repack: disable writing bitmaps when doing a local repack

In order to write a bitmap, we need to have full coverage of all objects
that are about to be packed. In the traditional non-multi-pack-index
world this meant we need to do a full repack of all objects into a
single packfile. But in the new multi-pack-index world we can get away
with writing bitmaps when we have multiple packfiles as long as the
multi-pack-index covers all objects.

This is not always the case though. When asked to perform a repack of
local objects, only, then we cannot guarantee to have full coverage of
all objects regardless of whether we do a full repack or a repack with a
multi-pack-index. The end result is that writing the bitmap will fail in
both worlds:

    $ git multi-pack-index write --stdin-packs --bitmap <packfiles
    warning: Failed to write bitmap index. Packfile doesn't have full closure (object 1529341d78cf45377407369acb0f4ff2b5cdae42 is missing)
    error: could not write multi-pack bitmap

Now there are two different ways to fix this. The first one would be to
amend git-multi-pack-index(1) to disable writing bitmaps when we notice
that we don't have full object coverage.

    - We don't have enough information in git-multi-pack-index(1) in
      order to tell whether the local repository _should_ have full
      coverage. Because even when connected to an alternate object
      directory, it may be the case that we still have all objects
      around in the main object database.

    - git-multi-pack-index(1) is quite a low-level tool. Automatically
      disabling functionality that it was asked to provide does not feel
      like the right thing to do.

We can easily fix it at a higher level in git-repack(1) though. When
asked to only include local objects via `-l` and when connected to an
alternate object directory then we will override the user's ask and
disable writing bitmaps with a warning. This is similar to what we do in
git-pack-objects(1), where we also disable writing bitmaps in case we
omit an object from the pack.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/object-store.h b/object-store.h
index 1a713d89d7..8ba010a9d6 100644
--- a/object-store.h
+++ b/object-store.h
@@ -56,6 +56,7 @@ KHASH_INIT(odb_path_map, const char * /* key: odb_path */,
 	struct object_directory *, 1, fspathhash, fspatheq)
 
 void prepare_alt_odb(struct repository *r);
+int has_alt_odb(struct repository *r);
 char *compute_alternate_path(const char *path, struct strbuf *err);
 struct object_directory *find_odb(struct repository *r, const char *obj_dir);
 typedef int alt_odb_fn(struct object_directory *, void *);