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

> 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.