gc debug "collectables" print can deadlock in free threading #161366

Bug report

import gc
import sys
import threading

stop = threading.Event()

def writer():
    while not stop.is_set():
        sys.stderr.write("x\n")

t = threading.Thread(target=writer)
t.start()

gc.set_debug(gc.DEBUG_COLLECTABLE)
for _ in range(1000):
    a = []
    a.append(a)
    del a
    gc.collect()
gc.set_debug(0)

stop.set()
t.join()
print("done")

From initial investigation, during a stop the world for GC debug collectables is printed which can occur whilst the BufferedWriter lock is being held. Debug for uncollectables is instead called outside this critical section and does not have this deadlock pattern.

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS

Linked PRs

• gh-161366: Fix deadlock with FT GC debug collectables print #161444