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From:   Junio C Hamano <gitster@pobox.com>
To:     "brian m. carlson" <sandals@crustytoothpaste.net>
Cc:     git@vger.kernel.org
Subject: Re: Design of multiple hash support
References: <20181105010032.GN731755@genre.crustytoothpaste.net>
Date:   Mon, 05 Nov 2018 11:36:01 +0900
In-Reply-To: <20181105010032.GN731755@genre.crustytoothpaste.net> (brian
        m. carlson's message of "Mon, 5 Nov 2018 01:00:33 +0000")
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"brian m. carlson" <sandals@crustytoothpaste.net> writes:

> I'm currently working on getting Git to support multiple hash algorithms
> in the same binary (SHA-1 and SHA-256).  In order to have a fully
> functional binary, we'll need to have some way of indicating to certain
> commands (such as init and show-index) that they should assume a certain
> hash algorithm.
>
> There are basically two approaches I can take.  The first is to provide
> each command that needs to learn about this with its own --hash
> argument.  So we'd have:
>
>   git init --hash=sha256
>   git show-index --hash=sha256 <some-file
>
> The other alternative is that we provide a global option to git, which
> is parsed by all programs, like so:
>
>   git --hash=sha256 init
>   git --hash=sha256 show-index <some-file

I am assuming that "show-index" above is a typo for something like
"hash-object"?

It is hard to answer the question without knowing what exactly does
"(to) support multiple hash algorithms" mean.  For example, inside
today's repository, what should this command do?

	git --hash=sha256 cat-file commit HEAD

It can work this way:

 - read HEAD, discover that I am on 'master' branch, read refs/heads/master
   to learn the object name in 40-hex, realize that it cannot be
   sha256 and report "corrupt ref".

Or it can work this way:

 - read repository format, realize it is a good old sha1 repository.

 - do the usual thing to get to read_object() to read the commit
   object data for the commit at HEAD, doing all of it in sha1.

 - in the commit object data, locate references to other objects
   that use sha1 name.

 - replace these sha1 references with their sha256 counterparts and
   show the result.

I am guessing that you are doing the former as a good first step, in
which case, as an option that changes/affects the behaviour of git
globally, I think "git --hash=sha256" would make sense, like other
global options like --literal-pathspecs and --no-replace-objects.

Thanks.