Re: Is the sha256 object format experimental or not?

From: "Ævar Arnfjörð Bjarmason" <avarab@gmail.com>
To: Junio C Hamano <gitster@pobox.com>
Cc: dwh@linuxprogrammer.org,
	"brian m. carlson" <sandals@crustytoothpaste.net>,
	git@vger.kernel.org
Subject: Re: Is the sha256 object format experimental or not?
Date: Fri, 14 May 2021 10:49:42 +0200	[thread overview]
Message-ID: <875yzlsngv.fsf@evledraar.gmail.com> (raw)
In-Reply-To: <xmqqo8de9wis.fsf@gitster.g>

On Fri, May 14 2021, Junio C Hamano wrote:

> dwh@linuxprogrammer.org writes:
>
>> I think Git should externalize the calculation of object digests just
>> like it externalizes the calcualtion of object digital signatures.
>
> The hashing algorithms used to generate object names has
> requirements fundamentally different from that of digital
> signatures.  I strongly suspect that that fact would change the
> equation when you rethink what you said above.
>
> We can "upgrade" digital signature algorithms fairly easily---nobody
> would complain if you suddenly choose different signing algorithm
> over a blob of data, as long as all project participants are aware
> (and self-describing datastream helps here) and are capable of
> grokking the new algorithm we are adopting.  But because object
> names are used by one object to refer to another, and most
> importantly, we do not want a single object to have multiple names,
> we cannot afford to introduce a new hashing algorithm every time we
> feel like it.  In other words, diversity of object naming algorithms
> is to be avoided as much as possible, while diversity of signature
> algorithms is naturally expected.

I agree insofar that I don't see a good reason for us to support some
plethora of hash algorithms, but I wouldn't have objections to adding
more if people find them useful for some reason. See e.g. [1] for an
implementation.

But I really don't see how anything you've said would present a
technical hurdle once we have SHA-1<->SHA-256 interop in a good enough
state. At that point we'll support re-hashing on arrival of content
hashed with algorithm X into Y, with a local lookup table between X<=>Y.

So if somebody wants to maintain content hashed with algorithm Z locally
we should easily be able to support that. The "diversity of naming"
won't matter past that local repository, any mention of Z will be
translated to X or Y on fetch/push.

1. https://lore.kernel.org/git/20191222064809.35667-1-michaeljclark@mac.com/