From 17420eafa9e2b7d69ff17dbf4d12088a2152fd9d Mon Sep 17 00:00:00 2001 From: "brian m. carlson" Date: Thu, 13 Aug 2020 22:49:00 +0000 Subject: docs: document SHA-256 pack and indices 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 Signed-off-by: Junio C Hamano --- Documentation/technical/pack-format.txt | 36 +++++++++++++++++++-------------- 1 file changed, 21 insertions(+), 15 deletions(-) (limited to 'Documentation/technical') 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. -- cgit v1.2.3 From 2ae12e568b14e068cd37afdd47dd46c2be8538fe Mon Sep 17 00:00:00 2001 From: "brian m. carlson" Date: Thu, 13 Aug 2020 22:49:01 +0000 Subject: docs: fix step in transition plan One of the required steps for the objectFormat extension is to implement the loose object index. However, without support for compatObjectFormat, we don't even know if the loose object index is needed, so it makes sense to move that step to the compatObjectFormat section. Do so. Signed-off-by: brian m. carlson Signed-off-by: Junio C Hamano --- Documentation/technical/hash-function-transition.txt | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'Documentation/technical') diff --git a/Documentation/technical/hash-function-transition.txt b/Documentation/technical/hash-function-transition.txt index 5b2db3be1e..6fd20ebbc2 100644 --- a/Documentation/technical/hash-function-transition.txt +++ b/Documentation/technical/hash-function-transition.txt @@ -650,7 +650,6 @@ Some initial steps can be implemented independently of one another: The first user-visible change is the introduction of the objectFormat extension (without compatObjectFormat). This requires: -- implementing the loose-object-idx - teaching fsck about this mode of operation - using the hash function API (vtable) when computing object names - signing objects and verifying signatures @@ -658,6 +657,7 @@ extension (without compatObjectFormat). This requires: repository Next comes introduction of compatObjectFormat: +- implementing the loose-object-idx - translating object names between object formats - translating object content between object formats - generating and verifying signatures in the compat format -- cgit v1.2.3