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From: "brian m. carlson" <sandals@crustytoothpaste.net>
To: Ian Jackson <ijackson@chiark.greenend.org.uk>
Cc: "Jeff King" <peff@peff.net>,
	"Ævar Arnfjörð Bjarmason" <avarab@gmail.com>,
	"Linus Torvalds" <torvalds@linux-foundation.org>,
	"Jason Cooper" <git@lakedaemon.net>,
	ankostis <ankostis@gmail.com>,
	"Junio C Hamano" <gitster@pobox.com>,
	"Git Mailing List" <git@vger.kernel.org>,
	"Stefan Beller" <sbeller@google.com>,
	"David Lang" <david@lang.hm>, "Joey Hess" <id@joeyh.name>
Subject: Re: Transition plan for git to move to a new hash function
Date: Tue, 28 Feb 2017 21:47:25 +0000	[thread overview]
Message-ID: <20170228214724.w7w5f6n4u6ehanzd@genre.crustytoothpaste.net> (raw)
In-Reply-To: <22708.8913.864049.452252@chiark.greenend.org.uk>

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On Mon, Feb 27, 2017 at 01:00:01PM +0000, Ian Jackson wrote:
> I said I was working on a transition plan.  Here it is.  This is
> obviously a draft for review, and I have no official status in the git
> project.  But I have extensive experience of protocol compatibility
> engineering, and I hope this will be helpful.
> 
> Ian.
> 
> 
> Subject: Transition plan for git to move to a new hash function
> 
> 
> BASIC PRINCIPLES
> ================
> 
> We run multiple hashes in parallel.  Each object is named by exactly
> one hash.  We define that objects with identical content, but named by
> different hash functions, are different objects.

I think this is fine.

> Objects of one hash may refer to objects named by a different hash
> function to their own.  Preference rules arrange that normally, new
> hash objects refer to other new hash objects.

The existing codebase isn't really intended with that in mind.

It's not that I am arguing against this because I think it's a bad idea,
I'm arguing against it because as a contributor, I'm doubtful that this
is easily achievable given the state of the codebase.

> The intention is that for most projects, the existing SHA-1 based
> history will be retained and a new history built on top of it.
> (Rewriting is also possible but means a per-project hard switch.)

I like Peff's suggested approach in which we essentially rewrite history
under the hood, but have a lookup table which looks up the old hash
based on the new hash.  That allows us to refer to old objects, but not
have to share serialized data that mentions both hashes.

Obviously only the SHA-1 versions of old tags and commits will be able
to be validated, but that shouldn't be an issue.  We can hook that code
into a conversion routine that can handle on-the-fly object conversion.

We also can implement (optionally disabled) fallback functionality to
look up old SHA-1 hash names based on the new hash.

> We extend the textual object name syntax to explicitly name the hash
> used.  Every program that invokes git or speaks git protocols will
> need to understand the extended object name syntax.
> 
> Packfiles need to be extended to be able to contain objects named by
> new hash functions.  Blob objects with identical contents but named by
> different hash functions would ideally share storage.
> 
> Safety catches preferent accidental incorporation into a project of
> incompatibly-new objects, or additional deprecatedly-old objects.
> This allows for incremental deployment.

We have a compatibility mechanism already in place: if the
repositoryFormatVersion option is set to 1, but an unknown extension
flag is set, Git will bail out.

For network protocols, we have the server offer a hash=foo extension,
and make the client echo it back, and either bail or convert on the fly.
This makes it fast for new clients, and slow for old clients, which
encourages migration.

We could also store old-style packs for easy fetch by clients.

> TEXTUAL SYNTAX
> ==============
> 
> The object name textual syntax is extended.  The new syntax may be
> used in all textual git objects and protocols (commits, tags, command
> lines, etc.).
> 
> We declare that the object name syntax is henceforth
>   [A-Z]+[0-9a-z]+ | [0-9a-f]+
> and that names [A-Z].* are deprecated as ref name components.

