"""Runtime guard enforcing the spec invariant: labels/masks are EVAL-ONLY.

Any code path that constructs, fits, or tunes the lesion subspace must run inside
`subspace_construction_guard()`. While that guard is active, any attempt to load a
lesion label or mask raises `LabelLeakError`. This ties the spec rule
("Never use lesion masks or labels to define, fit, or tune the lesion subspace")
to an enforceable runtime check, exercised by tests/test_label_leak.py.
"""
from __future__ import annotations

import threading
from contextlib import contextmanager

_state = threading.local()


class LabelLeakError(RuntimeError):
    """Raised when a lesion label/mask is accessed during subspace construction."""


def _in_construction() -> bool:
    return getattr(_state, "depth", 0) > 0


@contextmanager
def subspace_construction_guard():
    """Mark a region as label-free subspace construction.

    Mask/label loading inside this region is a spec violation and raises.
    """
    _state.depth = getattr(_state, "depth", 0) + 1
    try:
        yield
    finally:
        _state.depth -= 1


def assert_label_free(what: str = "lesion label/mask") -> None:
    """Call this at every label/mask read site. Raises inside subspace construction."""
    if _in_construction():
        raise LabelLeakError(
            f"Spec violation: attempted to access {what} during label-free "
            f"subspace construction. Masks/labels are EVAL-ONLY "
            f"(IMPLEMENTATION_SPEC §0.6, CLAUDE.md). "
            f"Move this access outside subspace_construction_guard()."
        )