A MIDX pseudo-pack assigns one bit position to each selected OID. Its
preferred-pack fast paths handle duplicate OIDs across packs in the
MIDX, where the MIDX chooses one copy, but assume that each individual
pack contains one entry per OID.
Consider a preferred pack ordered [A, B, A, C]. The MIDX retains one
copy of A, leaving three pseudo-pack positions for four physical
entries. This creates two problems:
- In MIDX-backed single-pack reuse, bitmap_nr came from
pack->num_objects and therefore counted both copies of A. Read the
selected count from the root MIDX layer BTMP chunk instead. If that
layer has no BTMP chunk, skip pack reuse and let the ordinary packing
path handle the bitmap-selected objects. MIDXs without BTMP lose
preferred-pack reuse until rewritten, rather than requiring a scan of
the entire pseudo-pack to retain an optional optimization.
- Correcting the range length still does not align the two orderings.
Once one copy of A is omitted, MIDX position N need not name physical
pack entry N.
That mismatch matters during both selection and output. Whole-word
selection bypasses the per-object check that the exact physical base of
a delta is present. Verbatim reuse copies the first N pack entries for
the first N bits. The per-object writer likewise used each bit as a
physical pack position.
Use the direct mapping only when the range begins at zero and its
selected count equals the physical entry count. Since selected entries
remain in pack-offset order, equal counts mean that none was omitted and
the two positions coincide.
For every other MIDX range, let whole-word selection fall through to
the existing per-bit path, which resolves the selected MIDX offset to a
physical pack position before checking the delta base. Disable verbatim
prefix reuse, and perform the same translation in the per-object writer.
Leave classic single-pack bitmap handling unchanged. The production
writer creates those bitmaps only for the pack it just wrote, which has
one entry per OID. The test helper can target an existing pack, but
rejects duplicate OIDs.
Cover the A, B, A, C case in a MIDX-backed preferred pack. Check reuse
with BTMP, then hide BTMP and check that the optional reuse path is
skipped. Also make B a delta against the omitted copy of A and require
normal packing to handle it.
Signed-off-by: Taylor Blau <ttaylorr@openai.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
fd2739b159