gh-120321: Lock gen_send by Fidget-Spinner · Pull Request #120327 · python/cpython

I wonder if we really only need the locks up to the transition to gen->gi_frame_state = FRAME_EXECUTING;. That would seem to work better with fixing SEND in the opcode as well as doing the unlocking there isn't going to be as easy.

I think other than the asserts that only leaves if (FRAME_STATE_SUSPENDED(gen->gi_frame_state)) { as the lone racing read. I'm not sure that this is fully really safe even w/ the GIL though. It seems like _PyEval_EvalFrameDefault wouldn't release the GIL between yielding a value and getting back to here, but something that's hooked the evaluation function could indeed release the GIL. So this is really already racing against another thread resuming the generator already (and you shouldn't get an interpreter crash, just odd results from trying to run a generator from multiple threads at the same time, which isn't really a sane thing to do anyway).