Use deferred reference counting in some _PyInterpreterFrame fields

[colesbury]

Feature or enhancement

The _PyInterpreterFrame struct contains a strong reference to:

• f_executable - the currently executing code object
• f_funcobj - the function object
• f_locals - the locals object (often NULL)
• frame_obj - the frame object (often NULL)

We should use deferred references (in the free-threaded build) for f_executable and f_funcobj because they are a common source of reference count contention. The pointed to objects (code and function) already support deferred reference counting, but the references in the frame are not deferred.

I think we don't need to bother with changing f_locals for now. I don't think it's used frequently enough to be a bottleneck, but we can revisit it later if necessary.

The frame_obj are typically unique -- not shared across threads -- so we don't need to bother with deferred reference counting for frame_obj.

Complications and hazards

_PyInterpreterFrame are also embedded in generators/coroutines and PyFrameObject, which are heap objects. We need to be careful that _PyStackRef fields are visible to the GC in order to be kept alive. Once an object is untracked, the _PyStackRef fields may no longer be valid: it's safe to call PyStackRef_CLOSE/CLEAR but not otherwise access or dereference those fields because the GC may have already collected them.

Linked PRs

• gh-123923: Defer refcounting for f_executable in _PyInterpreterFrame #123924
• gh-123923: Defer refcounting for f_funcobj in _PyInterpreterFrame #124026