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From: Jeff King <peff@peff.net>
To: Junio C Hamano <gitster@pobox.com>
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 21:36:32 -0400	[thread overview]
Message-ID: <20160413013632.GA10656@sigill.intra.peff.net> (raw)
In-Reply-To: <xmqqlh4imibd.fsf@gitster.mtv.corp.google.com>

On Tue, Apr 12, 2016 at 06:03:02PM -0700, Junio C Hamano wrote:

> > 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.

Yeah. I'd hope in general that there would be a single commit that does
the transition, and we'd only pay it when doing diffs across the
boundary. And even then, I think a local-only cache of aliases could
mitigate the worst of it.

> >   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.

Good point. I do think projects would do well to have a moment where
they switch to the new format, and don't freely intermingle. We could
possibly do some magic there to help things out. For example, if we are
building on a commit that is sha-2, we automatically use more sha-2
objects to point to them. And then the "flag day" for a project is
simply that somebody pushes to "master" using sha-2, and everybody
else's git (which learned long ago to speak the new algorithm) just
picks it up.

Of course that's not exactly a flag day for projects that branch from
old history for bugfixes. But it might be close enough.

-Peff

  reply	other threads:[~2016-04-13  1:36 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
2016-04-13  1:36     ` Jeff King [this message]
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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