copt: NULL-without-exception returns in SB_extends, IB__adapt__, CPB_descr_get

[jensens]

A detailed analysis of _zope_interface_coptimizations.c was published at
https://gist.github.com/devdanzin/5afbad864934b165a2cc080f110aa720 by @devdanzin.
We reviewed the report with agent support and verified the findings against the
current source (zope.interface 8.2). This issue is one of several extracted from
that review.

Problem

When _implied or _cls fields are NULL (possible because they use
T_OBJECT_EX which allows deletion), three functions return NULL without setting
a Python exception, causing SystemError:

• SB_extends line 331: if (implied == NULL) return NULL;
• IB__adapt__ line 789: same pattern for _implied
• CPB_descr_get line 618: if (self->_cls == NULL) return NULL;

Reproducer

from zope.interface._zope_interface_coptimizations import SpecificationBase
from zope.interface import Interface

sb = SpecificationBase()
sb.isOrExtends(Interface)
# SystemError: <method 'isOrExtends' of 'SpecificationBase' objects>
#   returned NULL without setting an exception

from zope.interface._zope_interface_coptimizations import ClassProvidesBase

cpb = ClassProvidesBase.__new__(ClassProvidesBase)
cpb.__get__(object(), type)
# SystemError: <slot wrapper '__get__' of 'ClassProvidesBase' objects>
#   returned NULL without setting an exception

Fix

Set an exception before returning NULL:

if (implied == NULL) {
    PyErr_SetString(PyExc_AttributeError, "_implied");
    return NULL;
}

Same for _cls in CPB_descr_get.

Scope

3 sites. No behavioral change for properly initialized objects.

Tests

Directly testable. The reproducers from the report work as-is as test cases:

def test_isOrExtends_raises_AttributeError_when_implied_is_None(self):
    from zope.interface.interface import SpecificationBase
    sb = SpecificationBase()
    # _implied is not set (NULL in C)
    with self.assertRaises(AttributeError):
        sb.isOrExtends(object())

def test_CPB_descr_get_raises_AttributeError_when_cls_is_None(self):
    from zope.interface._zope_interface_coptimizations import ClassProvidesBase
    cpb = ClassProvidesBase.__new__(ClassProvidesBase)
    with self.assertRaises(AttributeError):
        cpb.__get__(object(), type)

These currently raise SystemError (the bug). After the fix they raise
AttributeError. The tests verify the fix works.

Note: these test the C implementation specifically. The Python fallback may
behave differently (regular AttributeError from Python attribute access). Both
paths should be tested via the existing Fallback/Optimized test class pattern.

Performance impact

Zero. The NULL check already exists in the current code. The fix only adds a
PyErr_SetString call inside the existing never-taken branch. No new branches,
no new instructions on the happy path.