Reject unsupported subclass lock options #628

Severity: High. A subclass can accept construction while filelock discards a requested safety policy.

Problem

FileLockMeta._create_instance()
filters constructor arguments by the parameter names present in the subclass __init__ signature.

This supports older subclasses with narrow signatures, but it has two failure modes:

• A subclass with **kwargs does not receive base options because VAR_KEYWORD is not a named option.
• A narrow subclass discards non-default values such as on_acquired, preserve_lock_file, fallback_to_soft,
  context_error_policy, and close_error_policy.

Construction succeeds even though the requested behavior is inactive. The metaclass also calls inspect.signature() for
each construction.

Required contract

Pass all supported base options when the subclass accepts **kwargs. For a narrow signature, omit unsupported options
only when the caller supplied the documented default. Raise TypeError for each unsupported non-default option.

Cache the inspected signature and accepted-option model per subclass after the forwarding rules are correct. The cache
must not change singleton argument comparison.

Verification

• Define a virtualenv-style subclass with a narrow constructor and confirm that default construction still works.
• Pass each non-default base option to that subclass and confirm that construction raises before acquisition.
• Define a forwarding subclass with **kwargs and confirm that each option reaches BaseFileLock.
• Cover direct subclasses of sync and async base locks.
• Confirm that singleton reuse still rejects a changed hook object or policy.
• Benchmark repeated construction before and after signature caching with the same interpreter.

References

• Python inspect.signature()
• Python parameter kinds
• Compatibility reason recorded in source