midx: fix segfault with no packs and invalid preferred pack

When asked to write a multi-pack-index the user can specify a preferred
pack that is used as a tie breaker when multiple packs contain the same
objects. When this packfile cannot be found, we just pick the first pack
that is getting tracked by the newly written multi-pack-index as a
fallback.

Picking the fallback can fail in the case where we're asked to write a
multi-pack-index with no packfiles at all: picking the fallback value
will cause a segfault as we blindly index into the array of packfiles,
which would be empty.

Fix this bug by resetting the preferred packfile index to `-1` before
searching for the preferred pack. This fixes the segfault as we already
check for whether the index is `> - 1`. If it is not, we simply don't
pick a preferred packfile at all.

Helped-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 3 insertions, 3 deletions

diff --git a/midx.c b/midx.c
index 7cfad04a24..8455df4f35 100644
--- a/midx.c
+++ b/midx.c
@@ -1326,17 +1326,17 @@ static int write_midx_internal(const char *object_dir,
 	}
 
 	if (preferred_pack_name) {
-		int found = 0;
+		ctx.preferred_pack_idx = -1;
+
 		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)
+		if (ctx.preferred_pack_idx == -1)
 			warning(_("unknown preferred pack: '%s'"),
 				preferred_pack_name);
 	} else if (ctx.nr &&