midx.c: fix an integer underflow

When verifying a midx index with 0 objects, the
    m->num_objects - 1
underflows and wraps around to 4294967295.

Fix this both by checking that the midx contains at least one oid,
and also that we don't write any midx when there is no packfiles.

Update the tests to check that `git multi-pack-index write` does
not write an midx when there is no objects, and another to check
that `git multi-pack-index verify` warns when it verifies an midx with no
objects. For this last test, use t5319/no-objects.midx which was
generated by an older version of git.

Signed-off-by: Damien Robert <redacted>
Signed-off-by: Junio C Hamano <redacted>

diff --git a/midx.c b/midx.c
index 1527e464a7b43e352f352dfc6c6f2b011e35986d..a520e263956086a988b77d6e22a7262c9aeba9b8 100644 (file)
--- a/midx.c
+++ b/midx.c
@@ -923,6 +923,12 @@ static int write_midx_internal(const char *object_dir, struct multi_pack_index *
        cur_chunk = 0;
        num_chunks = large_offsets_needed ? 5 : 4;
 
+       if (packs.nr - dropped_packs == 0) {
+               error(_("no pack files to index."));
+               result = 1;
+               goto cleanup;
+       }
+
        written = write_midx_header(f, num_chunks, packs.nr - dropped_packs);
 
        chunk_ids[cur_chunk] = MIDX_CHUNKID_PACKNAMES;
@@ -1124,6 +1130,15 @@ int verify_midx_file(struct repository *r, const char *object_dir, unsigned flag
                                    i, oid_fanout1, oid_fanout2, i + 1);
        }
 
+       if (m->num_objects == 0) {
+               midx_report(_("the midx contains no oid"));
+               /*
+                * Remaining tests assume that we have objects, so we can
+                * return here.
+                */
+               return verify_midx_error;
+       }
+
        if (flags & MIDX_PROGRESS)
                progress = start_sparse_progress(_("Verifying OID order in multi-pack-index"),
                                                 m->num_objects - 1);

diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index 43a7a66c9d1b50640775d6b38df7a31b32e8e548..22240fd30b482f201daa69ada8ffcb6248a24638 100755 (executable)
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -42,10 +42,15 @@ test_expect_success 'setup' '
        EOF
 '
 
-test_expect_success 'write midx with no packs' '
-       test_when_finished rm -f pack/multi-pack-index &&
-       git multi-pack-index --object-dir=. write &&
-       midx_read_expect 0 0 4 .
+test_expect_success "don't write midx with no packs" '
+       test_must_fail git multi-pack-index --object-dir=. write &&
+       test_path_is_missing pack/multi-pack-index
+'
+
+test_expect_success "Warn if a midx contains no oid" '
+       cp "$TEST_DIRECTORY"/t5319/no-objects.midx $objdir/pack/multi-pack-index &&
+       test_must_fail git multi-pack-index verify &&
+       rm $objdir/pack/multi-pack-index
 '
 
 generate_objects () {

diff --git a/t/t5319/no-objects.midx b/t/t5319/no-objects.midx
new file mode 100644 (file)
index 0000000..e466b8e
Binary files /dev/null and b/t/t5319/no-objects.midx differ