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 exiting early in case we have determined that the MIDX
wouldn't have any packfiles to index. While the request itself does not
make much sense anyway, it is still preferable to exit gracefully than
to abort.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 midx.c                      |  6 ++++++
 t/t5319-multi-pack-index.sh | 11 +++++++++++
 2 files changed, 17 insertions(+)

diff --git a/midx.c b/midx.c
index 9af3e5de88..22ea7ffb75 100644
--- a/midx.c
+++ b/midx.c
@@ -1307,6 +1307,12 @@ static int write_midx_internal(const char *object_dir,
 	for_each_file_in_pack_dir(object_dir, add_pack_to_midx, &ctx);
 	stop_progress(&ctx.progress);
 
+	if (!ctx.nr) {
+		error(_("no pack files to index."));
+		result = 1;
+		goto cleanup;
+	}
+
 	if ((ctx.m && ctx.nr == ctx.m->num_packs) &&
 	    !(packs_to_include || packs_to_drop)) {
 		struct bitmap_index *bitmap_git;
diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index 499d5d4c78..be7f3c1e1f 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -183,6 +183,17 @@ test_expect_success 'write midx with --stdin-packs' '
 
 compare_results_with_midx "mixed mode (one pack + extra)"
 
+test_expect_success 'write with no objects and preferred pack' '
+	test_when_finished "rm -rf empty" &&
+	git init empty &&
+	test_must_fail git -C empty multi-pack-index write \
+		--stdin-packs --preferred-pack=does-not-exist </dev/null 2>err &&
+	cat >expect <<-EOF &&
+	error: no pack files to index.
+	EOF
+	test_cmp expect err
+'
+
 test_expect_success 'write progress off for redirected stderr' '
 	git multi-pack-index --object-dir=$objdir write 2>err &&
 	test_line_count = 0 err
-- 
2.40.0