Re: [PATCH 09/13] parse-options API: don't restrict OPT_SUBCOMMAND() to one *_fn type

> When the OPT_SUBCOMMAND() API was implemented in [1] it did so by
> adding a new "subcommand_fn" member to "struct option", rather than
> allowing the user of the API to pick the type of the function.
>
> An advantage of mandating that "parse_opt_subcommand_fn" must be used
> is that we'll get type checking for the function we're passing in, a
> disadvantage is that we can't convert e.g. "builtin/bisect--helper.c"
> easily to it, as its callbacks need their own argument.
>
> Let's generalize this interface, while leaving in place a small hack
> to give the existing API users their type safety. We assign to
> "typecheck_subcommand_fn", but don't subsequently use it for
> anything. Instead we use the "defval" and "value" members.
>
> A subsequent commit will add a OPT_SUBCOMMAND() variant where the
> "callback" isn't our default "parse_options_pick_subcommand" (and that
> caller won't be able to use the type checking).
>
> 1. fa83cc834da (parse-options: add support for parsing subcommands,
>    2022-08-19)
>
> Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
> ---
>  parse-options.c |  9 ++++++---
>  parse-options.h | 25 +++++++++++++++++++++----
>  2 files changed, 27 insertions(+), 7 deletions(-)
>
> diff --git a/parse-options.c b/parse-options.c
> index a1ec932f0f9..1d9e46c9dc7 100644
> --- a/parse-options.c
> +++ b/parse-options.c
> @@ -427,7 +427,8 @@ static enum parse_opt_result parse_subcommand(const char *arg,
>  	for (; options->type != OPTION_END; options++)
>  		if (options->type == OPTION_SUBCOMMAND &&
>  		    !strcmp(options->long_name, arg)) {
> -			*(parse_opt_subcommand_fn **)options->value = options->subcommand_fn;
> +			if (options->callback(options, arg, 0))
> +				BUG("OPT_SUBCOMMAND callback returning non-zero");
>  			return PARSE_OPT_SUBCOMMAND;
>  		}
>
> @@ -506,8 +507,10 @@ static void parse_options_check(const struct option *opts)
>  			       "That case is not supported yet.");
>  			break;
>  		case OPTION_SUBCOMMAND:
> -			if (!opts->value || !opts->subcommand_fn)
> -				optbug(opts, "OPTION_SUBCOMMAND needs a value and a subcommand function");
> +			if (!opts->value || !opts->callback)
> +				optbug(opts, "OPTION_SUBCOMMAND needs a value and a callback function");
> +			if (opts->ll_callback)
> +				optbug(opts, "OPTION_SUBCOMMAND uses callback, not ll_callback");
>  			if (!subcommand_value)
>  				subcommand_value = opts->value;
>  			else if (subcommand_value != opts->value)
> diff --git a/parse-options.h b/parse-options.h
> index b6ef86e0d15..61e3016c3fc 100644
> --- a/parse-options.h
> +++ b/parse-options.h
> @@ -128,19 +128,24 @@ typedef int parse_opt_subcommand_fn(int argc, const char **argv,
>   *			 the option takes optional argument.
>   *
>   * `callback`::
> - *   pointer to the callback to use for OPTION_CALLBACK
> + *   pointer to the callback to use for OPTION_CALLBACK and OPTION_SUBCOMMAND.
>   *
>   * `defval`::
>   *   default value to fill (*->value) with for PARSE_OPT_OPTARG.
>   *   OPTION_{BIT,SET_INT} store the {mask,integer} to put in the value when met.
> + *   OPTION_SUBCOMMAND stores the pointer the function selected for
> + *   the subcommand.
> + *
>   *   CALLBACKS can use it like they want.
>   *
>   * `ll_callback`::
>   *   pointer to the callback to use for OPTION_LOWLEVEL_CALLBACK
>   *
>   * `subcommand_fn`::
> - *   pointer to a function to use for OPTION_SUBCOMMAND.
> - *   It will be put in value when the subcommand is given on the command line.
> + *   pointer to the callback used with OPT_SUBCOMMAND() and
> + *   OPT_SUBCOMMAND_F(). Internally we store the same value in
> + *   `defval`. This is only here to give the OPT_SUBCOMMAND{,_F}()
> + *   common case type safety.
>   */
>  struct option {
>  	enum parse_opt_type type;
> @@ -217,12 +222,24 @@ struct option {
>  #define OPT_ALIAS(s, l, source_long_name) \
>  	{ OPTION_ALIAS, (s), (l), (source_long_name) }
>
> +static inline int parse_options_pick_subcommand_cb(const struct option *option,
> +						   const char *arg UNUSED,
> +						   int unset UNUSED)
> +{
> +	parse_opt_subcommand_fn *fn = (parse_opt_subcommand_fn *)option->defval;
> +	*(parse_opt_subcommand_fn **)option->value = fn;

> +	return 0;
> +}
> +
>  #define OPT_SUBCOMMAND_F(l, v, fn, f) { \
>  	.type = OPTION_SUBCOMMAND, \
>  	.long_name = (l), \
>  	.value = (v), \
>  	.flags = (f), \
> -	.subcommand_fn = (fn) }
> +	.defval = (intptr_t)(fn), \
> +	.subcommand_fn = (fn), \
> +	.callback = parse_options_pick_subcommand_cb, \