PyObject_RichCompareBool and PyObject_IsTrue error-as-truthy in signal disconnect, compare, and observer call

[devdanzin]

Several functions use the return value of PyObject_RichCompareBool or PyObject_IsTrue in a boolean if(...) context. Both APIs return -1 on error, which is truthy in C/C++ — comparison errors are silently treated as "true" matches.

1. _Disconnector_call — wrong slot disconnected (signaling.cpp:444-450)

slot.richcmp(other, Py_EQ) returns -1 on error, treated as truthy → the first slot in the list matches on comparison error, causing the wrong slot to be disconnected.

Reproducer:

import gc; gc.disable()
import os
from enaml.signaling import Signal

class BadSlot:
    def __eq__(self, other):
        raise TypeError("comparison bomb")
    def __hash__(self):
        return id(self)
    def __call__(self): pass

class Host: pass
host = Host()
sig = Signal()
bound = sig.__get__(host, type(host))

bound.connect(lambda: None)
bound.connect(BadSlot())
try:
    bound.disconnect(BadSlot())
except SystemError as e:
    print(f"SystemError (BUG - error-as-truthy): {e}")
except TypeError as e:
    print(f"TypeError propagated (correct): {e}")
os._exit(0)
# SystemError: <enaml.signaling._Disconnector object ...> returned a result with an exception set

2. BoundSignal_richcompare (signaling.cpp:622)

Same pattern — error during __eq__ treated as truthy.

3. CallableRef_richcompare (callableref.cpp:123,132)

Same pattern in both Py_EQ and Py_NE branches.

4. PyObject_IsTrue in SubscriptionObserver_call (subscription_observer.cpp:122)

PyObject_IsTrue(self->ref) returns -1 on error. Used in if(...) where -1 is truthy → proceeds with active exception.

Fix for all sites — check for error before branching:

int cmp = slot.richcmp( other, Py_EQ );
if( cmp < 0 )
    return 0;  // propagate exception
if( cmp ) { ... }

Found by cext-review-toolkit.