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>

diff --git a/midx.c b/midx.c
index 7cfad04a24027c1f0703dafdeeb31989414a13c5..8455df4f35c292d7ca6f89cdd4d62b36881d18b0 100644 (file)
--- 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 &&

diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index 499d5d4c78590ae16879d8dbd386259d847efb76..0883c7c6bd965d5506ca45a10c2c406a2700707a 100755 (executable)
--- 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