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, 12 insertions, 0 deletions

diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index 499d5d4c78..0883c7c6bd 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -183,6 +183,18 @@ 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 &&
+	warning: unknown preferred pack: ${SQ}does-not-exist${SQ}
+	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