[1/2] docs: document SHA-256 pack and indices
diff mbox series

Message ID 20200813224901.2652387-2-sandals@crustytoothpaste.net
State New
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Series
  • Documentation updates for SHA-256
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Commit Message

brian m. carlson Aug. 13, 2020, 10:49 p.m. UTC
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@crustytoothpaste.net>
---
 Documentation/technical/pack-format.txt | 36 ++++++++++++++-----------
 1 file changed, 21 insertions(+), 15 deletions(-)

Comments

Derrick Stolee Aug. 14, 2020, 2:26 a.m. UTC | #1
On 8/13/2020 6:49 PM, brian m. carlson wrote:
> 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@crustytoothpaste.net>
> ---
>  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.

I immediately got concerned and looked at the existing MIDX
format to see how we deal with hash length and the contents
missing in this diff are suitably vague about the hash-length:

  "The OIDs for all objects in the MIDX are stored..."

These changes are good.

Thanks,
-Stolee

Patch
diff mbox series

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.