From: Marc Branchaud <marcnarc@xiplink.com>
To: Junio C Hamano <gitster@pobox.com>,
"Robin H. Johnson" <robbat2@gentoo.org>
Cc: "Git Mailing List" <git@vger.kernel.org>,
"Michał Górny" <mgorny@gentoo.org>, "Jeff King" <peff@peff.net>,
"Lars Schneider" <larsxschneider@gmail.com>,
"Ævar Arnfjörð Bjarmason" <avarab@gmail.com>
Subject: Re: [RFC PATCH] checkout: Force matching mtime between files
Date: Wed, 25 Apr 2018 11:18:26 -0400 [thread overview]
Message-ID: <ad4d0d66-58f4-5cab-d314-a30a50e8ad32@xiplink.com> (raw)
In-Reply-To: <xmqqin8f4qoq.fsf@gitster-ct.c.googlers.com>
On 2018-04-25 04:48 AM, Junio C Hamano wrote:
> "Robin H. Johnson" <robbat2@gentoo.org> writes:
>
>> In the thread from 6 years ago, you asked about tar's behavior for
>> mtimes. 'tar xf' restores mtimes from the tar archive, so relative
>> ordering after restore would be the same, and would only rebuild if the
>> original source happened to be dirty.
>>
>> This behavior is already non-deterministic in Git, and would be improved
>> by the patch.
>
> But Git is not an archiver (tar), but is a source code control
> system, so I do not think we should spend any extra cycles to
> "improve" its behaviour wrt the relative ordering, at least for the
> default case. Only those who rely on having build artifact *and*
> source should pay the runtime (and preferrably also the
> maintainance) cost.
Anyone who uses "make" or some other mtime-based tool is affected by
this. I agree that it's not "Everyone" but it sure is a lot of people.
Are we all that sure that the performance hit is that drastic? After
all, we've just done write_entry(). Calling utime() at that point
should just hit the filesystem cache.
> The best approach to do so is to have those people do the "touch"
> thing in their own post-checkout hook. People who use Git as the
> source control system won't have to pay runtime cost of doing the
> touch thing, and we do not have to maintain such a hook script.
> Only those who use the "feature" would.
The post-checkout hook approach is not exactly straightforward.
Naively, it's simply
for F in `git diff --name-only $1 $2`; do touch "$F"; done
But consider:
* Symlinks can cause the wrong file to be touched. (Granted, Michał's
proposed patch also doesn't deal with symlinks.) Let's assume that a
hook can be crafted will all possible sophistication. There are still
some fundamental problems:
* In a "file checkout" ("git checkout -- path/to/file"), $1 and $2 are
identical so the above loop does nothing. Offhand I'm not even sure how
a hook might get the right files in this case.
* The hook has to be set up in every repo and submodule (at least until
something like Ævar's experiments come to fruition).
* A fresh clone can't run the hook. This is especially important when
dealing with submodules. (In one case where we were bit by this, make
though that half of a fresh submodule clone's files were stale, and
decided to re-autoconf the entire thing.)
I just don't think the hook approach can completely solve the problem.
I appreciate Ævar's concern that there are more than just two mtime
requests floating around. But I think git's users are best served by a
built-in approach, with a config setting to control the desired mtime
handling (defaulting to the current behaviour). People who want a
different mtime solution will at least have a clear place in the code to
propose a patch.
M.
next prev parent reply other threads:[~2018-04-25 15:18 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-04-13 17:01 [RFC PATCH] checkout: Force matching mtime between files Michał Górny
2018-04-23 20:07 ` Robin H. Johnson
2018-04-23 23:41 ` Junio C Hamano
2018-04-25 7:13 ` Robin H. Johnson
2018-04-25 8:48 ` Junio C Hamano
2018-04-25 15:18 ` Marc Branchaud [this message]
2018-04-25 20:07 ` Robin H. Johnson
2018-04-26 1:25 ` Junio C Hamano
2018-04-26 14:12 ` Marc Branchaud
2018-04-26 14:46 ` Michał Górny
2018-04-28 14:23 ` Duy Nguyen
2018-04-28 19:35 ` Michał Górny
2018-04-26 16:43 ` Duy Nguyen
2018-04-26 17:48 ` Robin H. Johnson
2018-04-26 18:44 ` Duy Nguyen
2018-04-29 23:56 ` Junio C Hamano
2018-04-30 15:10 ` Duy Nguyen
2018-04-27 17:03 ` Duy Nguyen
2018-04-27 21:08 ` Elijah Newren
2018-04-28 6:08 ` Duy Nguyen
2018-04-29 23:47 ` Junio C Hamano
2018-04-27 21:08 ` Marc Branchaud
2018-04-28 6:16 ` Duy Nguyen
2018-04-27 17:18 ` Michał Górny
2018-04-27 19:53 ` Ævar Arnfjörð Bjarmason
2018-04-25 8:41 ` Ævar Arnfjörð Bjarmason
2018-04-26 17:15 ` Duy Nguyen
2018-04-26 17:51 ` Robin H. Johnson
2018-04-26 17:53 ` SZEDER Gábor
2018-04-26 18:45 ` Duy Nguyen
2018-04-24 14:41 ` Marc Branchaud
2018-04-25 6:58 ` Robin H. Johnson
2018-04-25 7:13 ` Michał Górny
2018-05-05 18:44 ` Jeff King
2018-05-06 3:37 ` Junio C Hamano
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