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, 12 insertions, 0 deletions

diff --git a/builtin/repack.c b/builtin/repack.c
index 165fe1b..c26f067 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -885,6 +885,18 @@ int cmd_repack(int argc, const char **argv, const char *prefix)
 	if (write_bitmaps && !(pack_everything & ALL_INTO_ONE) && !write_midx)
 		die(_(incremental_bitmap_conflict_error));
 
+	if (write_bitmaps && po_args.local && has_alt_odb(the_repository)) {
+		/*
+		 * When asked to do a local repack, but we have
+		 * packfiles that are inherited from an alternate, then
+		 * we cannot guarantee that the multi-pack-index would
+		 * have full coverage of all objects. We thus disable
+		 * writing bitmaps in that case.
+		 */
+		warning(_("disabling bitmap writing, as some objects are not being packed"));
+		write_bitmaps = 0;
+	}
+
 	if (write_midx && write_bitmaps) {
 		struct strbuf path = STRBUF_INIT;