acquire()
if self._cache_complete:
    break                    # ← lock NOT released

try:
    for j in range(10):
        cache.append(advance_iterator(gen))
except StopIteration:
    self._cache_gen = gen = None
    self._cache_complete = True
    break                    # ← lock NOT released

release()                    # only reached on normal iteration path

import threading
from dateutil.rrule import rrule, DAILY
from datetime import datetime

r = rrule(DAILY, count=5, dtstart=datetime(2020, 1, 1), cache=True)

# Exhaust the cached iterator — triggers the StopIteration break path
list(r)

# At this point, the lock is held and never released.
# A second thread trying to iterate the same cached rrule will deadlock.