RFC move pruned objects to a separate repository

Now that cruft packs are available in v2.37.0, here is an interesting application of that new feature to enable a two-phase object pruning approach.

This came out of a discussion within GitHub about ways we could support storing a set of pruned objects in "limbo" so that they were not accessible from the repository which pruned them, but instead stored in a cruft pack in a separate repository which lists the original one as an alternate.

This makes it possible to take the collection of all pruned objects and store them in a cruft pack in a separate repository. This repository (which I have been referring to as the "expired.git") can then be used as a donor repository for any missing objects (like the ones described by the race in [1]).

The first few patches are preparatory. The final one implements writing the pruned objects separately. The trick is to write another cruft pack to a separate repository, with two tweaks:

- the --cruft-expiration value is set to "never", since we want to keep around all of the objects we expired in the previous step, and

- the original cruft pack appears as a pack that we are going to keep, meaning all unreachable objects that are stored in the original cruft pack are excluded from the one we write to the "expired.git" repository.

You can try this out yourself by doing something like:

    $ git init --bare ../expired.git $ git repack --cruft
    --cruft-expiration=1.day.ago -d \
    --expire-to=../expired.git/objects/pack/pack

which will create two cruft packs:

- one in the repository which ran git repack containing all unreachable objects written within the last day, and
- another in the "expired.git" repository which contains all unreachable objects written prior to the last day

This series is an RFC for now since I'm interested in discussing whether or not this is a feature that people would actually want to use or not. But if it is, I'm happy to polish this up and turn it into a non-RFC-quality series ;-).

In the meantime, thanks for your review!

[1]: https://lore.kernel.org/git/[email protected]/

Taylor Blau (4):
builtin/repack.c: pass "out" to prepare_pack_objects builtin/repack.c: pass "cruft_expiration" to write_cruft_pack builtin/repack.c: write cruft packs to arbitrary locations builtin/repack.c: implement --expire-to for storing pruned objects

 Documentation/git-repack.txt |   6 ++
 builtin/repack.c             |  67 ++++++++++++++++---
 t/t7700-repack.sh            | 121 +++++++++++++++++++++++++++++++++++
 3 files changed, 186 insertions(+), 8 deletions(-)

2.37.0.1.g1379af2e9d