I'd simply say that we have data always be in the new format if it's
available, and tag the old SHA-1 versions instead.  Otherwise, as Peff
pointed out, we're going to be stuck typing a bunch of identical stuff
every time.  Again, this encourages migration.
-- 
brian m. carlson / brian with sandals: Houston, Texas, US
+1 832 623 2791 | https://www.crustytoothpaste.net/~bmc | My opinion only
OpenPGP: https://keybase.io/bk2204

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  parent reply	other threads:[~2017-02-28 21:49 UTC|newest]

Thread overview: 136+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-02-23 16:43 SHA1 collisions found Joey Hess
2017-02-23 17:00 ` David Lang
2017-02-23 17:02 ` Junio C Hamano
2017-02-23 17:12   ` David Lang
2017-02-23 20:49     ` Jakub Narębski
2017-02-23 20:57       ` Jeff King
2017-02-23 17:18   ` Junio C Hamano
2017-02-23 17:35   ` Joey Hess
2017-02-23 17:52     ` Linus Torvalds
2017-02-23 18:21       ` Joey Hess
2017-02-23 18:31         ` Joey Hess
2017-02-23 19:13           ` Morten Welinder
2017-02-24 15:52             ` Geert Uytterhoeven
2017-02-23 18:40         ` Linus Torvalds
2017-02-23 18:46           ` Jeff King
2017-02-23 19:09             ` Linus Torvalds
2017-02-23 19:32               ` Jeff King
2017-02-23 19:47                 ` Linus Torvalds
2017-02-23 19:57                   ` Jeff King
     [not found]                     ` <alpine.LFD.2.20.1702231428540.30435@i7.lan>
2017-02-23 22:43                       ` Jeff King
2017-02-23 22:50                         ` Linus Torvalds
2017-02-23 23:05                         ` Jeff King
2017-02-23 23:05                           ` [PATCH 1/3] add collision-detecting sha1 implementation Jeff King
2017-02-23 23:15                             ` Stefan Beller
2017-02-24  0:01                               ` Jeff King
2017-02-24  0:12                                 ` Linus Torvalds
2017-02-24  0:16                                   ` Jeff King
2017-02-23 23:05                           ` [PATCH 2/3] sha1dc: adjust header includes for git Jeff King
2017-02-23 23:06                           ` [PATCH 3/3] Makefile: add USE_SHA1DC knob Jeff King
2017-02-24 18:36                             ` HW42
2017-02-24 18:57                               ` Jeff King
2017-02-23 23:14                           ` SHA1 collisions found Linus Torvalds
2017-02-28 18:41                           ` Junio C Hamano
2017-02-28 19:07                             ` Junio C Hamano
2017-02-28 19:20                               ` Jeff King
2017-03-01  8:57                                 ` Dan Shumow
2017-02-28 19:34                               ` Linus Torvalds
2017-02-28 19:52                                 ` Shawn Pearce
2017-02-28 22:56                                   ` Linus Torvalds
2017-02-28 21:22                                 ` Dan Shumow
2017-02-28 22:50                                   ` Marc Stevens
2017-02-28 23:11                                     ` Linus Torvalds
2017-03-01 19:05                                       ` Jeff King
2017-02-23 20:47               ` Øyvind A. Holm
2017-02-23 20:46             ` Joey Hess
2017-02-23 18:42         ` Jeff King
2017-02-23 17:52     ` David Lang
2017-02-23 19:20   ` David Lang
2017-02-23 17:19 ` Linus Torvalds
2017-02-23 17:29   ` Linus Torvalds
2017-02-23 18:10   ` Joey Hess
2017-02-23 18:29     ` Linus Torvalds
2017-02-23 18:38     ` Junio C Hamano
2017-02-24  9:42 ` Duy Nguyen
2017-02-25 19:04   ` brian m. carlson
2017-02-27 13:29     ` René Scharfe
2017-02-28 13:25       ` brian m. carlson
2017-02-24 15:13 ` Ian Jackson
2017-02-24 17:04   ` ankostis
