Re: [PATCH 1/7] chunk-format: introduce `pair_chunk_expect()` helper

From: Junio C Hamano <gitster@pobox.com>
Date: Nov 10, 2023, 23:38 UTC
Message-ID: <xmqqa5rluqk1.fsf@gitster.g>
In-Reply-To: <ZU5Z/Z4PcdNP5U1r@nand.local>

Taylor Blau <me@ttaylorr.com> writes:

Show 7 quoted lines

> But I don't think we enjoy the same benefits in the MIDX scenario. In
> this case, the num_objects field is just:
>
>     m->num_objects = ntohl(m->chunk_oid_fanout[255])
>
> so I don't think we can make the same guarantees about what is and isn't
> save to compute under size_t arithmetic.

So ..., from the correctness's point of view, if we do not mind st_mult() dying, the "multiply-and-compare" should give us much more robust results. If we do mind st_mult() dying, we could pair the "truncating-division-and-compare" you wrote with "ensure that chunk size is a multiple of record size" to achieve the same, I would think. I.e.,

	if (chunk_size % pcd->record_size ||
	    chunk_size / pcd->record_size != pcd->record_nr)
		return -1;

or something like that.