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

> On Tue, Aug 27, 2024 at 05:14:14PM -0700, Junio C Hamano wrote:
> > > 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.
> 
> Good question, and good answer ;-).
> 
> This is only relevant in a read-only path where we're generating a new
> pack from existing packs and not altering those pack or rewriting /
> deleting the MIDX attached to them. So I think we're OK here and don't
> have any lifetime/scope issues.