You can not select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
104 lines
4.1 KiB
104 lines
4.1 KiB
Git commit graph format |
|
======================= |
|
|
|
The Git commit graph stores a list of commit OIDs and some associated |
|
metadata, including: |
|
|
|
- The generation number of the commit. Commits with no parents have |
|
generation number 1; commits with parents have generation number |
|
one more than the maximum generation number of its parents. We |
|
reserve zero as special, and can be used to mark a generation |
|
number invalid or as "not computed". |
|
|
|
- The root tree OID. |
|
|
|
- The commit date. |
|
|
|
- The parents of the commit, stored using positional references within |
|
the graph file. |
|
|
|
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 |
|
(1 << 30) + (1 << 29) + (1 << 28) - 1 (around 1.8 billion) commits. |
|
|
|
== 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. |
|
|
|
All 4-byte numbers are in network order. |
|
|
|
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 (1 = SHA-1) |
|
We infer the hash length (H) from this value. |
|
|
|
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 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). |
|
|
|
OID Lookup (ID: {'O', 'I', 'D', 'L'}) (N * H bytes) |
|
The OIDs for all commits in the graph, sorted in ascending order. |
|
|
|
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 |
|
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 generation number 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. |
|
|
|
Extra Edge List (ID: {'E', 'D', 'G', 'E'}) [Optional] |
|
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. |
|
|
|
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 |
|
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.
|
|
|