When pruning objects with --cruft, git repack(man) offers some flexibility when selecting the set of which objects are pruned via the --cruft-expiration option.
This is useful for expiring objects which are older than the grace period, making races where to-be-pruned objects become reachable and then ancestors of freshly pushed objects, leaving the repository in a corrupt state after pruning substantially less likely.
But in practice, such races are impossible to avoid entirely, no matter how long the grace period is.
To prevent this race, it is often advisable to temporarily put a repository into a read-only state.
But in practice, this is not always practical, and so some middle ground would be nice.
This patch introduces a new option, --expire-to, which teaches git repack to write an additional cruft pack containing just the objects which were pruned from the repository.
The caller can specify a directory outside of the current repository as the destination for this second cruft pack.
This makes it possible to prune objects from a repository, while still holding onto a supplemental copy of them outside of the original repository.
Having this copy on-disk makes it substantially easier to recover objects when the aforementioned race is encountered.
--expire-to is implemented in a somewhat convoluted manner, which is to take advantage of the fact that the first time write_cruft_pack() is called, it adds the name of the cruft pack to the names string list.
That means the second time we call write_cruft_pack(), objects in the previously-written cruft pack will be excluded.
As long as the caller ensures that no objects are expired during the second pass, this is sufficient to generate a cruft pack containing all objects which don't appear in any of the new packs written by git repack, including the cruft pack.
In other words, all of the objects which are about to be pruned from the repository.
It is important to note that the destination in --expire-to does not necessarily need to be a Git repository (though it can be) Notably, the expired packs do not contain all ancestors of expired objects.
So if the source repository contains something like:
<unreachable>
/
C1 --- C2
\
ads/master
where C2 is unreachable, but has a parent (C1) which is reachable, and C2 would be pruned, then the expiry pack will contain only C2, not C1.
git gc也会运行git repack”;) 因此，是的，git gc就足够了。 - michas