Re: [PATCH 2/2] builtin/pack-objects.c: ensure pack validity from MIDX bitmap objects

[Taylor Blau <me@ttaylorr.com>]

On Fri, May 13, 2022 at 04:06:39PM -0700, Jonathan Tan wrote:
> Taylor Blau <me@ttaylorr.com> writes:
> > An alternative approach to closing this race would have been to call
> > `is_pack_valid()` on _all_ packs in a multi-pack bitmap on load. This
> > has a couple of problems:
> >
> >   - it is unnecessarily expensive in the cases where we don't actually
> >     need to open any packs (e.g., in `git rev-list --use-bitmap-index
> >     --count`)
> >
> >   - more importantly, it means any time we would have hit this race,
> >     we'll avoid using bitmaps altogether, leading to significant
> >     slowdowns by forcing a full object traversal
>
> This answers a question I had about why we're only opening the preferred
> pack instead of all packs. (You mention in [1] that it's also answered
> in that patch message, but I didn't see it.) In any case, it might be
> clearer to move this part to the 1st commit.
>
> [1] https://lore.kernel.org/git/Yn63nDhSBIEa3%2F+%2F@nand.local/

Makes sense, will do. In [1] I was referring to why we wanted to call
`is_pack_valid()` as early as we did, and not in
`reuse_partial_packfile_from_bitmap()`.

But the quoted part here is useful context for the first patch, too, so
I moved it up.

> > Work around this by calling `is_pack_valid()` from within
> > `want_found_object()`, matching the behavior in
> > `want_object_in_pack_one()` (which has an analogous call). Most calls to
> > `is_pack_valid()` should be basically no-ops, since only the first call
> > requires us to open a file (subsequent calls realize the file is already
> > open, and return immediately).
> >
> > This does require us to make a small change in
> > `want_object_in_pack_one()`, since `want_found_object()` may return `-1`
> > (indicating "keep searching for other packs containing this object")
> > when `*found_pack` is non-NULL. Force `want_object_in_pack_one()` to
> > call `is_pack_valid()` when `p != *found_pack`, not just when
> > `*found_pack` is non-NULL.
>
> It took me a while to realize that the relevant want_found_object()
> invocation that may return -1 is not the one in
> want_object_in_pack_one(), but in the latter's caller
> want_object_in_pack(). But even in this case, couldn't
> want_found_object() return -1 (see the very end of the function) even
> before this patch?

Perhaps changing the parenthetical to be:

    (indicating that `want_object_in_pack()` should continue searching
    for other packs containing this object)

Yes, `want_found_object()` could have returned -1 before, but the only
time when `*found_pack != NULL` and `want_found_object()` would have returned
-1 is when given `--local` with at least one non-local pack.

I actually think it's possible we have a bug there, since AFAICT the
pre-image of this patch would have left the non-NULL `*found_pack` alone
but picked a copy of the given object from a _different_ pack. So I
think this change inadvertently resolves that bug.

> > @@ -1424,14 +1427,15 @@ static int want_object_in_pack_one(struct packed_git *p,
> >  				   off_t *found_offset)
> >  {
> >  	off_t offset;
> > +	int use_found = p == *found_pack;
> >
> > -	if (p == *found_pack)
> > +	if (use_found)
> >  		offset = *found_offset;
> >  	else
> >  		offset = find_pack_entry_one(oid->hash, p);
> >
> >  	if (offset) {
> > -		if (!*found_pack) {
> > +		if (!use_found) {
> >  			if (!is_pack_valid(p))
> >  				return -1;
> >  			*found_offset = offset;
>
> My understanding of the purpose of this code change is that if we reach
> here with a non-NULL *found_pack, it is likely that *found_pack contains
> an invalid pack, and this part overwrites *found_pack (and
> *found_offset) if it finds a valid pack. This seems like a good change,
> but I don't see how this is a result of something that "does require
> us" (as far as I can tell, *found_pack could have already been invalid
> before this patch, so the downstream code should already be able to
> handle it). Maybe it's just that we couldn't tell if the pack is invalid
> previously, but now we can; but if so, it would be better to say "use
> this added information to overwrite *found_pack when it makes sense" or
> something like that.

I think my reply above indicates why this change is necessary, but if
we're talking about separate issues, let me know.

Thanks,
Taylor