On 9/19/2022 9:55 PM, Taylor Blau wrote:

The `expire` sub-command unlinks any packs that are (a) contained in the
MIDX, but (b) have no objects referenced by the MIDX.

It is important to note that this can only happen if all objects in
the pack have duplicates in other pack-files.
 

This sub-command ignores `.keep` packs, which remain on-disk even if
they have no objects referenced by the MIDX. Cruft packs, however,
aren't given the same treatment: if none of the objects contained in the
cruft pack are selected from the cruft pack by the MIDX, then the cruft
pack is eligible to be expired.

This is less than desireable, since the cruft pack has important

s/desireable/desirable/
(according to my spell-checker)

metadata about the individual object mtimes, which is useful to
determine how quickly an object should age out of the repository when
pruning.

Ordinarily, we wouldn't expect the contents of a cruft pack to
duplicated across non-cruft packs (and we'd expect to see the MIDX
select all cruft objects from other sources even less often). But
nonetheless, it is still possible to trick the `expire` sub-command into
removing the `.mtimes` file in this circumstance.

I was initially unconvinced that this scenario was super-critical
to keeping the .mtimes file, but I was able to think of cases where
objects are duplicated out of the cruft pack due to de-thinning or
otherwise inefficiently packing unreachable objects into these
other pack-files.

Thanks,
-Stolee