gh-159130: _remote_debugging: Do not return another thread when the selected thread is filtered out

[maurycy]

Please see #159130 for more details.

Repro no longer holds:

maurycy@gimel cpython-issue1 (remote-debugging-next-thread b20e3d1*?) % cat /tmp/repro_filter.py
import os, threading, time
from _remote_debugging import RemoteUnwinder

PROFILING_MODE_EXCEPTION = 4


def sample():
    time.sleep(0.2)
    unwinder = RemoteUnwinder(
        os.getpid(), only_active_thread=True, mode=PROFILING_MODE_EXCEPTION
    )
    print("sampler:", threading.get_native_id())
    print(
        "sampled:",
        [
            (t.thread_id, t.frame_info[0].funcname)
            for i in unwinder.get_stack_trace()
            for t in i.threads
        ],
    )


t = threading.Thread(target=sample)
t.start()
try:
    raise ValueError
except ValueError:
    t.join()
maurycy@gimel cpython-issue1 (remote-debugging-next-thread b20e3d1*?) % ./python.exe /tmp/repro_filter.py
sampler: 10865679
sampled: []
maurycy@gimel cpython-issue1 (remote-debugging-next-thread b20e3d1*?) %

• Issue: _remote_debugging: only_active_thread=True can return a different thread after mode filtering #159130

[maurycy]

Suggested change

	if (self->tstate_addr || self->only_active_thread) {
	if (self->only_active_thread) {

I think this could be sufficient but I kept this the same as this condition for symmetry:

https://github.com/python/cpython/pull/159132/changes#diff-9676fd8cee6e0d841f42f405a8e33a434fbebbc861207051571c97cef17c4108R848-R850