Subject: Re: [PATCH 1/2] sequencer: truncate labels to accommodate loose refs

From: Junio C Hamano <[email-masked]>
Date: Thu, 10 Aug 2023 10:12:12 -0700

"Mark Ruvald Pedersen via GitGitGadget" <[email-masked]>
writes:

> +/*
> + * To accommodate common filesystem limitations, where the loose refs' file
> + * names must not exceed `NAME_MAX`, the labels generated by `git rebase
> + * --rebase-merges` need to be truncated if the corresponding commit subjects
> + * are too long.
> + * Add some margin to stay clear from reaching `NAME_MAX`.
> + */
> +#define GIT_MAX_LABEL_LENGTH ((NAME_MAX) - (LOCK_SUFFIX_LEN) - 16)

OK.  Hopefully no systems defien NAME_MAX shorter than 20 bytes ;-).

We may suffix "-%d" to make it unique after this truncation, so
there definitely is a need for some slop, and 16-bytes should
sufficiently be long.

> @@ -5404,14 +5415,34 @@ static const char *label_oid(struct object_id *oid, const char *label,
>  		 *
>  		 * Note that we retain non-ASCII UTF-8 characters (identified
>  		 * via the most significant bit). They should be all acceptable
> -		 * in file names. We do not validate the UTF-8 here, that's not
> -		 * the job of this function.
> +		 * in file names.
> +		 *
> +		 * As we will use the labels as names of (loose) refs, it is
> +		 * vital that the name not be longer than the maximum component
> +		 * size of the file system (`NAME_MAX`). We are careful to
> +		 * truncate the label accordingly, allowing for the `.lock`
> +		 * suffix and for the label to be UTF-8 encoded (i.e. we avoid
> +		 * truncating in the middle of a character).
>  		 */
> -		for (; *label; label++)
> -			if ((*label & 0x80) || isalnum(*label))
> +		for (; *label && buf->len + 1 < max_len; label++)
> +			if (isalnum(*label) ||
> +			    (!label_is_utf8 && (*label & 0x80)))
>  				strbuf_addch(buf, *label);
> +			else if (*label & 0x80) {
> +				const char *p = label;
> +
> +				utf8_width(&p, NULL);
> +				if (p) {
> +					if (buf->len + (p - label) > max_len)
> +						break;
> +					strbuf_add(buf, label, p - label);
> +					label = p - 1;
> +				} else {
> +					label_is_utf8 = 0;
> +					strbuf_addch(buf, *label);
> +				}

Utf8_width() does let you advance one unicode character at a time as
its side effect, but it may be a bit overkill, as its primary
function is to compute the display width of that character.

We could take advantage of the fact that the first byte of a UTF-8
character has two high-bits set (i.e. 11xxxxxx) while the second and
subsequent bytes have only the top-bit set and the second highest
bit clear (i.e. 10xxxxxx) to simplify/optimize it.  If this were in
a performance sensitive codepath, that is.

I'll queue it as-is for now, as we are in "regression fix only"
phase of the cycle, and have enough time to polish it.

Thanks.

>  			/* avoid leading dash and double-dashes */
> -			else if (buf->len && buf->buf[buf->len - 1] != '-')
> +			} else if (buf->len && buf->buf[buf->len - 1] != '-')
>  				strbuf_addch(buf, '-');
>  		if (!buf->len) {
>  			strbuf_addstr(buf, "rev-");