GeezerVison

The type watcher implementation adds char tp_watched; to the PyTypeObject struct, and then in PyType_Modified it assigns int bits = type->tp_watched; and then treats bits as a bitset, expecting that only the low 8 bits should ever be set.

On platforms where char is signed (e.g. x86), if the top bit in tp_watched is set, the cast to int bits will sign-extend with 1s, and we will thus have a lot of high bits set in bits that are not supposed to ever be set.

On x86 in a debug build, without the fix in this PR, the new test hits the assertion in PyType_Modified that only bit positions < TYPE_MAX_WATCHERS should be set, and aborts.

The fix is simple: tp_watched should be defined as unsigned char so that it is zero-extended, not sign-extended.

Note that the test is carefully designed to not assume there are no type watchers already active. It doesn't attempt to register a fixed number of type watchers, instead it just registers type watchers until it reaches TYPE_MAX_WATCHERS - 1, the last available slot and the one that can trigger this bug (since it will set the highest bit in tp_watched when watching a type.)

• Issue: Allow extensions to set a callback to be invoked when a type is modified #91051