accepts:
nil / none / full / {last_n, _} / {last_t, _, _} /
{since_cid, _} — pass through or normalize
Proplist with :mode + :limit -> {last_n, N}
Proplist with :mode + :duration -> {last_t, T, fun() -> 0 end}
Proplist with :mode = full -> full
Anything else -> none (open-world default)
Substrate gotchas re-confirmed and worked around:
- lists:nthtail/2 not registered — rolled drop_n/2
- Pattern-alias 'Pat = Var' not supported by this port's
parser — parse_mode/1 clauses use explicit deconstruction
20/20 in next/tests/backfill.sh covering all five modes plus
edge cases (N=0, N>length, T=0 -> empty window, since_cid
hit/miss/unknown), wrap_backfill semantics, parse_mode for
atoms / tuple shapes / proplists / unknown / nil.
Step 9b (outbox listing ?since=Cid&limit=N pagination) and
Step 9c (Follow-Accept-backfill wiring) layer on top.
Conformance preserved at 761/761.