bug#71179: [PATCH] In rgrep, check matching files before excluding files

[Eli Zaretskii <eliz@gnu.org>]

> From: Spencer Baugh <sbaugh@janestreet.com>
> Date: Fri, 24 May 2024 16:14:39 -0400
> 
> In my benchmarking, this takes (rgrep "foo" "*.el" "~/src/emacs/trunk/")
> from ~410ms to ~130ms.

Which is a minor improvement at best, possibly a negligible one.  In
my testing (on MS-Windows), I see a barely-tangible improvement: 0.7%.

> Date: Fri, 24 May 2024 23:45:00 +0300
> From: Dmitry Gutov <dmitry@gutov.dev>
> 
> > In my benchmarking, this takes (rgrep "foo" "*.el" "~/src/emacs/trunk/")
> > from ~410ms to ~130ms.
> 
> I can confirm improvement here (though not exactly 3x).
> 
> 1.9s to 1.3s in a Linux checkout, for example. Nice.

Which is still quite minor.

> Moving the files exclude instructions to the <F> placeholder is a slight 
> incompatibility

Right, and for that reason, we cannot install this change as-is.  We
need either a different command or a user option controlling the order
(with a good explanation of the effect of the difference).

> Cc: 71179@debbugs.gnu.org
> From: Spencer Baugh <sbaugh@janestreet.com>
> Date: Fri, 24 May 2024 16:54:37 -0400
> 
> > Moving the files exclude instructions to the <F> placeholder is a
> > slight incompatibility, but I wonder if there are any custom
> > grep-find-template values which would be bothered by it (that's the
> > only incompatibility I could think of). Perhaps those that currently
> > don't include <X> at all?
> 
> A grep-find-template that doesn't include <X> will indeed start seeing
> ignores based on grep-find-ignored-files in rgrep.  But, such a user can
> just set grep-find-ignored-files to nil and then they'll stop seeing
> ignores again.

That's not a valid argument for changing the default behavior.
Because I could counter-argue that if you don't care about the order
and want those few hundreds of millisecond at all costs, then _you_
can customize the template to your liking, leaving the default
behavior intact.

> Also, for what it's worth, note that the documentation for
> grep-find-template says this:
> 
>   <X> - find options to restrict or expand the directory list
>   <F> - find options to limit the files matched
> 
> So this change makes the documentation more accurate: <X> previously
> also affected the files matched, but now it only affects the directory
> list, as documented.  <F> continues to limit the files matched, as
> before.

Sorry, such incompatible changes are not acceptable, definitely when
the gain is so small.  Correctness trumps speed.