Inaccessible lock files now imply temporary directories can't be removed

Fix #7500

Co-authored-by: Ran Benita <ran@unusedvar.com>

changelog/7491.bugfix.rst

@@ -0,0 +1,2 @@
+:fixture:`tmpdir` and :fixture:`tmp_path` no longer raise an error if the lock to check for
+stale temporary directories is not accessible.

src/_pytest/pathlib.py

@@ -286,12 +286,17 @@ def maybe_delete_a_numbered_dir(path: Path) -> None:
 
 
 def ensure_deletable(path: Path, consider_lock_dead_if_created_before: float) -> bool:
-    """checks if a lock exists and breaks it if its considered dead"""
+    """checks if `path` is deletable based on whether the lock file is expired"""
     if path.is_symlink():
         return False
     lock = get_lock_path(path)
-    if not lock.exists():
-        return True
+    try:
+        if not lock.is_file():
+            return True
+    except OSError:
+        # we might not have access to the lock file at all, in this case assume
+        # we don't have access to the entire directory (#7491).
+        return False
     try:
         lock_time = lock.stat().st_mtime
     except Exception:

testing/test_pathlib.py

@@ -358,17 +358,25 @@ def test_get_extended_length_path_str():
 
 
 def test_suppress_error_removing_lock(tmp_path):
-    """ensure_deletable should not raise an exception if the lock file cannot be removed (#5456)"""
+    """ensure_deletable should be resilient if lock file cannot be removed (#5456, #7491)"""
     path = tmp_path / "dir"
     path.mkdir()
     lock = get_lock_path(path)
     lock.touch()
     mtime = lock.stat().st_mtime
 
-    with unittest.mock.patch.object(Path, "unlink", side_effect=OSError):
+    with unittest.mock.patch.object(Path, "unlink", side_effect=OSError) as m:
         assert not ensure_deletable(
             path, consider_lock_dead_if_created_before=mtime + 30
         )
+        assert m.call_count == 1
+    assert lock.is_file()
+
+    with unittest.mock.patch.object(Path, "is_file", side_effect=OSError) as m:
+        assert not ensure_deletable(
+            path, consider_lock_dead_if_created_before=mtime + 30
+        )
+        assert m.call_count == 1
     assert lock.is_file()
 
     # check now that we can remove the lock file in normal circumstances