From e2d003dbedf05906c0cc22493a96124c94521973 Mon Sep 17 00:00:00 2001 From: Derrick Stolee Date: Wed, 8 Mar 2023 18:47:32 +0000 Subject: [PATCH] object-file: reprepare alternates when necessary When an object is not found in a repository's object store, we sometimes call reprepare_packed_git() to see if the object was temporarily moved into a new pack-file (and its old pack-file or loose object was deleted). This process does a scan of each pack directory within each odb, but does not reevaluate if the odb list needs updating. Extend reprepare_packed_git() to also reprepare the alternate odb list by setting loaded_alternates to zero and calling prepare_alt_odb(). This will add newly-discoverd odbs to the linked list, but will not duplicate existing ones nor will it remove existing ones that are no longer listed in the alternates file. Do this under the object read lock to avoid readers from interacting with a potentially incomplete odb being added to the odb list. If the alternates file was edited to _remove_ some alternates during the course of the Git process, Git will continue to see alternates that were ever valid for that repository. ODBs are not removed from the list, the same as the existing behavior before this change. Git already has protections against an alternate directory disappearing from the filesystem during the lifetime of a process, and those are still in effect. This change is specifically for concurrent changes to the repository, so it is difficult to create a test that guarantees this behavior is correct. I manually verified by introducing a reprepare_packed_git() call into get_revision() and stepped into that call in a debugger with a parent 'git log' process. Multiple runs of prepare_alt_odb() kept the_repository->objects->odb as a single-item chain until I added a .git/objects/info/alternates file in a different process. The next run added the new odb to the chain and subsequent runs did not add to the chain. Signed-off-by: Derrick Stolee Signed-off-by: Junio C Hamano --- packfile.c | 10 ++++++++++ 1 file changed, 10 insertions(+) diff --git a/packfile.c b/packfile.c index c0d7dd93f4..9d4eecfc1c 100644 --- a/packfile.c +++ b/packfile.c @@ -1008,6 +1008,16 @@ void reprepare_packed_git(struct repository *r) struct object_directory *odb; obj_read_lock(); + + /* + * Reprepare alt odbs, in case the alternates file was modified + * during the course of this process. This only _adds_ odbs to + * the linked list, so existing odbs will continue to exist for + * the lifetime of the process. + */ + r->objects->loaded_alternates = 0; + prepare_alt_odb(r); + for (odb = r->objects->odb; odb; odb = odb->next) odb_clear_loose_cache(odb);