python
PySequence_Fast needs new macros to be safe in a nogil world
Feature or enhancement
Proposal:
Right now, most uses of PySequence_Fast are invalid in a nogil context when it is passed an existing list; PySequence_FAST_ITEMS returns a reference to the internal array of PyObject*s that can be resized at any time if other threads add or delete items, PySequence_FAST_GET_SIZE similarly reports a size that is invalid an instant after it's reported.
But when the argument passed is a tuple (incref-ed and returned unchanged, but safe due to immutability) or any non-list type (converted to new list) no lock is needed. Per conversation with Dino, going to create macros, to be called after a call to PySequence_Fast, to conditionally lock and unlock the original list when applicable, while avoiding locks in all other cases, before any other PySequence* APIs are used.
Preliminary (subject to bike-shedding) macro names are:
Py_BEGIN_CRITICAL_SECTION_SEQUENCE_FAST
Py_END_CRITICAL_SECTION_SEQUENCE_FAST
both defined in pycore_critical_section.h (again, subject to bike-shedding; maybe belongs in abstract.h with the rest of the PySequence APIs?).
Has this already been discussed elsewhere?
This is a minor feature, which does not need previous discussion elsewhere
Links to previous discussion of this feature:
Discussion occurred with @DinoV during CPython core sprints.
