Bug report

Bug description:

Very similar to #151424, but this is not about impossible stack traces but implausible. :-)

Found while wrapping my head around #151424 and checking the source code for internals of last_profiled_frame.

Mechansim

The interpr has to advance tstate->last_profiled_frame for us. It does so in _PyEval_FrameClearAndPop:

if (tstate->last_profiled_frame != NULL && tstate->last_profiled_frame == frame) {

But the helper not always used when a frame is removed from the chain.

Naively:

_YIELD_VALUE missed:

	frame = tstate->current_frame = frame->previous;
	gen_frame->previous = NULL;

RETURN_GENERATOR missed:

_PyThreadState_PopFrame(tstate, frame);

_gen_throw missed:

tstate->current_frame = prev;

frame-init fail missed:

clear_thread_frame(tstate, frame);

Reproduction

def gen():
    while True:
        n = yield
        sum(range(n))
g = gen()
next(g)
def drv_a(): g.send(5000)
def drv_b(): g.send(50000)
while True:
    drv_a(); drv_b()

2026-06-13T11:41:14.582916000+0200 maurycy@gimel /Users/maurycy/work/cpython-main (main f4f1020?) % sudo ./python.exe -m profiling.sampling run --collapsed -o /tmp/stacks.txt -d  10 genrepro.py&& cat /tmp/stacks.txt
Captured 10,000 samples in 10.00 seconds
Sample rate: 999.99 samples/sec
Error rate: 0.01
Collapsed stack output written to /tmp/stacks.txt
tid:13788812;<frozen runpy>:_run_module_as_main:201;<frozen runpy>:_run_code:87;genrepro.py:<module>:13;genrepro.py:drv_b:10;genrepro.py:gen:4 9990
tid:13788812;<frozen runpy>:_run_module_as_main:201;<frozen runpy>:_run_code:87;genrepro.py:<module>:13;genrepro.py:drv_b:10;genrepro.py:gen:3 6
tid:13788812;<frozen runpy>:_run_module_as_main:201;<frozen runpy>:_run_code:87;genrepro.py:<module>:13 3
2026-06-13T11:41:27.285300000+0200 maurycy@gimel /Users/maurycy/work/cpython-main (main f4f1020?) %

Where is drv_a? TOTALLY_GONE.

CPython versions tested on:

CPython main branch

Operating systems tested on: