Subject: Re: [PATCH 2/5] index-format.txt: document SHA-256 index format

From: Derrick Stolee <[email-masked]>
Date: Fri, 14 Aug 2020 08:28:13 -0400

On 8/14/2020 8:21 AM, Martin Ågren wrote:
> Similar to a recent commit, document that in SHA-1 repositories, we use
> SHA-1 and in SHA-256 repositories, we use SHA-256, then replace all
> other uses of "SHA-1" with something more neutral.
> 
> Signed-off-by: Martin Ågren <[email-masked]>
> ---
>  Documentation/technical/index-format.txt | 27 +++++++++++++-----------
>  1 file changed, 15 insertions(+), 12 deletions(-)
> 
> diff --git a/Documentation/technical/index-format.txt b/Documentation/technical/index-format.txt
> index faa25c5c52..827ece2ed1 100644
> --- a/Documentation/technical/index-format.txt
> +++ b/Documentation/technical/index-format.txt
> @@ -3,8 +3,11 @@ Git index format
>  
>  == The Git index file has the following format
>  
> -  All binary numbers are in network byte order. Version 2 is described
> -  here unless stated otherwise.
> +  All binary numbers are in network byte order.
> +  In a repository using the traditional SHA-1, 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.
> +  Version 2 is described here unless stated otherwise.
>  
>     - A 12-byte header consisting of
>  
> @@ -32,7 +35,7 @@ Git index format
>  
>       Extension data
>  
> -   - 160-bit SHA-1 over the content of the index file before this
> +   - 160-bit hash checksum over the content of the index file before this
>       checksum.

If this hash is flexible, then "160-bit" is not correct anymore, right?

>  == Index entry
> @@ -80,7 +83,7 @@ Git index format
>    32-bit file size
>      This is the on-disk size from stat(2), truncated to 32-bit.
>  
> -  160-bit SHA-1 for the represented object
> +  160-bit object name for the represented object

Same here. The later instances of "160-bit" were dropped.

>    A 16-bit 'flags' field split into (high to low bits)
>  
> @@ -211,8 +214,8 @@ Git index format
>  
>    The extension consists of:
>  
> -  - 160-bit SHA-1 of the shared index file. The shared index file path
> -    is $GIT_DIR/sharedindex.<SHA-1>. If all 160 bits are zero, the
> +  - Hash of the shared index file. The shared index file path
> +    is $GIT_DIR/sharedindex.<hash>. If all bits are zero, the
>      index does not require a shared index file.
>  
>    - An ewah-encoded delete bitmap, each bit represents an entry in the
> @@ -253,10 +256,10 @@ Git index format
>  
>    - 32-bit dir_flags (see struct dir_struct)
>  
> -  - 160-bit SHA-1 of $GIT_DIR/info/exclude. Null SHA-1 means the file
> +  - Hash of $GIT_DIR/info/exclude. A null hash means the file
>      does not exist.
>  
> -  - 160-bit SHA-1 of core.excludesfile. Null SHA-1 means the file does
> +  - Hash of core.excludesfile. A null hash means the file does
>      not exist.
>  
>    - NUL-terminated string of per-dir exclude file name. This usually
> @@ -285,13 +288,13 @@ The remaining data of each directory block is grouped by type:
>    - An ewah bitmap, the n-th bit records "check-only" bit of
>      read_directory_recursive() for the n-th directory.
>  
> -  - An ewah bitmap, the n-th bit indicates whether SHA-1 and stat data
> +  - An ewah bitmap, the n-th bit indicates whether hash and stat data
>      is valid for the n-th directory and exists in the next data.
>  
>    - An array of stat data. The n-th data corresponds with the n-th
>      "one" bit in the previous ewah bitmap.
>  
> -  - An array of SHA-1. The n-th SHA-1 corresponds with the n-th "one" bit
> +  - An array of hashes. The n-th hash corresponds with the n-th "one" bit
>      in the previous ewah bitmap.
>  
>    - One NUL.
> @@ -330,12 +333,12 @@ The remaining data of each directory block is grouped by type:
>  
>    - 32-bit offset to the end of the index entries
>  
> -  - 160-bit SHA-1 over the extension types and their sizes (but not
> +  - Hash over the extension types and their sizes (but not
>  	their contents).  E.g. if we have "TREE" extension that is N-bytes
>  	long, "REUC" extension that is M-bytes long, followed by "EOIE",
>  	then the hash would be:
>  
> -	SHA-1("TREE" + <binary representation of N> +
> +	Hash("TREE" + <binary representation of N> +
>  		"REUC" + <binary representation of M>)
>  
>  == Index Entry Offset Table
> 

Thanks,
-Stolee