Re: [PATCH v2 2/3] diff: add -I<regex> that ignores matching changes

[Johannes Schindelin <Johannes.Schindelin@gmx.de>]

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Hi Junio,

On Mon, 12 Oct 2020, Junio C Hamano wrote:

> Johannes Schindelin <Johannes.Schindelin@gmx.de> writes:
>
> > Hi Michał,
> >
> > On Mon, 12 Oct 2020, Michał Kępień wrote:
> >
> >> @@ -5203,6 +5207,22 @@ static int diff_opt_patience(const struct option *opt,
> >>  	return 0;
> >>  }
> >>
> >> +static int diff_opt_ignore_regex(const struct option *opt,
> >> +				 const char *arg, int unset)
> >> +{
> >> +	struct diff_options *options = opt->value;
> >> +	regex_t *regex;
> >> +
> >> +	BUG_ON_OPT_NEG(unset);
> >> +	regex = xcalloc(1, sizeof(*regex));
> >> +	if (regcomp(regex, arg, REG_EXTENDED | REG_NEWLINE))
> >> +		die("invalid regex: %s", arg);
> >> +	ALLOC_GROW(options->ignore_regex, options->ignore_regex_nr + 1,
> >> +		   options->ignore_regex_alloc);
> >> +	options->ignore_regex[options->ignore_regex_nr++] = regex;
> >
> > A slightly more elegant way would be to have `ignore_regex` have the type
> > `regex_t *` and use `ALLOC_GROW_BY()` (which zeroes the newly-added
> > elements automagically).
>
> It may be "elegant", but we we know if it is "correct" on
> everybody's implementation of regex_t?
>
> A struct like
>
> 	struct foo {
> 		char *str;
> 		char in_place_buffer[10];
> 	};
>
> where str points at in_place_buffer[] only when it needs to point at
> a very short string, and points at an allocated memory on heap, can
> not be safely copied and used, and an array of such a struct breaks
> when ALLOC_GROW_BY() needs to call realloc().  Keeping an array of
> pointers to regex_t and growing it would not break even for such a
> structure type.
>
> Since we cannot rely on the implementation detail of regex_t on a
> single platform, I'd rather not to see anybody suggest such an
> "elegant" approach.

True, such an implementation detail is totally conceivable.

Thanks for the sanity check.

Having said that, my suggestion to use `ALLOC_GROW_BY()` was triggered by
the use of `xcalloc()`, and now that I think further about it, an
`xmalloc()` should be plenty enough: `regcomp()` does not need that struct
to be initialized to all zero.

Ciao,
Dscho