Subject: Re: [PATCH 1/2] FIX: use utf8_strnwidth for line_prefix in diff.c

From: Junio C Hamano <[email-masked]>
Date: Wed, 24 Jan 2024 12:08:02 -0800

"Md Isfarul Haque via GitGitGadget" <[email-masked]> writes:

> From: Md Isfarul Haque <[email-masked]>
>
> This patch adresses diff.c:2721 and proposes the fix using a new function.

Once the issue has fully been addressed, it is expected that the
NEEDSWORK comment there would be removed, making this proposed log
message useless.  Make it a habit to write a log message that is
self-contained enough to help readers (including yourself in the
future when you have forgotten the details of what you did in this
commit).

> +const struct strbuf *diff_line_prefix_buf(struct diff_options *opt)
> +{

Given that there is only one caller of this function in the same
file, I do not see a reason why this needs to be extern and exported
in diff.h (actually I do not see a need for this helper at all).

When dealing with a string buffer, it is much more common in this
codebase for the caller to prepare a strbuf (often on its stack) and
pass a pointer to it to helper functions.  I.e.

	static void prepare_diff_line_prefix_buf(struct strbuf *buf,
						struct diff_options *opt)
	{
		... stuff whatever you need into the string buffer ...
                strbuf_add(buf, ...);
	}

	/* in show_stats() */
	struct strbuf line_prefix = STRBUF_INIT;
	...
	prepare_diff_line_prefix_buf(&line_prefix, options);
	... use line_prefix and ...
	... release the resource before returning ...
	strbuf_release(&line_prefix);
	
is more common and less prone to resource leak over time.

> @@ -2635,7 +2649,7 @@ static void show_stats(struct diffstat_t *data, struct diff_options *options)
>  	int width, name_width, graph_width, number_width = 0, bin_width = 0;
>  	const char *reset, *add_c, *del_c;
>  	int extra_shown = 0;
> -	const char *line_prefix = diff_line_prefix(options);
> +	const struct strbuf *line_prefix = diff_line_prefix_buf(options);
>  	struct strbuf out = STRBUF_INIT;
>  
>  	if (data->nr == 0)
> @@ -2718,7 +2732,7 @@ static void show_stats(struct diffstat_t *data, struct diff_options *options)
>  	 * used to correctly count the display width instead of strlen().
>  	 */
>  	if (options->stat_width == -1)
> -		width = term_columns() - strlen(line_prefix);
> +		width = term_columns() - utf8_strnwidth(line_prefix->buf, line_prefix->len, 1);

I do not see the need for any of the diff_line_prefix_buf() related
changes, only to do this change.  You have a const char *line_prefix
at this point, and utf8_strnwidth() wants to know its length, so
what you need is to massage the parameter to match what it wants.
Perhaps even something simple and dumb like

	utf8_strnwidth(line_prefix, strlen(line_prefix), 1);

might be sufficient to replace strlen(line_prefix) in the original?

This patch hopefully will change the behaviour of the command.  A
patch needs to also protect the change from future breakages by
adding a test or two to demonstrate the desired behaviour.  Such a
test should pass with the code change in the patch, and should fail
when the code change in the patch gets reverted.

Thanks.