Subject: Re: [PATCH 2/5] pack-bitmap: tag bitmapped packs with their corresponding MIDX

From: Junio C Hamano <[email-masked]>
Date: Tue, 27 Aug 2024 17:14:14 -0700

Taylor Blau <[email-masked]> writes:

> The next commit will need to use the bitmap's MIDX (if one exists) to
> translate bit positions into pack-relative positions in the source pack.
>
> Ordinarily, we'd use the "midx" field of the bitmap_index struct. But
> since that struct is defined within pack-bitmap.c, and our caller is in
> a separate compilation unit, we do not have access to the MIDX field.
>
> Instead, add a "from_midx" field to the bitmapped_pack structure so that
> we can use that piece of data from outside of pack-bitmap.c. The caller
> that uses this new piece of information will be added in the following
> commit.
>
> Signed-off-by: Taylor Blau <[email-masked]>
> ---
>  midx.c        | 1 +
>  pack-bitmap.c | 1 +
>  pack-bitmap.h | 1 +
>  3 files changed, 3 insertions(+)
>
> diff --git a/midx.c b/midx.c
> index ca98bfd7c6..67e0d64004 100644
> --- a/midx.c
> +++ b/midx.c
> @@ -496,6 +496,7 @@ int nth_bitmapped_pack(struct repository *r, struct multi_pack_index *m,
>  				 MIDX_CHUNK_BITMAPPED_PACKS_WIDTH * local_pack_int_id +
>  				 sizeof(uint32_t));
>  	bp->pack_int_id = pack_int_id;
> +	bp->from_midx = m;

Do multi_pack_index objects live as long as bitmapped_pack objects
that point at them live?  If m later goes away without letting the
bitmapped_pack know about it, the borrowed pointer in from_midx
would become dangling, which is not what we want to see.

"None of these objects are released or relocated while we are
running pack-objects, so once the .from_midx member is assigned
here, it will always be pointing at a valid multi_pack_index object"
is a satisfactory answer, I guess.