Re: [PATCH 2/3] [GSOC] ref-filter: support %(contents) for blob, tree

[ZheNing Hu <adlternative@gmail.com>]

Junio C Hamano <gitster@pobox.com> 于2021年5月26日周三 上午1:11写道：
>
> ZheNing Hu <adlternative@gmail.com> writes:
>
> > So I don't know if adding %(header) will cause duplication of functions.
>
> I do not think you can duplicate the feature of %(header) with
> other existing placeholders.  You may want to dump the headers of
> series of commits so that you can find if there is any commit with
> unusal header elements, but %(tree), %(parents), etc. would by
> definition be the known ones, so any combination of them will not be
> a substitute.
>

Well, "find if there is any commit with unusal header elements"
is indeed one thing that may need to be done (not now).

I tried to build "%(header)" yesterday and found some problems:

1. These atoms are too "flat":
"tree", "parent", "numparent", "object"... they are all part of the
header, why not use "%(header:tree)" like "%(contents:subject)"
does? Similarly, why not use "%(author:name)" instead of "%(authorname)"...

2. Why is there no state machine to save the parsing state of an object?
`parse_tag_buffer()`, `parse_commit_buffer()`, they only parse part of the
content of the object. E.g. tag parsed "object", "type", "tag" and "taggerdate"
in `parse_tag_buffer()`, but it did not save the starting position and length of
the parsed fields, which caused `grab_person()`, `grab_date()` to parse the
object again and again. I think this may be an optimization point.

> But nobody is asking for it right now, and your cat-file --batch
> does not need it right away.
>

Or we only provide the simplest "%(header)" and "%(header:size)"
(it can be easily supported with "%(raw)"), leaving the other feature
to the future.

> What I wanted to say in the message you are responding to was *not*
> that you would want to add %(header) for completeness right now.
> But thinking beyond your immediate need (i.e. the "whole thing") and
> imagining how various pieces, including the ones that do not exist
> yet, would fit together, would help you to avoid designing the parts
> you immediately need in a way you would regret in the future.

As I said before, many atomic designs are not very regular.
Our ultimate goal may be we can simultaneously support:

"%(raw)"
"%(raw:header)"
"%(raw:header:tagger)"
"%(raw:header:tagger:eamil)"
"%(raw:header:taggereamil)"
"%(header:taggereamil)"
"%(header:tagger:eamil)"
"%(taggereamil)"
"%(tagger:eamil)"
...

Each layer is a namespace. Of course, it may take a long
distance to support this.

Thank.
--
ZheNing Hu