Re: [PATCH v2 2/3] parse-options: make OPT_ARGUMENT() more useful

[Jeff King <peff@peff.net>]

On Thu, Mar 14, 2019 at 04:25:04AM -0700, Johannes Schindelin via GitGitGadget wrote:

> diff --git a/Documentation/technical/api-parse-options.txt b/Documentation/technical/api-parse-options.txt
> index 2b036d7838..2e2e7c10c6 100644
> --- a/Documentation/technical/api-parse-options.txt
> +++ b/Documentation/technical/api-parse-options.txt
> @@ -198,8 +198,10 @@ There are some macros to easily define options:
>  	The filename will be prefixed by passing the filename along with
>  	the prefix argument of `parse_options()` to `prefix_filename()`.
>  
> -`OPT_ARGUMENT(long, description)`::
> +`OPT_ARGUMENT(long, &int_var, description)`::
>  	Introduce a long-option argument that will be kept in `argv[]`.
> +	If this option was seen, `int_var` will be set to one (except
> +	if a `NULL` pointer was passed).

So this effectively makes it into a "bool" that we keep. I think that's
fine. It always uses NOARG, so it is not like we would ever need to see
"we got --foo, and this is the argument it had".

I did wonder if it was possible for "--no-foo" to trigger this (leaving
the caller who looks at the int unsure if they saw "--foo" or
"--no-foo"), but it seems that the parse-options code checks for
OPTION_ARGUMENT before it ever looks at negation.

Curiously, it also checks it before doing the usual prefix-matching
magic. So you could otherwise say "--no-inde", but OPT_ARGUMENT() will
not allow it. I think that's probably sane and not worth thinking
further about, but it is an interesting quirk that a user could possibly
run into.

> diff --git a/parse-options.c b/parse-options.c
> index cec74522e5..1d57802da0 100644
> --- a/parse-options.c
> +++ b/parse-options.c
> @@ -286,6 +286,8 @@ static enum parse_opt_result parse_long_opt(
>  					     optname(options, flags));
>  			if (*rest)
>  				continue;
> +			if (options->value)
> +				*(int *)options->value = options->defval;

Cute. You could actually assign any defval you like, though of course
the convenient OPT_ARGUMENT() macro just always uses 1.

I wondered if you might need another cast for defval itself, but it's an
intptr_t (so it's the types that use it as a string that need to cast to
"const char *").

This looks very clean overall, and I agree it's much nicer than the
alternatives for your use case.

-Peff