odb: reject reading and writing null OIDs

The null OID (hash with all zeroes) indicates a missing object in
upstream git and is thus not a valid object ID. Add defensive
measurements to avoid writing such a hash to the object database in the
very unlikely case where some data results in the null OID. Furthermore,
add shortcuts when reading the null OID from the ODB to avoid ever
returning an object when a faulty repository may contain the null OID.

1 files changed, 18 insertions, 0 deletions

diff --git a/tests/odb/backend/simple.c b/tests/odb/backend/simple.c
index c0fcd403b..f4d29cc76 100644
--- a/tests/odb/backend/simple.c
+++ b/tests/odb/backend/simple.c
@@ -230,3 +230,21 @@ void test_odb_backend_simple__exists_with_highly_ambiguous_prefix(void)
 	cl_git_pass(git_odb_exists_prefix(&found, _odb, &_oid, 40));
 	cl_assert(git_oid_equal(&found, &_oid));
 }
+
+void test_odb_backend_simple__null_oid_is_ignored(void)
+{
+	const fake_object objs[] = {
+		{ "0000000000000000000000000000000000000000", "null oid content" },
+		{ NULL, NULL }
+	};
+	git_oid null_oid = {{0}};
+	git_odb_object *obj;
+
+	setup_backend(objs);
+
+	cl_git_pass(git_libgit2_opts(GIT_OPT_ENABLE_STRICT_HASH_VERIFICATION, 0));
+	cl_assert(!git_odb_exists(_odb, &null_oid));
+
+	cl_git_fail_with(GIT_ENOTFOUND, git_odb_read(&obj, _odb, &null_oid));
+	cl_assert(giterr_last() && strstr(giterr_last()->message, "null OID"));
+}