tests: odb: add tests with multiple backends

Previous to pulling out and extending the fake backend, it was quite
cumbersome to write tests for very specific scenarios regarding
backends. But as we have made it more generic, it has become much easier
to do so. As such, this commit adds multiple tests for scenarios with
multiple backends for the ODB.

The changes also include a test for a very targeted scenario. When one
backend found a matching object via `read_prefix`, but the last backend
returns `GIT_ENOTFOUND` and when object hash verification is turned off,
we fail to reset the error code to `GIT_OK`. This causes us to segfault
later on, when doing a double-free on the returned object.

tests/odb/backend/multiple.c

+#include "clar_libgit2.h"
+#include "repository.h"
+#include "backend_helpers.h"
+
+#define EXISTING_HASH "e69de29bb2d1d6434b8b29ae775ad8c2e48c5391"
+
+static git_repository *_repo;
+static git_odb_object *_obj;
+static fake_backend *_fake_empty;
+static fake_backend *_fake_filled;
+
+static git_oid _existing_oid;
+
+static const fake_object _objects_filled[] = {
+	{ EXISTING_HASH, "" },
+	{ NULL, NULL }
+};
+
+static const fake_object _objects_empty[] = {
+	{ NULL, NULL }
+};
+
+void test_odb_backend_multiple__initialize(void)
+{
+	git_odb_backend *backend;
+
+	git_oid_fromstr(&_existing_oid, EXISTING_HASH);
+
+	_obj = NULL;
+	_repo = cl_git_sandbox_init("testrepo.git");
+
+	cl_git_pass(build_fake_backend(&backend, _objects_filled));
+	_fake_filled = (fake_backend *)backend;
+
+	cl_git_pass(build_fake_backend(&backend, _objects_empty));
+	_fake_empty = (fake_backend *)backend;
+}
+
+void test_odb_backend_multiple__cleanup(void)
+{
+	git_odb_object_free(_obj);
+	cl_git_sandbox_cleanup();
+}
+
+void test_odb_backend_multiple__read_with_empty_first_succeeds(void)
+{
+	git_odb *odb;
+
+	cl_git_pass(git_repository_odb__weakptr(&odb, _repo));
+	cl_git_pass(git_odb_add_backend(odb, (git_odb_backend *)_fake_filled, 10));
+	cl_git_pass(git_odb_add_backend(odb, (git_odb_backend *)_fake_empty, 50));
+
+	cl_git_pass(git_odb_read(&_obj, odb, &_existing_oid));
+
+	cl_assert_equal_i(1, _fake_filled->read_calls);
+	cl_assert_equal_i(1, _fake_empty->read_calls);
+}
+
+void test_odb_backend_multiple__read_with_first_matching_stops(void)
+{
+	git_odb *odb;
+
+	cl_git_pass(git_repository_odb__weakptr(&odb, _repo));
+	cl_git_pass(git_odb_add_backend(odb, (git_odb_backend *)_fake_empty, 10));
+	cl_git_pass(git_odb_add_backend(odb, (git_odb_backend *)_fake_filled, 50));
+
+	cl_git_pass(git_odb_read(&_obj, odb, &_existing_oid));
+
+	cl_assert_equal_i(1, _fake_filled->read_calls);
+	cl_assert_equal_i(0, _fake_empty->read_calls);
+}
+
+void test_odb_backend_multiple__read_prefix_with_first_empty_succeeds(void)
+{
+	git_odb *odb;
+
+	cl_git_pass(git_repository_odb__weakptr(&odb, _repo));
+	cl_git_pass(git_odb_add_backend(odb, (git_odb_backend *)_fake_filled, 10));
+	cl_git_pass(git_odb_add_backend(odb, (git_odb_backend *)_fake_empty, 50));
+
+	cl_git_pass(git_odb_read_prefix(&_obj, odb, &_existing_oid, 7));
+
+	cl_assert_equal_i(1, _fake_filled->read_prefix_calls);
+	cl_assert_equal_i(1, _fake_empty->read_prefix_calls);
+}
+
+void test_odb_backend_multiple__read_prefix_with_first_matching_reads_both(void)
+{
+	git_odb *odb;
+
+	cl_git_pass(git_repository_odb__weakptr(&odb, _repo));
+	cl_git_pass(git_odb_add_backend(odb, (git_odb_backend *)_fake_empty, -10));
+	cl_git_pass(git_odb_add_backend(odb, (git_odb_backend *)_fake_filled, 50));
+
+	cl_git_pass(git_odb_read_prefix(&_obj, odb, &_existing_oid, 7));
+
+	cl_assert_equal_i(1, _fake_filled->read_prefix_calls);
+	cl_assert_equal_i(1, _fake_empty->read_prefix_calls);
+}
+
+void test_odb_backend_multiple__read_prefix_with_first_matching_succeeds_without_hash_verification(void)
+{
+	git_odb *odb;
+
+	git_libgit2_opts(GIT_OPT_ENABLE_STRICT_HASH_VERIFICATION, 0);
+
+	cl_git_pass(git_repository_odb__weakptr(&odb, _repo));
+	cl_git_pass(git_odb_add_backend(odb, (git_odb_backend *)_fake_empty, -10));
+	cl_git_pass(git_odb_add_backend(odb, (git_odb_backend *)_fake_filled, 50));
+
+	cl_git_pass(git_odb_read_prefix(&_obj, odb, &_existing_oid, 7));
+
+	/*
+	 * Both backends should be checked as we have to check
+	 * for collisions
+	 */
+	cl_assert_equal_i(1, _fake_filled->read_prefix_calls);
+	cl_assert_equal_i(1, _fake_empty->read_prefix_calls);
+
+	git_libgit2_opts(GIT_OPT_ENABLE_STRICT_HASH_VERIFICATION, 1);
+}