[BUG] Race condition writing to exttype attributes #7907

Describe the bug

Writing to object attributes of extension types uses decref before assignment, leaving the attribute value in an invalid state for a split second:

  __Pyx_GOTREF((PyObject *)__pyx_v_obj->attr);
  __Pyx_DECREF((PyObject *)__pyx_v_obj->attr);  // <- dead reference !
  __pyx_v_obj->attr = __pyx_v_value;

Code to reproduce the behaviour:

cdef class Value:
    pass

cdef class Ext:
    cdef Value attr

def update():
    x = Ext()
    value = Value()
    x.attr = value  # <-

Expected behaviour

We should use ExprNode.generate_decref_set() to generate "read-update-decref" ordering. That's still not completely safe (race condition between read and update) but avoids leaving the potentially dead reference in the attribute before updating it. Thus, concurrent updates would stay unsafe, but an update with only concurrent reads would become safe, which brings it to a reasonable and cheap level of safety that is user controllable.

OS

No response

Python version

No response

Cython version

master

Additional context

No response