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authorTaylor Blau <me@ttaylorr.com>2021-09-14 22:06:04 (GMT)
committerJunio C Hamano <gitster@pobox.com>2021-09-14 23:34:17 (GMT)
commit8de300e1f7ebe7099ce6ce60f6c6fb494e6703b2 (patch)
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pack-bitmap.c: propagate namehash values from existing bitmaps
When an old bitmap exists while writing a new one, we load it and build a "reposition" table which maps bit positions of objects from the old bitmap to their respective positions in the new bitmap. This can help when we encounter a commit which was selected in both the old and new bitmap, since we only need to permute its bit (not recompute it from scratch). We do not, however, repurpose existing namehash values in the case of the hash-cache extension. There has been thus far no good reason to do so, since all of the namehash values for objects in the new bitmap would be populated during the traversal that was just performed by pack-objects when generating single-pack reachability bitmaps. But this isn't the case for multi-pack bitmaps, which are written via `git multi-pack-index write --bitmap` and do not perform any traversal. In this case all namehash values are set to zero, but we don't even bother to check the `pack.writeBitmapHashcache` option anyway, so it fails to matter. There are two approaches we could take to fill in non-zero hash-cache values: - have either the multi-pack-index builtin run its own traversal to attempt to fill in some values, or let a hypothetical caller (like `pack-objects` when `repack` eventually drives the `multi-pack-index` builtin) fill in the values they found during their traversal - or copy any existing namehash values that were stored in an existing bitmap to their corresponding positions in the new bitmap In a system where a repository is generally repacked with `git repack --geometric=<d>` and occasionally repacked with `git repack -a`, the hash-cache coverage will tend towards all objects. Since populating the hash-cache is additive (i.e., doing so only helps our delta search), any intermediate lack of full coverage is just fine. So let's start by just propagating any values from the existing hash-cache if we see one. The next patch will respect the `pack.writeBitmapHashcache` option while writing MIDX bitmaps, and then test this new behavior. Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
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