Re: [PATCH v4 1/4] gitformat-commit-graph: describe version 2 of BDAT
From: Junio C Hamano <hidden>
Date: 2023-06-13 21:59:23
Jonathan Tan [off-list ref] writes:
quoted hunk
The code change to Git to support version 2 will be done in subsequent commits. Signed-off-by: Jonathan Tan <redacted> --- Documentation/gitformat-commit-graph.txt | 9 ++++++--- 1 file changed, 6 insertions(+), 3 deletions(-)diff --git a/Documentation/gitformat-commit-graph.txt b/Documentation/gitformat-commit-graph.txt index 31cad585e2..112e6d36a6 100644 --- a/Documentation/gitformat-commit-graph.txt +++ b/Documentation/gitformat-commit-graph.txt@@ -142,13 +142,16 @@ All multi-byte numbers are in network byte order. ==== Bloom Filter Data (ID: {'B', 'D', 'A', 'T'}) [Optional] * It starts with header consisting of three unsigned 32-bit integers: - - Version of the hash algorithm being used. We currently only support - value 1 which corresponds to the 32-bit version of the murmur3 hash + - Version of the hash algorithm being used. We currently support + value 2 which corresponds to the 32-bit version of the murmur3 hash implemented exactly as described in https://en.wikipedia.org/wiki/MurmurHash#Algorithm and the double hashing technique using seed values 0x293ae76f and 0x7e646e2 as described in https://doi.org/10.1007/978-3-540-30494-4_26 "Bloom Filters - in Probabilistic Verification" + in Probabilistic Verification". Version 1 bloom filters have a bug that appears
"bloom" -> "Bloom", probably, as the name comes from the name of its inventor (just like we spell "Boolean", not "boolean").
+ when char is signed and the repository has path names that have characters >= + 0x80; Git supports reading and writing them, but this ability will be removed + in a future version of Git.
Makes sense. I wonder if we want to mention what the undesired misbehaviour the "bug" causes and what we do to avoid getting affected by the bug here. If we can say something like "When querying for a pathname with a byte with high-bit set, the buggy filter may produce false negative, making the filter unusable, but asking for a pathname without such a byte produces no false negatives (even though we may get false positives). When Git reads version 1 filter data, it refrains from using it for processing paths with high-bit set to avoid triggering the bug", then it would be ideal. Or "When the repository has even a single pathname with high-bit set anywhere in its history, version 1 Bloom can give false negative when querying any paths and becomes unusable. You can use $THIS configuration variable to disable use of Bloom filter data in such a case" would also be fine. The point is to give actionable piece of information to the readers. Thanks.