bpo-38091: Import deadlock detection causes deadlock (GH-17518)
Automerge-Triggered-By: @brettcannon
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@ -67,6 +67,7 @@ class _ModuleLock:
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# Deadlock avoidance for concurrent circular imports.
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me = _thread.get_ident()
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tid = self.owner
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seen = set()
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while True:
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lock = _blocking_on.get(tid)
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if lock is None:
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@ -74,6 +75,14 @@ class _ModuleLock:
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tid = lock.owner
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if tid == me:
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return True
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if tid in seen:
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# bpo 38091: the chain of tid's we encounter here
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# eventually leads to a fixpoint or a cycle, but
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# does not reach 'me'. This means we would not
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# actually deadlock. This can happen if other
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# threads are at the beginning of acquire() below.
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return False
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seen.add(tid)
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def acquire(self):
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"""
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@ -436,16 +436,24 @@ class ImportTests(unittest.TestCase):
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os.does_not_exist
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def test_concurrency(self):
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# bpo 38091: this is a hack to slow down the code that calls
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# has_deadlock(); the logic was itself sometimes deadlocking.
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def delay_has_deadlock(frame, event, arg):
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if event == 'call' and frame.f_code.co_name == 'has_deadlock':
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time.sleep(0.1)
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sys.path.insert(0, os.path.join(os.path.dirname(__file__), 'data'))
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try:
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exc = None
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def run():
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sys.settrace(delay_has_deadlock)
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event.wait()
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try:
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import package
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except BaseException as e:
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nonlocal exc
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exc = e
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sys.settrace(None)
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for i in range(10):
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event = threading.Event()
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@ -0,0 +1 @@
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Tweak import deadlock detection code to not deadlock itself.
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