GeezerVison

…le locking

Fix a deadlock in the import system that occurs when concurrent threads
import modules at different levels of the same package hierarchy while
`__init__.py` files have circular imports.

The deadlock scenario (correctly analyzed by @emmatyping):
- Thread 1 imports `package.subpackage`, which imports `package.subpackage.module`
  in its `__init__.py`
- Thread 2 imports `package.subpackage.module`, which needs to ensure
  `package.subpackage` is loaded first
- Each thread holds a lock the other needs, causing deadlock

The fix introduces _HierarchicalLockManager that acquires all necessary
module locks in a consistent order (parent before child) for nested modules.
This ensures all threads acquire locks in the same order, preventing
circular wait conditions.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Resolved conflict in Lib/test/test_importlib/test_threaded_import.py:
both branches added a new test method at the same location. Kept both
test_import_failure_race_condition (from upstream) and
test_hierarchical_import_deadlock (from this PR).

…partial parent on cycle

If lock.acquire() raises (e.g. _DeadlockError or KeyboardInterrupt) after
some chain locks are already held, __exit__ is never called and the held
locks leak permanently. Wrap the loop so any exception releases what was
acquired before re-raising.

When acquiring a parent's lock would deadlock with another thread that is
loading that parent, skip the parent and proceed - the same accept-a-
partial-parent policy that _lock_unlock_module() applies. Without this,
concurrent cross-package circular imports that previously succeeded via
that path now raise _DeadlockError to user code. The leaf's lock is still
required; a deadlock there propagates as before.

Also lift _get_module_chain to a module-level function as it does not use
self.

…rcular test

test_hierarchical_import_deadlock: synchronise thread start with a
Barrier; assert errors == [] (the prior isinstance(error, ImportError)
check never matched _DeadlockError, a RuntimeError); record exception
type and message rather than the bare object so failures self-describe.

test_cross_package_circular_import: new. Two packages whose __init__.py
each import a submodule of the other; concurrent imports of further
submodules must not raise _DeadlockError.