From: Junio C Hamano <gitster@pobox.com>
To: Jeff King <peff@peff.net>
Cc: "H. Peter Anvin" <hpa@zytor.com>,
"git\@vger.kernel.org" <git@vger.kernel.org>
Subject: Re: Migrating away from SHA-1?
Date: Tue, 12 Apr 2016 18:03:02 -0700 [thread overview]
Message-ID: <xmqqlh4imibd.fsf@gitster.mtv.corp.google.com> (raw)
In-Reply-To: <20160412234251.GB2210@sigill.intra.peff.net> (Jeff King's message of "Tue, 12 Apr 2016 19:42:52 -0400")
Jeff King <peff@peff.net> writes:
> So a slightly nicer thing is to parameterize the algorithm for every
> object name reference. So commits look like:
>
> tree sha256:1234abcd...
> parent sha256:1234abcd...
>
> and so on. Of course trees don't have any space for this; they have a
> fixed-length for the hash part of each record, which is basically:
>
> <mode> <name> NUL <20-byte-sha1>
>
> So we'd probably need a "treev2" object type that gives room for an
> algorithm byte (or we'd have to try to shove it into the mode, but since
> old versions won't know the new algorithm anyway, I don't think it
> solves that much...). Or you can just define for the whole tree object
> (either implicit in its type, or in a header) that it always uses
> algorithm X.
This will hurt the performance a lot during the transition period as
it no longer will be possible to rely on "most of the time a fine
grained commit changes only a small part of the tree, and we can
cheaply avoid descending into trees that haven't changed because we
can tell that the corresponding tree objects in the pre- and post-
trees have the same object name" optimization. But we cannot avoid
it.
> Transitioning to that would be something like:
>
> 0. Overhaul all of the git code to handle arbitrary-sized object ids.
>
> 1. Decide on the new algorithm and implement it in git.
>
> 2. Recognize parameterized object ids in commits and tags (designing
> format, implementing the reading side).
>
> 3. Recognize parameterized object ids somehow in trees (designing
> format, implementing the reading side).
>
> 4. Teach the object database to index objects by the new algorithm (or
> possibly both algorithms).
>
> 5. Add a protocol extension so that both sides can decide which
> algorithm is being used when they talk about oids.
>
> 6. Add a config option to write references in objects using the new
> algorithm.
>
> 7. After a while, flip the config option on. Hopefully the readers
> from steps 1-5 have percolated to the masses by then, and it's not
> a horrible flag day.
>
> We're basically on step 0 right now. I'm sure I'm missing some
> subtleties in there, too.
One subtlety is that 7. "not a flag day" may not be a good thing.
There has to be a section of a history that spans the transition,
set of commits and trees that have pointers to both kinds of object
names. The narrower such a section of the history, the more
pleasant to use the result of the transition would be.
Different projects that can have their own flag days at their own
pace is a good thing, so the above observation does not invalidate
your transition plan, though.
next prev parent reply other threads:[~2016-04-13 1:03 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-04-12 22:38 Migrating away from SHA-1? H. Peter Anvin
2016-04-12 23:00 ` Stefan Beller
2016-04-12 23:06 ` H. Peter Anvin
2016-04-12 23:15 ` Jeff King
2016-04-12 23:15 ` David Turner
2016-04-12 23:44 ` Jeff King
2016-04-14 1:53 ` Theodore Ts'o
2016-04-14 16:47 ` Joey Hess
2016-04-14 17:23 ` David Turner
2016-04-14 17:28 ` H. Peter Anvin
2016-04-14 22:40 ` Theodore Ts'o
2016-04-15 2:13 ` Jeff King
2016-04-15 2:18 ` Junio C Hamano
2016-04-15 2:22 ` Jeff King
2016-04-12 23:42 ` Jeff King
2016-04-13 1:03 ` Junio C Hamano [this message]
2016-04-13 1:36 ` Jeff King
2016-04-13 1:38 ` H. Peter Anvin
2016-04-13 1:51 ` Duy Nguyen
2016-04-13 1:58 ` H. Peter Anvin
2016-04-15 1:50 ` brian m. carlson
-- strict thread matches above, loose matches on Subject: below --
2016-06-18 2:10 Leo Gaspard
2016-06-18 3:30 ` Eric Wong
2016-06-24 18:17 ` brian m. carlson
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