2017-02-24 17:23   ` Jason Cooper
2017-02-25 23:22     ` ankostis
2017-02-24 17:32   ` Junio C Hamano
2017-02-24 17:45     ` David Lang
2017-02-24 18:14       ` Junio C Hamano
2017-02-24 18:58         ` Stefan Beller
2017-02-24 19:20           ` Junio C Hamano
2017-02-24 20:05             ` ankostis
2017-02-24 20:32               ` Junio C Hamano
2017-02-25  0:31                 ` ankostis
2017-02-26  0:16                   ` Jason Cooper
2017-02-26 17:38                     ` brian m. carlson
2017-02-26 19:11                       ` Linus Torvalds
2017-02-26 21:38                         ` Ævar Arnfjörð Bjarmason
2017-02-26 21:52                           ` Jeff King
2017-02-27 13:00                             ` Transition plan for git to move to a new hash function Ian Jackson
2017-02-27 14:37                               ` Why BLAKE2? Markus Trippelsdorf
2017-02-27 15:42                                 ` Ian Jackson
2017-02-27 19:26                               ` Transition plan for git to move to a new hash function Tony Finch
2017-02-28 21:47                               ` brian m. carlson [this message]
2017-03-02 18:13                                 ` Ian Jackson
2017-03-04 22:49                                   ` brian m. carlson
2017-03-05 13:45                                     ` Ian Jackson
2017-03-05 23:45                                       ` brian m. carlson
2017-02-24 20:05             ` SHA1 collisions found Junio C Hamano
2017-02-24 20:33           ` Philip Oakley
2017-02-24 23:39     ` Jeff King
2017-02-25  0:39       ` Linus Torvalds
2017-02-25  0:54         ` Linus Torvalds
2017-02-25  1:16         ` Jeff King
2017-02-26 18:55           ` Junio C Hamano
2017-02-25  6:10         ` Junio C Hamano
2017-02-26  1:13           ` Jason Cooper
2017-02-26  5:18             ` Jeff King
2017-02-26 18:30               ` brian m. carlson
2017-03-02 21:46               ` Brandon Williams
2017-03-03 11:13                 ` Jeff King
2017-03-03 14:54                   ` Ian Jackson
2017-03-03 22:18                     ` Jeff King
2017-03-02 19:55         ` Linus Torvalds
2017-03-02 20:43           ` Junio C Hamano
2017-03-02 21:21             ` Linus Torvalds
2017-03-02 21:54               ` Joey Hess
2017-03-02 22:27                 ` Linus Torvalds
2017-03-03  1:50                   ` Mike Hommey
2017-03-03  2:19                     ` Linus Torvalds
2017-03-03 11:04           ` Jeff King
2017-03-03 21:47           ` Stefan Beller
2017-02-25  1:00       ` David Lang
2017-02-25  1:15         ` Stefan Beller
2017-02-25  1:21         ` Jeff King
2017-02-25  1:39           ` David Lang
2017-02-25  1:47             ` Jeff King
2017-02-25  1:56               ` David Lang
2017-02-25  2:28             ` Jacob Keller
2017-02-25  2:26           ` Jacob Keller
2017-02-25  5:39             ` grarpamp
2017-02-24 23:43     ` Ian Jackson
2017-02-25  0:06       ` Ian Jackson
2017-02-25 18:50     ` brian m. carlson
2017-02-25 19:26       ` Jeff King
2017-02-25 22:09         ` Mike Hommey
2017-02-26 17:38           ` brian m. carlson
2017-02-24 22:47 ` Jakub Narębski
2017-02-24 22:53   ` Santiago Torres
2017-02-24 23:05     ` Jakub Narębski
2017-02-24 23:24       ` Øyvind A. Holm
2017-02-24 23:06   ` Jeff King
2017-02-24 23:35     ` Jakub Narębski
2017-02-25 22:35     ` Lars Schneider
2017-02-26  0:46       ` Jeff King
2017-02-26 18:22         ` Junio C Hamano
2017-02-26 18:57     ` Thomas Braun
2017-02-26 21:30       ` Jeff King
2017-02-27  9:57         ` Geert Uytterhoeven
2017-02-27 10:43           ` Jeff King
2017-02-27 12:39             ` Morten Welinder

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