🐛 fix(cache): retry after cache I/O errors

docs/changelog/216.bugfix.rst

@@ -0,0 +1 @@
+Retry interpreter discovery without the cache when cache access fails.

docs/explanation.rst

@@ -272,6 +272,9 @@ with `filelock <https://py-filelock.readthedocs.io/>`_-based locking for safe co
 can also pass ``cache=None`` to disable caching, or implement your own backend (see
 :doc:`/how-to/standalone-usage`).
 
+If cache access raises an OS error, discovery retries the interpreter query without that cache. A failed cache write
+can therefore cause a second query.
+
 On POSIX, ``DiskCache`` uses a cache root only when it and both ``py_info`` directories are real directories owned by
 the current user or root and cannot be written by other users. Its ancestor directories must also prevent other users
 from replacing the cache root; a sticky shared ancestor such as ``/tmp`` is allowed. Discovery continues without disk

src/python_discovery/_cached_py_info.py

@@ -62,7 +62,14 @@ def from_exe(  # ruff:ignore[too-many-arguments]
     executable_args: str | None = None,
 ) -> PythonInfo | None:
     env = os.environ if env is None else env
-    result = _get_from_cache(cls, cache, exe, env, ignore_cache=ignore_cache, executable_args=executable_args)
+    if cache is None:
+        result = _get_from_cache(cls, cache, exe, env, ignore_cache=ignore_cache, executable_args=executable_args)
+    else:
+        try:
+            result = _get_from_cache(cls, cache, exe, env, ignore_cache=ignore_cache, executable_args=executable_args)
+        except OSError:
+            _LOGGER.debug("failed to use cache for %s", exe, exc_info=True)
+            result = _get_from_cache(cls, None, exe, env, ignore_cache=True, executable_args=executable_args)
     if isinstance(result, Exception):
         if raise_on_error:
             raise result

tests/test_cached_py_info.py

@@ -329,6 +329,27 @@ def test_from_exe_cache_entry_with_bad_content(tmp_path: Path) -> None:
     assert isinstance(store.read(), dict)
 
 
+@pytest.mark.parametrize(
+    ("folder", "suffix"),
+    [
+        pytest.param(".", ".json", id="entry"),
+        pytest.param("locks-v2" if os.name == "posix" else ".", ".lock", id="lock"),
+    ],
+)
+def test_from_exe_recovers_unusable_cache_path(tmp_path: Path, folder: str, suffix: str) -> None:
+    cache = DiskCache(tmp_path)
+    expected = PythonInfo.from_exe(sys.executable, cache, ignore_cache=True, resolve_to_host=False)
+    assert expected is not None
+    entry = next(tmp_path.rglob("*.json"))
+    blocked = entry.parent / folder / f"{entry.stem}{suffix}"
+    blocked.unlink(missing_ok=True)
+    blocked.mkdir()
+
+    result = PythonInfo.from_exe(sys.executable, cache, ignore_cache=True, resolve_to_host=False)
+    assert result is not None
+    assert result.version_info == expected.version_info
+
+
 _LAUNCHER: Final[str] = r"C:\Program Files\Python Launcher\py.exe"
 _ARGS: Final[str] = '-3.11 --config "settings folder\\python.json"'
 _SELECTED: Final[str] = r"C:\Python311\python.exe"