[voidstackloop]

Description

Fixes #15295.

Since 9.2.0, Cache._join_within checks if the resolved path is inside �ase using path.is_relative_to(base). However, path is normalized using os.path.normpath(), whereas �ase was left un-normalized.

When cache_dir contains .. segments (e.g. cache_dir = ../.pytest_cache in pytest.ini or generated via tox oxworkdir), path.is_relative_to(base) fails because the prefix parts of the normalized path do not match the un-normalized �ase parts containing ... This triggered an immediate INTERNALERROR> ValueError: 'cache/lastfailed' is not allowed to escape the cache directory on startup.

Solution

1. In Cache._join_within, normalize �ase = Path(os.path.normpath(base)) before resolving and verifying containment with path.is_relative_to(base).
2. In Cache.init and Cache.cache_dir_from_config, wrap the configured cache directory with Path(os.path.normpath(...)) so that internal paths derived from _cachedir are clean and normalized.
3. Added unit tests for _join_within with unnormalized base paths and a regression test for cache_dir containing .. in configuration.
4. Added towncrier changelog newsfragment.