Re: [PATCH] attr: do not mark queried macros as unset

[Junio C Hamano <gitster@pobox.com>]

Jeff King <peff@peff.net> writes:

> And here's a resurrection of the optimization that _seems_ to work, but
> I'm not 100% confident in.
>
> In particular, it does not care about macros at all. It simply asks: is
> this queried attribute a thing which was ever mentioned in the
> attributes files (either as a path match or as a possible macro
> expansion). If not, then we know we do not need to look further for it.

So, if we are looking for 'diff' and we know no .gitattributes (or
$GIT_DIR/info/attributes) entry for 'diff' or any macro that expands
to touch 'diff' (e.g. 'binary') is in use, we know for any path
governed by the current attr-stack 'diff' attribute is unspecified.
But if we see an entry, say, "*.exe binary", then we do need to be
aware of the possibility that 'diff' may be unset for some paths.

Makes sense.

> I guess maybe what I'm missing is that asking for "diff" means that we
> need to care about:
>
>   - whether "diff" was mentioned in the stack
>
>   - whether "binary" was mentioned in the stack
>
> But just "binary" mentioning "diff" is not interesting without somebody
> actually mentioning "binary".

Yeah, that matches my understanding (which mostly comes from the
original design before even Duy's optimization).

> I'm not sure how to do it robustly without being able to reverse-map all
> of the macros after we've resolved them (i.e., to know that "diff" gets
> mentioned by "binary", and then check if "binary" is actually
> mentioned). I think that would be possible now, as we should know that
> after determine_macros(). But I also wonder if we are hitting
> diminishing returns (after all, determine_macros() is already walking
> the attr stack).