Now that we have SHA-256 support for packs and indices, let's document
that in SHA-256 repositories, we use SHA-256 instead of SHA-1 for object
names and checksums. Instead of duplicating this information throughout
the document, let's just document that in SHA-1 repositories, we use
SHA-1 for these purposes, and in SHA-256 repositories, we use SHA-256.
Signed-off-by: brian m. carlson <sandals@xxxxxxxxxxxxxxxxxxxx>
---
Documentation/technical/pack-format.txt | 36 ++++++++++++++-----------
1 file changed, 21 insertions(+), 15 deletions(-)
diff --git a/Documentation/technical/pack-format.txt b/Documentation/technical/pack-format.txt
index d3a142c652..f4c8d94f73 100644
--- a/Documentation/technical/pack-format.txt
+++ b/Documentation/technical/pack-format.txt
@@ -1,6 +1,12 @@
Git pack format
===============
+== Checksums and object IDs
+
+In a repository using the traditional SHA-1, pack checksums, index checksums,
+and object IDs (object names) mentioned below are all computed using SHA-1.
+Similarly, in SHA-256 repositories, these values are computed using SHA-256.
+
== pack-*.pack files have the following format:
- A header appears at the beginning and consists of the following:
@@ -26,7 +32,7 @@ Git pack format
(deltified representation)
n-byte type and length (3-bit type, (n-1)*7+4-bit length)
- 20-byte base object name if OBJ_REF_DELTA or a negative relative
+ base object name if OBJ_REF_DELTA or a negative relative
offset from the delta object's position in the pack if this
is an OBJ_OFS_DELTA object
compressed delta data
@@ -34,7 +40,7 @@ Git pack format
Observation: length of each object is encoded in a variable
length format and is not constrained to 32-bit or anything.
- - The trailer records 20-byte SHA-1 checksum of all of the above.
+ - The trailer records a pack checksum of all of the above.
=== Object types
@@ -58,8 +64,8 @@ ofs-delta and ref-delta, which is only valid in a pack file.
Both ofs-delta and ref-delta store the "delta" to be applied to
another object (called 'base object') to reconstruct the object. The
-difference between them is, ref-delta directly encodes 20-byte base
-object name. If the base object is in the same pack, ofs-delta encodes
+difference between them is, ref-delta directly encodes base object
+name. If the base object is in the same pack, ofs-delta encodes
the offset of the base object in the pack instead.
The base object could also be deltified if it's in the same pack.
@@ -143,14 +149,14 @@ This is the instruction reserved for future expansion.
object is stored in the packfile as the offset from the
beginning.
- 20-byte object name.
+ one object name of the appropriate size.
- The file is concluded with a trailer:
- A copy of the 20-byte SHA-1 checksum at the end of
- corresponding packfile.
+ A copy of the pack checksum at the end of the corresponding
+ packfile.
- 20-byte SHA-1-checksum of all of the above.
+ Index checksum of all of the above.
Pack Idx file:
@@ -198,7 +204,7 @@ Pack file entry: <+
If it is not DELTA, then deflated bytes (the size above
is the size before compression).
If it is REF_DELTA, then
- 20-byte base object name SHA-1 (the size above is the
+ base object name (the size above is the
size of the delta data that follows).
delta data, deflated.
If it is OFS_DELTA, then
@@ -227,9 +233,9 @@ Pack file entry: <+
- A 256-entry fan-out table just like v1.
- - A table of sorted 20-byte SHA-1 object names. These are
- packed together without offset values to reduce the cache
- footprint of the binary search for a specific object name.
+ - A table of sorted object names. These are packed together
+ without offset values to reduce the cache footprint of the
+ binary search for a specific object name.
- A table of 4-byte CRC32 values of the packed object data.
This is new in v2 so compressed data can be copied directly
@@ -248,10 +254,10 @@ Pack file entry: <+
- The same trailer as a v1 pack file:
- A copy of the 20-byte SHA-1 checksum at the end of
+ A copy of the pack checksum at the end of
corresponding packfile.
- 20-byte SHA-1-checksum of all of the above.
+ Index checksum of all of the above.
== multi-pack-index (MIDX) files have the following format:
@@ -329,4 +335,4 @@ CHUNK DATA:
TRAILER:
- 20-byte SHA1-checksum of the above contents.
+ Index checksum of the above contents.
