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From: "brian m. carlson" <sandals@crustytoothpaste.net>
To: "Ævar Arnfjörð Bjarmason" <avarab@gmail.com>
Cc: git@vger.kernel.org, Emily Shaffer <emilyshaffer@google.com>,
	Johannes Schindelin <Johannes.Schindelin@gmx.de>,
	Konstantin Ryabitsev <konstantin@linuxfoundation.org>,
	Jason Pyeron <jpyeron@pdinc.us>
Subject: Re: [PATCH 4/4] docs: note that archives are not stable
Date: Sun, 28 Feb 2021 18:19:48 +0000	[thread overview]
Message-ID: <YDvexO2NFM0KZ1Jo@camp.crustytoothpaste.net> (raw)
In-Reply-To: <87h7lwl5mv.fsf@evledraar.gmail.com>

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On 2021-02-28 at 12:48:56, Ævar Arnfjörð Bjarmason wrote:
> Perhaps something like this instead:
> 
>     The output of 'git archive' is guaranteed to be the same across
>     versions of git, but the archive itself is not guaranteed to be
>     bit-for-bit identical.
> 
>     In practice the output of 'git archive' is relatively stable across
>     git versions, but has changed in the past, and most likely will in
>     the future.
> 
>     Since the tar format provides multiple ways to encode the same
>     output (ordering, headers, padding etc.) you should not rely on
>     output being bit-for-bit identical across versions of git for
>     e.g. GPG signing a SHA-256 hash of an archive generated with one
>     version of git, and then expecting to be able to validate that GPG
>     signature with a freshly generated archive made with same arguments
>     on another version of git.

I think something like this is good.  I'm a bit nervous about telling
people that the output is relatively stable because that will likely
push people in the direction that we don't want to encourage.  I might
rephrase the first two paragraphs as so:

  The output of 'git archive' is guaranteed to be the same across
  versions of git, but the archive itself is not guaranteed to be
  bit-for-bit identical.  The output of 'git archive' has changed
  in the past, and most likely will in the future.

I'm not very familiar with the zip format, but I assume that it also has
features that allow equivalent but not bit-for-bit equal archives.
Looking at Wikipedia leads me to believe that one could indeed create
different archives just by either writing a Zip64 record or not, and if
we store the SHA-1 revision ID in a comment, then we would also produce
a different archive when using an equivalent SHA-256 repo.  And of
course there's compression, which allows many different but equivalent
serializations.  So we'd probably need to say the same thing about zip
files as well.
-- 
brian m. carlson (he/him or they/them)
Houston, Texas, US

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  reply	other threads:[~2021-02-28 18:22 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-02-27 19:18 [PATCH 0/4] Documentation updates to FAQ and git-archive brian m. carlson
2021-02-27 19:18 ` [PATCH 1/4] docs: add a question on syncing repositories to the FAQ brian m. carlson
2021-02-28 13:01   ` Ævar Arnfjörð Bjarmason
2021-03-15 20:40     ` brian m. carlson
2021-02-27 19:18 ` [PATCH 2/4] docs: add line ending configuration article to FAQ brian m. carlson
2021-02-27 19:18 ` [PATCH 3/4] docs: add a FAQ section on push and fetch problems brian m. carlson
2021-02-28 12:37   ` Ævar Arnfjörð Bjarmason
2021-02-28 18:07     ` brian m. carlson
2021-03-01 18:02   ` Junio C Hamano
2021-02-27 19:18 ` [PATCH 4/4] docs: note that archives are not stable brian m. carlson
2021-02-28 12:48   ` Ævar Arnfjörð Bjarmason
2021-02-28 18:19     ` brian m. carlson [this message]
2021-02-28 18:46       ` Ævar Arnfjörð Bjarmason
2021-03-01 18:15       ` Junio C Hamano
2021-03-03  0:36         ` brian m. carlson
2021-03-03  6:55           ` Junio C Hamano

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