From f4609b896fac842433bd495c166d5987852a6a73 Mon Sep 17 00:00:00 2001 From: Vincent Ambo Date: Sat, 21 Nov 2020 19:20:35 +0100 Subject: merge(3p/git): Merge git subtree at v2.29.2 This also bumps the stable nixpkgs to 20.09 as of 2020-11-21, because there is some breakage in the git build related to the netrc credentials helper which someone has taken care of in nixpkgs. The stable channel is not used for anything other than git, so this should be fine. Change-Id: I3575a19dab09e1e9556cf8231d717de9890484fb --- .../technical/commit-graph-format.txt | 43 ++++++++++++++++++++-- 1 file changed, 39 insertions(+), 4 deletions(-) (limited to 'third_party/git/Documentation/technical/commit-graph-format.txt') diff --git a/third_party/git/Documentation/technical/commit-graph-format.txt b/third_party/git/Documentation/technical/commit-graph-format.txt index a4f17441aed3..b3b58880b926 100644 --- a/third_party/git/Documentation/technical/commit-graph-format.txt +++ b/third_party/git/Documentation/technical/commit-graph-format.txt @@ -17,6 +17,9 @@ metadata, including: - The parents of the commit, stored using positional references within the graph file. +- The Bloom filter of the commit carrying the paths that were changed between + the commit and its first parent, if requested. + These positional references are stored as unsigned 32-bit integers corresponding to the array position within the list of commit OIDs. Due to some special constants we use to track parents, we can store at most @@ -29,7 +32,7 @@ the body into "chunks" and provide a binary lookup table at the beginning of the body. The header includes certain values, such as number of chunks and hash type. -All 4-byte numbers are in network order. +All multi-byte numbers are in network byte order. HEADER: @@ -39,8 +42,13 @@ HEADER: 1-byte version number: Currently, the only valid version is 1. - 1-byte Hash Version (1 = SHA-1) - We infer the hash length (H) from this value. + 1-byte Hash Version + We infer the hash length (H) from this value: + 1 => SHA-1 + 2 => SHA-256 + If the hash type does not match the repository's hash algorithm, the + commit-graph file should be ignored with a warning presented to the + user. 1-byte number (C) of "chunks" @@ -74,7 +82,7 @@ CHUNK DATA: Commit Data (ID: {'C', 'D', 'A', 'T' }) (N * (H + 16) bytes) * The first H bytes are for the OID of the root tree. * The next 8 bytes are for the positions of the first two parents - of the ith commit. Stores value 0x7000000 if no parent in that + of the ith commit. Stores value 0x70000000 if no parent in that position. If there are more than two parents, the second value has its most-significant bit on and the other bits store an array position into the Extra Edge List chunk. @@ -93,6 +101,33 @@ CHUNK DATA: positions for the parents until reaching a value with the most-significant bit on. The other bits correspond to the position of the last parent. + Bloom Filter Index (ID: {'B', 'I', 'D', 'X'}) (N * 4 bytes) [Optional] + * The ith entry, BIDX[i], stores the number of bytes in all Bloom filters + from commit 0 to commit i (inclusive) in lexicographic order. The Bloom + filter for the i-th commit spans from BIDX[i-1] to BIDX[i] (plus header + length), where BIDX[-1] is 0. + * The BIDX chunk is ignored if the BDAT chunk is not present. + + 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 + 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" + - The number of times a path is hashed and hence the number of bit positions + that cumulatively determine whether a file is present in the commit. + - The minimum number of bits 'b' per entry in the Bloom filter. If the filter + contains 'n' entries, then the filter size is the minimum number of 64-bit + words that contain n*b bits. + * The rest of the chunk is the concatenation of all the computed Bloom + filters for the commits in lexicographic order. + * Note: Commits with no changes or more than 512 changes have Bloom filters + of length one, with either all bits set to zero or one respectively. + * The BDAT chunk is present if and only if BIDX is present. + Base Graphs List (ID: {'B', 'A', 'S', 'E'}) [Optional] This list of H-byte hashes describe a set of B commit-graph files that form a commit-graph chain. The graph position for the ith commit in this -- cgit 1.4.1