Re: [PATCH 4/6] [GSOC] ref-filter: add %(rest) atom and --rest option

From: ZheNing Hu <adlternative@gmail.com>
To: Junio C Hamano <gitster@pobox.com>
Cc: ZheNing Hu via GitGitGadget <gitgitgadget@gmail.com>,
	Git List <git@vger.kernel.org>,
	Christian Couder <christian.couder@gmail.com>,
	Hariom Verma <hariom18599@gmail.com>
Subject: Re: [PATCH 4/6] [GSOC] ref-filter: add %(rest) atom and --rest option
Date: Mon, 7 Jun 2021 21:02:28 +0800	[thread overview]
Message-ID: <CAOLTT8S+5m+-XF-AcQi9t8njTvyDYzHt=BU+4OPcvTT27RP6dw@mail.gmail.com> (raw)
In-Reply-To: <xmqq7dj6w7a6.fsf@gitster.g>

Junio C Hamano <gitster@pobox.com> 于2021年6月7日周一 下午1:52写道：
>
> "ZheNing Hu via GitGitGadget" <gitgitgadget@gmail.com> writes:
>
> > From: ZheNing Hu <adlternative@gmail.com>
> >
> > In order to let "cat-file --batch=%(rest)" use the ref-filter
> > interface, add %(rest) atom for ref-filter and --rest option
> > for "git for-each-ref", "git branch", "git tag" and "git verify-tag".
> > `--rest` specify a string to replace %(rest) placeholders of
> > the --format option.
>
> I cannot think of a sane reason why we need to allow "%(rest)" in
> anithing but "cat-file --batch", where a natural source of %(rest)
> exists in its input stream (i.e. each input record begins with an
> object name to be processed, and the rest of the record can become
> "%(rest)").
>

First of all, although %(rest) is meaningless in ordinary circumstances,
ref-filter must learn %(rest), it is impossible for us to leave the parsing
of %(rest) in cat-file.c alone.

Then, `--rest` is a strategy that make %(rest) can use in `git for-each-ref`
or `git branch -l`. As you said, it is just a boring placeholder used for string
replacement. We can make it output only empty content, If we really don’t
need `--rest`.

> The "cat-file --batch" thing is much more understandable.  You could
> for example:
>
>     git ls-files -s |
>     sed -e 's/^[0-7]* \([0-9a-f]*\) [0-3]       /\1 /' |
>     git cat-file --batch='%(objectname) %(objecttype) %(rest)'
>

s/[0-3]       /[0-3]\t/

> to massage output from "ls-files -s" like this
>
>     100644 c2f5fe385af1bbc161f6c010bdcf0048ab6671ed 0   .cirrus.yml
>     100644 c592dda681fecfaa6bf64fb3f539eafaf4123ed8 0   .clang-format
>     100644 f9d819623d832113014dd5d5366e8ee44ac9666a 0   .editorconfig
>     ...
>
> into recods of "<objectname> <path>", and each output record will
> replay the <path> part from each corresponding input record.
>

Yeah, the <path> in the input will be treated as "rest".

> Unless for-each-ref family of commands read the list of refs that it
> shows from their standard input (they do not, and I do not think it
> makes any sense to teach them to), there is no place to feed the
> "rest" information that is associated with each output record.  The
> only thing the commands taught about %(rest) by this patch can do is
> to parrot the same string into each and every output record.  I am
> not seeing what this new feature is attempting to give us.
>

"parrot the same string"? I think we should use an empty string here,
"parrot the same string" more like what the "git log --format" family does.

> If anything, I would imagine that it would be a very useful addition
> to teach the ref-filter machinery an ability to optionally error out
> depending on the caller when the caller attempts to use certain
> placeholder.  Then, we can reject "git branch --sort=rest" sensibly,
> instead of accepting "git branch --sort=rest --rest=constant", which
> is not technically wrong per-se, but smells like a total nonsense from
> practical usefulness's point of view.
>

This sounds like it might help `cat-file` to reject some useless atoms
like %(refname). So something like:

$ git for-each-ref --format="%(objectname) %(objectsize)"
--refject-atoms="%(objectsize) %(objectname)"

will fail.

"git for-each-ref" family use hardcoded to reject %(rest).
I can try to achieve this function.

> > -     [--list] [<pattern>...]
> > +     [--list] [<pattern>...] [--rest=<rest>]
> >  'git branch' [--track | --no-track] [-f] <branchname> [<start-point>]
> >  'git branch' (--set-upstream-to=<upstream> | -u <upstream>) [<branchname>]
> >  'git branch' --unset-upstream [<branchname>]
> > @@ -298,6 +298,10 @@ start-point is either a local or remote-tracking branch.
> >       and the object it points at.  The format is the same as
> >       that of linkgit:git-for-each-ref[1].
> >
> > +--rest=<rest>::
> > +     If given, the `%(rest)` placeholders in the `--format` option
> > +     will be replaced.
>
> If not given, what happens?

Will output an empty string.

Hope we can reach an agreement:
delete `--rest` and add `--reject-atoms`. ;-)

Thanks.
--
ZheNing Hu