GeezerVison

Synchronize FrameLocalsProxy accesses with the frame's owning thread: fast locals are accessed under the frame object's critical section, and with the world stopped when the frame may be executing on another thread.

Changes:

• Only the fast locals read and write run in the synchronized region. Key lookup (__hash__ / __eq__) runs outside it, so no Python code is executed while the world is stopped.
• The extra locals dict is never touched by the running frame, so it does not need stop-the-world. It is fetched under the frame's critical section and used with the dict's own locking.
• The value replaced in a cell is decref'd after the region, since its destructor can run arbitrary code.
• keys(), values(), items() and len() snapshot the fast locals in the region and hold the extra locals dict's critical section while iterating it.
• PyFrame_GetVar is synchronized the same way.

Co-authored-by: Sam Gross [email protected]

• Issue: Data races with frame objects #144446