midx: preliminary support for `--refs-snapshot`

To figure out which commits we can write a bitmap for, the multi-pack
index/bitmap code does a reachability traversal, marking any commit
which can be found in the MIDX as eligible to receive a bitmap.

This approach will cause a problem when multi-pack bitmaps are able to
be generated from `git repack`, since the reference tips can change
during the repack. Even though we ignore commits that don't exist in
the MIDX (when doing a scan of the ref tips), it's possible that a
commit in the MIDX reaches something that isn't.

This can happen when a multi-pack index contains some pack which refers
to loose objects (e.g., if a pack was pushed after starting the repack
but before generating the MIDX which depends on an object which is
stored as loose in the repository, and by definition isn't included in
the multi-pack index).

By taking a snapshot of the references before we start repacking, we can
close that race window. In the above scenario (where we have a packed
object pointing at a loose one), we'll either (a) take a snapshot of the
references before seeing the packed one, or (b) take it after, at which
point we can guarantee that the loose object will be packed and included
in the MIDX.

This patch does just that. It writes a temporary "reference snapshot",
which is a list of OIDs that are at the ref tips before writing a
multi-pack bitmap. References that are "preferred" (i.e,. are a suffix
of at least one value of the 'pack.preferBitmapTips' configuration) are
marked with a special '+'.

The format is simple: one line per commit at each tip, with an optional
'+' at the beginning (for preferred references, as described above).

When provided, the reference snapshot is used to drive bitmap selection
instead of the MIDX code doing its own traversal. When it isn't
provided, the usual traversal takes place instead.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

2 files changed, 9 insertions, 4 deletions

diff --git a/builtin/multi-pack-index.c b/builtin/multi-pack-index.c
index 047647b5f2..4b827a07c0 100644
--- a/builtin/multi-pack-index.c
+++ b/builtin/multi-pack-index.c
@@ -7,7 +7,8 @@
 #include "object-store.h"
 
 #define BUILTIN_MIDX_WRITE_USAGE \
-	N_("git multi-pack-index [<options>] write [--preferred-pack=<pack>]")
+	N_("git multi-pack-index [<options>] write [--preferred-pack=<pack>]" \
+	   "[--refs-snapshot=<path>]")
 
 #define BUILTIN_MIDX_VERIFY_USAGE \
 	N_("git multi-pack-index [<options>] verify")
@@ -45,6 +46,7 @@ static char const * const builtin_multi_pack_index_usage[] = {
 static struct opts_multi_pack_index {
 	const char *object_dir;
 	const char *preferred_pack;
+	const char *refs_snapshot;
 	unsigned long batch_size;
 	unsigned flags;
 	int stdin_packs;
@@ -83,6 +85,8 @@ static int cmd_multi_pack_index_write(int argc, const char **argv)
 			MIDX_WRITE_BITMAP | MIDX_WRITE_REV_INDEX),
 		OPT_BOOL(0, "stdin-packs", &opts.stdin_packs,
 			 N_("write multi-pack index containing only given indexes")),
+		OPT_FILENAME(0, "refs-snapshot", &opts.refs_snapshot,
+			     N_("refs snapshot for selecting bitmap commits")),
 		OPT_END(),
 	};
 
@@ -106,7 +110,8 @@ static int cmd_multi_pack_index_write(int argc, const char **argv)
 		read_packs_from_stdin(&packs);
 
 		ret = write_midx_file_only(opts.object_dir, &packs,
-					   opts.preferred_pack, opts.flags);
+					   opts.preferred_pack,
+					   opts.refs_snapshot, opts.flags);
 
 		string_list_clear(&packs, 0);
 
@@ -114,7 +119,7 @@ static int cmd_multi_pack_index_write(int argc, const char **argv)
 
 	}
 	return write_midx_file(opts.object_dir, opts.preferred_pack,
-			       opts.flags);
+			       opts.refs_snapshot, opts.flags);
 }
 
 static int cmd_multi_pack_index_verify(int argc, const char **argv)
diff --git a/builtin/repack.c b/builtin/repack.c
index 82ab668272..27158a897b 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -733,7 +733,7 @@ int cmd_repack(int argc, const char **argv, const char *prefix)
 		unsigned flags = 0;
 		if (git_env_bool(GIT_TEST_MULTI_PACK_INDEX_WRITE_BITMAP, 0))
 			flags |= MIDX_WRITE_BITMAP | MIDX_WRITE_REV_INDEX;
-		write_midx_file(get_object_directory(), NULL, flags);
+		write_midx_file(get_object_directory(), NULL, NULL, flags);
 	}
 
 	string_list_clear(&names, 0);