From 91badeba32d31d4dcc695a8888e5b697b4c3d90c Mon Sep 17 00:00:00 2001 From: Taylor Blau Date: Mon, 24 Oct 2022 14:43:12 -0400 Subject: builtin/repack.c: implement `--expire-to` for storing pruned objects When pruning objects with `--cruft`, `git repack` 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 [1]. 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: / C1 --- C2 \ refs/heads/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. [1]: https://lore.kernel.org/git/20190319001829.GL29661@sigill.intra.peff.net/ Signed-off-by: Taylor Blau Signed-off-by: Junio C Hamano --- Documentation/git-repack.txt | 6 ++++++ 1 file changed, 6 insertions(+) (limited to 'Documentation') diff --git a/Documentation/git-repack.txt b/Documentation/git-repack.txt index 0bf13893d8..4017157949 100644 --- a/Documentation/git-repack.txt +++ b/Documentation/git-repack.txt @@ -74,6 +74,12 @@ to the new separate pack will be written. immediately instead of waiting for the next `git gc` invocation. Only useful with `--cruft -d`. +--expire-to=:: + Write a cruft pack containing pruned objects (if any) to the + directory ``. This option is useful for keeping a copy of + any pruned objects in a separate directory as a backup. Only + useful with `--cruft -d`. + -l:: Pass the `--local` option to 'git pack-objects'. See linkgit:git-pack-objects[1]. -- cgit v1.3