Re: [PATCH v3 1/7] git-compat-util: introduce skip_to_optional_arg()

[Jeff King <peff@peff.net>]

On Sun, Dec 10, 2017 at 09:31:18AM -0500, Jeff King wrote:

> On Sat, Dec 09, 2017 at 09:40:07PM +0100, Christian Couder wrote:
> 
> > The changes compared to v2 are:
> > 
> >   - s/_val/_arg/ in the name of the functions
> >   - s/val/arg/ in the name of the third argument of the functions
> >   - works with NULL as third argument of the functions
> 
> This whole series looks OK to me, but this third point made me wonder:
> what would be the use of allowing NULL for the "arg" parameter?
> 
> I didn't see any use of this in the series, and I'm having trouble
> figuring out how it would be useful. E.g., if I do:
> 
>   if (skip_to_optional_arg(arg, "--foo", NULL))
>      ...
> 
> what can I do in "..."? I know we matched _some_ type of "--foo", but I
> cannot know whether it was "--foo" or "--foo=bar", nor what "bar" is. It
> could only be used by some kind of vague validator to say "well, at
> least this looks like an option that I _could_ parse if I wanted to".
> 
> So I guess I don't mind it, as it does the most reasonable thing it can
> when passed NULL, but I would be surprised if we ever actually exercise
> the code path.

And of course as soon as I sent this, I went back and double-checked.
And indeed I totally missed this call:

  +       else if (starts_with(arg, "-B") ||
  +                skip_to_optional_arg(arg, "--break-rewrites", NULL)) {
	  if ((options->break_opt = diff_scoreopt_parse(arg)) == -1)

So that's kind-of weird, because we are parsing "-B", etc, and then
expecting it to be _reparsed_ by diff_scoreopt_parse. So the two
callsites must always match. IMHO this ought to do either:

  - we should just ask diff_scoreopt_parser to tell us if this was a
    valid option that it understood

or

  - parse up to the "=", and then ask the scoreopt parser to parse the
    remainder. This would require us passing 'B'/'C'/'M' to the
    function ourselves, I think that's a better pattern. It means we
    could reuse the parser for things like config values if we wanted to
    (our current diff.renames is a bool, but it would not be
    unreasonable for it to take a score).

None of that is a mess of your creation, though, so I'm OK punting on it
for now.

-Peff