E_assert
E_assert(fdflags >= 0, "fcntl F_GETFL of new fd");
E_assert(otherflags >= 0, "fcntl F_GETFD of new fd");
E_assert(hardcoded_fd >= 0, "open fd to hardcode");
E_assert(dfd, "failed to openat root '%s': %m", test->dir);