From: Jeff King <peff@peff•net>
To: Elijah Newren <newren@gmail•com>
Cc: Elijah Newren via GitGitGadget <gitgitgadget@gmail•com>,
Git Mailing List <git@vger•kernel.org>,
Jonathan Nieder <jrnieder@gmail•com>,
Sergey Organov <sorganov@gmail•com>
Subject: Re: [PATCH 5/7] tmp-objdir: new API for creating and removing primary object dirs
Date: Fri, 1 Oct 2021 01:21:47 -0400 [thread overview]
Message-ID: <YVaa650FehPmk0QD@coredump.intra.peff.net> (raw)
In-Reply-To: <CABPp-BEqcq0wYqNP1xUJj8+E5HUTnip7+asadRoeFLAZ34rTdQ@mail.gmail.com>
On Thu, Sep 30, 2021 at 07:31:44PM -0700, Elijah Newren wrote:
> > Some ways it could go wrong:
> >
> > - is it possible for the merge code to ever write an object? I kind of
> > wonder if we'd ever do any cache-able transformations as part of a
> > content-level merge. I don't think we do now, though.
>
> Yes, of course -- otherwise there would have been no need for the
> tmp_objdir in the first place. In particular, it needs to write
> three-way-content merges of individual files, and it needs to write
> new tree objects. (And it needs to do this both for creating the
> virtual merge bases if the merge is recursive, as well as doing it for
> the outer merge.)
Right, sorry, I should have added: ...in addition to the merge-results
we're writing. What I'm getting at is whether there might be _other_
parts of the code that the merge code would invoke. Say, if a
.gitattribute caused us to convert a file's encoding in order to perform
a more semantic merge, and we wanted to store that result somehow (e.g.,
in a similar cache).
I don't think anything like that exists now, but I don't find it outside
the realm of possibility in the future. It's sort of the merge
equivalent of "textconv"; we have external diff and external merge
drivers, but being able to convert contents and feed them to the regular
text diff/merge code simplifies things.
> > - in step 5, write_object_file() may still be confused by the presence
> > of the to-be-thrown-away objects in the alternate. This is pretty
> > unlikely, as it implies that the remerge-diff wrote a blob or tree
> > that is byte-identical to something that the diff wants to write.
>
> That's one reason it could be confused. The textconv filtering in
> particular was creating a new object based on an existing one, and a
> tree, and a ref. What if there was some other form of caching or
> statistic gathering that didn't write a new object based on an
> existing one, but did add trees and especially refs that referenced
> the existing object? It's not that the diff wanted to write something
> byte-identical to what the merge wrote, it's just that the diff wants
> to reference the object somehow.
Yes, good point. It can help a little if the alternate-odb added by
tmp_objdir was marked with a flag to say "hey, this is temporary". That
would help write_object_file() decide not to depend on it. But the
problem is so much wider than that that I think it will always be a bit
dangerous; any code could say "can I access this object" and we don't
know if its intent is simply to read it, or to create a new object that
references it.
-Peff
next prev parent reply other threads:[~2021-10-01 5:21 UTC|newest]
Thread overview: 64+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-08-31 2:26 [PATCH 0/7] Add a new --remerge-diff capability to show & log Elijah Newren via GitGitGadget
2021-08-31 2:26 ` [PATCH 1/7] merge-ort: mark a few more conflict messages as omittable Elijah Newren via GitGitGadget
2021-08-31 21:06 ` Junio C Hamano
2021-09-01 0:03 ` Elijah Newren
2021-09-01 17:19 ` Junio C Hamano
2021-08-31 2:26 ` [PATCH 2/7] merge-ort: add ability to record conflict messages in a file Elijah Newren via GitGitGadget
2021-09-28 22:29 ` Jeff King
2021-09-29 6:25 ` Elijah Newren
2021-09-29 16:14 ` Junio C Hamano
2021-09-29 16:31 ` Elijah Newren
2021-09-30 7:58 ` Jeff King
2021-09-30 8:09 ` Ævar Arnfjörð Bjarmason
2021-10-01 2:07 ` Elijah Newren
2021-10-01 5:28 ` Jeff King
2021-08-31 2:26 ` [PATCH 3/7] ll-merge: add API for capturing warnings in a strbuf instead of stderr Elijah Newren via GitGitGadget
2021-09-28 22:37 ` Jeff King
2021-09-28 23:49 ` Junio C Hamano
2021-09-29 4:03 ` Elijah Newren
2021-08-31 2:26 ` [PATCH 4/7] merge-ort: capture and print ll-merge warnings in our preferred fashion Elijah Newren via GitGitGadget
2021-09-28 22:39 ` Jeff King
2021-09-30 16:53 ` Ævar Arnfjörð Bjarmason
2021-10-01 1:54 ` Elijah Newren
2021-10-01 7:23 ` Ævar Arnfjörð Bjarmason
2021-08-31 2:26 ` [PATCH 5/7] tmp-objdir: new API for creating and removing primary object dirs Elijah Newren via GitGitGadget
2021-09-28 7:55 ` Ævar Arnfjörð Bjarmason
2021-09-29 4:22 ` Elijah Newren
2021-09-30 7:41 ` Jeff King
2021-09-30 14:17 ` Ævar Arnfjörð Bjarmason
2021-10-01 3:55 ` Elijah Newren
2021-09-28 23:17 ` Jeff King
2021-09-29 4:08 ` Junio C Hamano
2021-09-30 7:33 ` Jeff King
2021-09-30 13:16 ` Ævar Arnfjörð Bjarmason
2021-09-30 21:00 ` Jeff King
2021-10-01 3:11 ` Elijah Newren
2021-10-01 7:30 ` Ævar Arnfjörð Bjarmason
2021-10-01 8:03 ` Elijah Newren
2021-10-01 4:26 ` Elijah Newren
2021-10-01 5:27 ` Jeff King
2021-10-01 7:43 ` Ævar Arnfjörð Bjarmason
2021-09-29 5:05 ` Elijah Newren
2021-09-30 7:26 ` Jeff King
2021-09-30 7:46 ` Jeff King
2021-09-30 20:06 ` Junio C Hamano
2021-10-01 3:59 ` Elijah Newren
2021-10-01 16:36 ` Junio C Hamano
2021-10-01 2:31 ` Elijah Newren
2021-10-01 5:21 ` Jeff King [this message]
2021-09-30 19:25 ` Neeraj Singh
2021-09-30 20:19 ` Junio C Hamano
2021-09-30 20:00 ` Junio C Hamano
2021-10-01 2:23 ` Elijah Newren
2021-08-31 2:26 ` [PATCH 6/7] show, log: provide a --remerge-diff capability Elijah Newren via GitGitGadget
2021-08-31 9:19 ` Sergey Organov
2021-09-28 8:01 ` Ævar Arnfjörð Bjarmason
2021-09-29 4:00 ` Elijah Newren
2021-08-31 2:26 ` [PATCH 7/7] doc/diff-options: explain the new --remerge-diff option Elijah Newren via GitGitGadget
2021-08-31 11:05 ` [PATCH 0/7] Add a new --remerge-diff capability to show & log Bagas Sanjaya
2021-08-31 16:16 ` Elijah Newren
2021-08-31 20:03 ` Junio C Hamano
2021-08-31 20:23 ` Elijah Newren
2021-09-01 21:07 ` Junio C Hamano
2021-09-01 21:42 ` Elijah Newren
2021-09-01 21:55 ` Junio C Hamano
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