midx: allow marking a pack as preferred

When multiple packs in the multi-pack index contain the same object, the
MIDX machinery must make a choice about which pack it associates with
that object. Prior to this patch, the lowest-ordered[1] pack was always
selected.

Pack selection for duplicate objects is relatively unimportant today,
but it will become important for multi-pack bitmaps. This is because we
can only invoke the pack-reuse mechanism when all of the bits for reused
objects come from the reuse pack (in order to ensure that all reused
deltas can find their base objects in the same pack).

To encourage the pack selection process to prefer one pack over another
(the pack to be preferred is the one a caller would like to later use as
a reuse pack), introduce the concept of a "preferred pack". When
provided, the MIDX code will always prefer an object found in a
preferred pack over any other.

No format changes are required to store the preferred pack, since it
will be able to be inferred with a corresponding MIDX bitmap, by looking
up the pack associated with the object in the first bit position (this
ordering is described in detail in a subsequent commit).

[1]: the ordering is specified by MIDX internals; for our purposes we
can consider the "lowest ordered" pack to be "the one with the
most-recent mtime.

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

1 files changed, 16 insertions, 3 deletions

diff --git a/builtin/multi-pack-index.c b/builtin/multi-pack-index.c
index 8711174fae..5d3ea445fd 100644
--- a/builtin/multi-pack-index.c
+++ b/builtin/multi-pack-index.c
@@ -4,9 +4,10 @@
 #include "parse-options.h"
 #include "midx.h"
 #include "trace2.h"
+#include "object-store.h"
 
 #define BUILTIN_MIDX_WRITE_USAGE \
-	N_("git multi-pack-index [<options>] write")
+	N_("git multi-pack-index [<options>] write [--preferred-pack=<pack>]")
 
 #define BUILTIN_MIDX_VERIFY_USAGE \
 	N_("git multi-pack-index [<options>] verify")
@@ -43,6 +44,7 @@ static char const * const builtin_multi_pack_index_usage[] = {
 
 static struct opts_multi_pack_index {
 	const char *object_dir;
+	const char *preferred_pack;
 	unsigned long batch_size;
 	unsigned flags;
 } opts;
@@ -61,7 +63,15 @@ static struct option *add_common_options(struct option *prev)
 
 static int cmd_multi_pack_index_write(int argc, const char **argv)
 {
-	struct option *options = common_opts;
+	struct option *options;
+	static struct option builtin_multi_pack_index_write_options[] = {
+		OPT_STRING(0, "preferred-pack", &opts.preferred_pack,
+			   N_("preferred-pack"),
+			   N_("pack for reuse when computing a multi-pack bitmap")),
+		OPT_END(),
+	};
+
+	options = add_common_options(builtin_multi_pack_index_write_options);
 
 	trace2_cmd_mode(argv[0]);
 
@@ -72,7 +82,10 @@ static int cmd_multi_pack_index_write(int argc, const char **argv)
 		usage_with_options(builtin_multi_pack_index_write_usage,
 				   options);
 
-	return write_midx_file(opts.object_dir, opts.flags);
+	FREE_AND_NULL(options);
+
+	return write_midx_file(opts.object_dir, opts.preferred_pack,
+			       opts.flags);
 }
 
 static int cmd_multi_pack_index_verify(int argc, const char **argv)