Bug report
The update to the invalidation counter is not thread-safe and can lose updates in some Python implementations:
Failures seen on:
- Python 3.14t
- Python 3.9
- pypy3.10
- pypy3.11
But not on Python 3.10-3.14 with GIL due to limited GIL switch opportunities.
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def register(cls, subclass): |
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"""Register a virtual subclass of an ABC. |
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|
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Returns the subclass, to allow usage as a class decorator. |
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""" |
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if not isinstance(subclass, type): |
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raise TypeError("Can only register classes") |
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if issubclass(subclass, cls): |
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return subclass # Already a subclass |
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# Subtle: test for cycles *after* testing for "already a subclass"; |
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# this means we allow X.register(X) and interpret it as a no-op. |
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if issubclass(cls, subclass): |
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# This would create a cycle, which is bad for the algorithm below |
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raise RuntimeError("Refusing to create an inheritance cycle") |
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cls._abc_registry.add(subclass) |
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ABCMeta._abc_invalidation_counter += 1 # Invalidate negative cache |
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return subclass |
For example, consider the following repro, adapted from test_abc.test_registration_basics:
import _py_abc as abc # Use Python implementation of ABCs!!
import threading
import os
import sys
sys.setswitchinterval(1e-6)
N = 5
def run(b):
b.wait()
class A(metaclass=abc.ABCMeta):
pass
A.register(int)
if not isinstance(42, A):
print("Oops!")
os._exit(1)
def main():
for _ in range(10000):
threads = []
b = threading.Barrier(N)
for _ in range(N):
t = threading.Thread(target=run, args=(b,))
threads.append(t)
t.start()
for t in threads:
t.join()
if __name__ == "__main__":
main()Linked PRs
Bug report
The update to the invalidation counter is not thread-safe and can lose updates in some Python implementations:
Failures seen on:
But not on Python 3.10-3.14 with GIL due to limited GIL switch opportunities.
cpython/Lib/_py_abc.py
Lines 54 to 70 in 05e89c3
For example, consider the following repro, adapted from
test_abc.test_registration_basics:Linked PRs
_py_abctests as not thread-safe #130131