Re: [RFC PATCH] multi-pack-index: allow operating without pack files

On Sun, Aug 22, 2021 at 08:34:43PM -0400, Derrick Stolee wrote:

On 8/20/2021 3:55 PM, Johannes Berg wrote:

quoted

Technically, multi-pack-index doesn't need pack files to exist,
but add_packed_git() today checks whether it exists or not.

Having a multi-pack-index is supposed to indicate that we have
these objects in the objects/pack directory within the specified
pack-files.

I understand your goal to relax a condition of the multi-pack-index
file, but it's triggered by a flag during write and that choice
isn't persisted into the file. There is no way for a later Git
process to understand that the multi-pack-index doesn't actually
guarantee object existence.

We're going to run into problems much earlier than that, though: the
MIDX needs to load information about objects from packs in order to
break ties when multiple copies of the same object exist in multiple
packs (according to that pack's mtime).

So I'm not sure how we would even write a MIDX without physical packs on
disk that we can open and stat, let along how we would teach Git to
handle a situation where packs that did exist when writing a MIDX went
away when we tried to read from the same MIDX later on.

Thanks,
Taylor