builtin/repack.c: make largest pack preferred

When repacking into a geometric series and writing a multi-pack bitmap,
it is beneficial to have the largest resulting pack be the preferred
object source in the bitmap's MIDX, since selecting the large packs can
lead to fewer broken delta chains and better compression.

Teach 'git repack' to identify this pack and pass it to the MIDX write
machinery in order to mark it as preferred.

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

1 files changed, 26 insertions, 1 deletions

diff --git a/builtin/repack.c b/builtin/repack.c
index dbbb14b3b4..f940e2bafe 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -423,6 +423,25 @@ static void split_pack_geometry(struct pack_geometry *geometry, int factor)
 	geometry->split = split;
 }
 
+static struct packed_git *get_largest_active_pack(struct pack_geometry *geometry)
+{
+	if (!geometry) {
+		/*
+		 * No geometry means either an all-into-one repack (in which
+		 * case there is only one pack left and it is the largest) or an
+		 * incremental one.
+		 *
+		 * If repacking incrementally, then we could check the size of
+		 * all packs to determine which should be preferred, but leave
+		 * this for later.
+		 */
+		return NULL;
+	}
+	if (geometry->split == geometry->pack_nr)
+		return NULL;
+	return geometry->pack[geometry->pack_nr - 1];
+}
+
 static void clear_pack_geometry(struct pack_geometry *geometry)
 {
 	if (!geometry)
@@ -468,10 +487,12 @@ static void midx_included_packs(struct string_list *include,
 }
 
 static int write_midx_included_packs(struct string_list *include,
+				     struct pack_geometry *geometry,
 				     int show_progress, int write_bitmaps)
 {
 	struct child_process cmd = CHILD_PROCESS_INIT;
 	struct string_list_item *item;
+	struct packed_git *largest = get_largest_active_pack(geometry);
 	FILE *in;
 	int ret;
 
@@ -492,6 +513,10 @@ static int write_midx_included_packs(struct string_list *include,
 	if (write_bitmaps)
 		strvec_push(&cmd.args, "--bitmap");
 
+	if (largest)
+		strvec_pushf(&cmd.args, "--preferred-pack=%s",
+			     pack_basename(largest));
+
 	ret = start_command(&cmd);
 	if (ret)
 		return ret;
@@ -783,7 +808,7 @@ int cmd_repack(int argc, const char **argv, const char *prefix)
 		midx_included_packs(&include, &existing_nonkept_packs,
 				    &existing_kept_packs, &names, geometry);
 
-		ret = write_midx_included_packs(&include,
+		ret = write_midx_included_packs(&include, geometry,
 						show_progress, write_bitmaps > 0);
 
 		string_list_clear(&include, 0);