The store's flag field is a substrate, and this is its read half: a v4
index entry carries a 16-bit class mask, and a scan by class is a walk that
reads the mask and tests it. Nothing about the merge changes — the same log
overlay, the same order — so a caller pays for its own class rather than for
the store.
The op takes a space, a mask, a mode (any/all), a cursor and a buffer, in
its own size-versioned block. A mask of zero matches nothing, because naming
no class is asking no question. Records come back as
key | flags(2) | value_len | value: the mask travels, since a record can
carry bits the scan did not name and no other op returns a mask.
Both layouts the mask can be in are read. With the record still in the log
it comes from the v2 log entry; after a fold it comes from the v4 index
entry. The non-indexed path is not a corner — it is the state of every store
between the write that classified something and the fold, so answering it
with 'nothing' would make the substrate work only after a checkpoint.
Also settles what an append writes into which log, since the entry's frame
has to match the header a reader frames it by: a log that already holds v1
entries keeps taking v1 entries (a device folds first — that is the v4
migration — and a bare image keeps the log it has), an empty log is framed
v2 on a device and left alone on a bare image, and a log with no header is
framed v2 on a device and v1 on a bare image. The bare layout is the legacy
one: it has no index for a mask to be folded into, so a mask written there
could only be scanned and never checkpointed — half a feature, bought by
making every existing reader of that layout grow a version it cannot use.
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