bpo-43977: Make sure that tp_flags for pattern matching are inherited correctly.

[markshannon]

Makes sure that Py_TPFLAGS_SEQUENCE and Py_TPFLAGS_MAPPING are mutually exclusive.

https://bugs.python.org/issue43977

[brandtbucher]

Thanks!

It looks like there might a typo (base instead of type) in typeobject.c:

[brandtbucher]

I think you mean type->tp_flags on the LHS of the last assignment, right?

Also, the base->tp_flags & COLLECTION_FLAGS check seems unnecessary.

Suggested change

	if ((base->tp_flags & COLLECTION_FLAGS) == 0) {
	return;
	}
	base->tp_flags |= base->tp_flags & COLLECTION_FLAGS;
	type->tp_flags |= base->tp_flags & COLLECTION_FLAGS;

[brandtbucher]

This might read better if the nested "if" conditions are merged. Also (looking at this again), I don't think the first PyType_Check call is needed.

So maybe something like:

    /* Set Py_TPFLAGS_SEQUENCE or Py_TPFLAGS_MAPPING flag */
    if (PyType_Check(self) &&
        PyType_HasFeature((PyTypeObject *)self, COLLECTION_FLAGS) &&
        !PyType_HasFeature((PyTypeObject *)subclass, Py_TPFLAGS_IMMUTABLETYPE))
    {
        ((PyTypeObject *)subclass)->tp_flags &= ~COLLECTION_FLAGS;
        ((PyTypeObject *)subclass)->tp_flags |= (((PyTypeObject *)self)->tp_flags & COLLECTION_FLAGS);
    }

(GitHub won't let me suggest changing lines that aren't part of the diff.)

[bedevere-bot]

When you're done making the requested changes, leave the comment: I have made the requested changes; please review again.

[markshannon]

@brandtbucher I've made your requested changes.
The additional tests I added showed another error: inherit_special was setting the flags before the MRO inheritance, so I've fixed that as well.

[brandtbucher]

Something we have yet to consider (can wait until after the freeze though): how do we want to handle classes that are registered as mappings or sequences after being subclassed? It's a strange case... do we go through and recursively update the flags of all children, or just say "that won't work for your previously-defined children" (perhaps with a warning at register time)?