Re: [PATCH v3 0/2] check-attr: add support to work with revisions

From: "Ævar Arnfjörð Bjarmason" <avarab@gmail.com>
To: Junio C Hamano <gitster@pobox.com>
Cc: Karthik Nayak <karthik.188@gmail.com>,
	git@vger.kernel.org, toon@iotcl.com
Subject: Re: [PATCH v3 0/2] check-attr: add support to work with revisions
Date: Mon, 19 Dec 2022 09:45:34 +0100	[thread overview]
Message-ID: <221219.86fsdb22hb.gmgdl@evledraar.gmail.com> (raw)
In-Reply-To: <xmqq7cyrm0tr.fsf@gitster.g>

On Sat, Dec 17 2022, Junio C Hamano wrote:

> Ævar Arnfjörð Bjarmason <avarab@gmail.com> writes:
>
>> So we could invent that as this series currently does with:
>>
>> 	git check-attrs --revision <rev> <attr>... <path>...
>>
>> Or, as I suggested:
>>
>>         git check-attr [<rev>:]<attr>... -- <path>...
>
> What does <rev>:<attr> really mean?  As the syntax for the proposed
> feature, I do not think it makes much sense.  For example:
>
>   $ git check-attr HEAD:text HEAD^:text -- README.txt
>
>  - With which README.txt are we checking the attribute?  The one
>    taken from HEAD or HEAD^ or the index or the working tree?

All of them, but I do think this rightly points out that the "rev before
path" part of this doesn't make sense, but shouldn't we be making this
work like "git grep" with <rev>/<path> combinations? I.e.:

	$ git -P grep -m 1 oid HEAD~:cache.h v2.26.0:cache.h v1.6.0:cache.h
	HEAD~:cache.h:#include "oid-array.h"
	v2.26.0:cache.h:void git_inflate_init(git_zstream *);
	v1.6.0:cache.h:static inline void copy_cache_entry(struct cache_entry *dst, struct cache_entry *src)

I.e. we currently support:

	git check-attr [-a | --all | <attr>...] [--] <pathname>...
	git check-attr --stdin [-z] [-a | --all | <attr>...]

So if we add to that:

	git check-attr --stdin [-z] <rev>:<pathname>...

We'd have this do the right thing:

	$ git check-attr diff -- README.md HEAD:git-send-email.perl v1.6.0:git-send-email.perl
	README.md: diff: unspecified
	HEAD:git-send-email.perl: diff: perl
	v1.6.0:git-send-email.perl: diff: perl

Which would technically break backwards compatibility, as we now
"support" it (we just interpret the whole thing as a path), but I think
such revision-looking paths aren't worth worrying about

>  - When we say "README.txt has the text attribute", how does the
>    user tell which "text" applies to the path?  From HEAD?  From
>    HEAD^?

Regardless of what I'm suggesting here, the "git check-attr" output
already has a one-to-one line output correspondance with its input, so
just as it does now we'd print both.

This looks like a bug though (on master, the missing "\n" is there in
the output):

	$ ./git check-attr diffgit-send-email.perl foo.perl git-send-email.perl
	foo.perl: diffgit-send-email.perl: unspecified
	git-send-email.perl: diffgit-send-email.perl: unspecified

>  - Does the same attribute 'text' have different meaning when coming
>    from two different tree-ish?

Yes, just like "git grep", we'd need to parse & apply the .gitattributes
for that revision. Whether we call it "<rev>:<path>", "--revision <rev>
<path>" or whatever we'd always want to do that, otherwise what's the
point?