1 files changed, 44 insertions, 6 deletions

diff --git a/midx.c b/midx.c
index 26089ec9c7..67de1dbaeb 100644
--- a/midx.c
+++ b/midx.c
@@ -969,15 +969,57 @@ static int write_midx_internal(const char *object_dir, struct multi_pack_index *
 	if (ctx.m && ctx.nr == ctx.m->num_packs && !packs_to_drop)
 		goto cleanup;
 
-	ctx.preferred_pack_idx = -1;
 	if (preferred_pack_name) {
+		int found = 0;
 		for (i = 0; i < ctx.nr; i++) {
 			if (!cmp_idx_or_pack_name(preferred_pack_name,
 						  ctx.info[i].pack_name)) {
 				ctx.preferred_pack_idx = i;
+				found = 1;
 				break;
 			}
 		}
+
+		if (!found)
+			warning(_("unknown preferred pack: '%s'"),
+				preferred_pack_name);
+	} else if (ctx.nr && (flags & MIDX_WRITE_REV_INDEX)) {
+		struct packed_git *oldest = ctx.info[ctx.preferred_pack_idx].p;
+		ctx.preferred_pack_idx = 0;
+
+		if (packs_to_drop && packs_to_drop->nr)
+			BUG("cannot write a MIDX bitmap during expiration");
+
+		/*
+		 * set a preferred pack when writing a bitmap to ensure that
+		 * the pack from which the first object is selected in pseudo
+		 * pack-order has all of its objects selected from that pack
+		 * (and not another pack containing a duplicate)
+		 */
+		for (i = 1; i < ctx.nr; i++) {
+			struct packed_git *p = ctx.info[i].p;
+
+			if (!oldest->num_objects || p->mtime < oldest->mtime) {
+				oldest = p;
+				ctx.preferred_pack_idx = i;
+			}
+		}
+
+		if (!oldest->num_objects) {
+			/*
+			 * If all packs are empty; unset the preferred index.
+			 * This is acceptable since there will be no duplicate
+			 * objects to resolve, so the preferred value doesn't
+			 * matter.
+			 */
+			ctx.preferred_pack_idx = -1;
+		}
+	} else {
+		/*
+		 * otherwise don't mark any pack as preferred to avoid
+		 * interfering with expiration logic below
+		 */
+		ctx.preferred_pack_idx = -1;
 	}
 
 	if (ctx.preferred_pack_idx > -1) {
@@ -1058,11 +1100,7 @@ static int write_midx_internal(const char *object_dir, struct multi_pack_index *
 						      ctx.info, ctx.nr,
 						      sizeof(*ctx.info),
 						      idx_or_pack_name_cmp);
-
-		if (!preferred)
-			warning(_("unknown preferred pack: '%s'"),
-				preferred_pack_name);
-		else {
+		if (preferred) {
 			uint32_t perm = ctx.pack_perm[preferred->orig_pack_int_id];
 			if (perm == PACK_EXPIRED)
 				warning(_("preferred pack '%s' is expired"),