NAME
gitformat-commit-graph - Git commit-graph formatSYNOPSIS
$GIT_DIR/objects/info/commit-graph $GIT_DIR/objects/info/commit-graphs/*
DESCRIPTION
The Git commit-graph stores a list of commit OIDs and some associated metadata, including:•The generation number of the
commit.
•The root tree OID.
•The commit date.
•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.
COMMIT-GRAPH FILES HAVE THE FOLLOWING FORMAT:
In order to allow extensions that add extra data to the graph, we organize 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.HEADER:
4-byte signature: The signature is: {'C', 'G', 'P', 'H'}
1-byte version number: Currently, the only valid version is 1.
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"
1-byte number (B) of base commit-graphs We infer the length (H*B) of the Base Graphs chunk from this value.
CHUNK LOOKUP:
(C + 1) * 12 bytes listing the table of contents for the chunks: First 4 bytes describe the chunk id. Value 0 is a terminating label. Other 8 bytes provide the byte-offset in current file for chunk to start. (Chunks are ordered contiguously in the file, so you can infer the length using the next chunk position if necessary.) Each chunk ID appears at most once.
The CHUNK LOOKUP matches the table of contents from the chunk-based file format, see linkgit:gitformat-chunk[5]
The remaining data in the body is described one chunk at a time, and these chunks may be given in any order. Chunks are required unless otherwise specified.
CHUNK DATA:
OID Fanout (ID: {O, I, D, F}) (256 * 4 bytes)The ith entry, F[i], stores the number of OIDs with first byte at most i. Thus F[255] stores the total number of commits (N).
The OIDs for all commits in the graph, sorted in ascending order.
•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 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.
•The next 8 bytes store the topological
level (generation number v1) of the commit and the commit time in seconds
since EPOCH. The generation number uses the higher 30 bits of the first 4
bytes, while the commit time uses the 32 bits of the second 4 bytes, along
with the lowest 2 bits of the lowest byte, storing the 33rd and 34th bit of
the commit time.
•This list of 4-byte values store
corrected commit date offsets for the commits, arranged in the same order as
commit data chunk.
•If the corrected commit date offset
cannot be stored within 31 bits, the value has its most-significant bit on and
the other bits store the position of corrected commit date into the Generation
Data Overflow chunk.
•Generation Data chunk is present only
when commit-graph file is written by compatible versions of Git and in case of
split commit-graph chains, the topmost layer also has Generation Data
chunk.
•This list of 8-byte values stores the
corrected commit date offsets for commits with corrected commit date offsets
that cannot be stored within 31 bits.
•Generation Data Overflow chunk is
present only when Generation Data chunk is present and atleast one corrected
commit date offset cannot be stored within 31 bits.
This list of 4-byte values store the second through nth parents for all octopus merges. The second parent value in the commit data stores an array position within this list along with the most-significant bit on. Starting at that array position, iterate through this list of commit 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.
•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.
•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.
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 file's OID Lookup chunk is equal to i plus the number of commits in all base graphs. If B is non-zero, this chunk must exist.
TRAILER:
H-byte HASH-checksum of all of the above.
HISTORICAL NOTES:
The Generation Data (GDA2) and Generation Data Overflow (GDO2) chunks have the number 2 in their chunk IDs because a previous version of Git wrote possibly erroneous data in these chunks with the IDs "GDAT" and "GDOV". By changing the IDs, newer versions of Git will silently ignore those older chunks and write the new information without trusting the incorrect data.GIT
Part of the git(1) suite02/28/2023 | Git 2.39.2 |