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Description
Bug report
Bug description:
First of all, I know exactly how this bug is going to sound: batshit insane. I implore you to keep reading.
I've had a timer application for ages that uses signal.alarm() concurrently with input() so that it has a rudimentary way of displaying things in the background while still allowing user interaction on the CLI. The gist of it is this:
@staticmethod
def _input_timeout(prompt, timeout_secs):
class TimeExpiredException(Exception): pass
def raise_exception(signum, frame):
raise TimeExpiredException()
signal.signal(signal.SIGALRM, raise_exception)
try:
signal.alarm(timeout_secs)
result = input(prompt)
signal.alarm(0)
except TimeExpiredException:
result = None
return result
This program has mysteriously stopped working with Python 3.13.3 with an exceptionally strange error mode: my signal handler is called from somewhere I apparently do not expect (HelpFormatter), causing the whole process to terminate. This always happens after exactly 29 seconds when I run my process as user and after exactly 25 seconds when I run as root. It happens only when there is no __pycache__ present, i.e., when the Python bytecode is generated on startup.
I tried creating a minimal reproducer, which was super difficult. Even when I remove dead code from the Python script, the bug stops triggering. Some code removal was fine, but for example if I remove the wrapper around argparse, the bug disappears.
As I said. This sounds batshit crazy.
When the mystery signal is caught, it gives one final indication where it comes from:
reliant [/tmp/heisenbug]: rm -fr __pycache__/ && time ./stopwatch
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Exception ignored in tp_clear of HelpFormatter:
Traceback (most recent call last):
File "/tmp/heisenbug/./stopwatch", line 69, in raise_exception
raise TimeExpiredException()
StopWatch._input_timeout.<locals>.TimeExpiredException:
FINALLY
real 0m25,035s
user 0m0,025s
sys 0m0,011s
Note the message: Exception ignored in tp_clear of HelpFormatter: (this was running as root, you see that it took 25 seconds.)
Just so you believe me that it always triggers after the same time, here are three consecutive runs as root (I swear this is not copy paste, timings are accurate to millisecond level!):
reliant [/tmp/heisenbug]: rm -fr __pycache__/ && time ./stopwatch
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Exception ignored in tp_clear of HelpFormatter:
Traceback (most recent call last):
File "/tmp/heisenbug/./stopwatch", line 69, in raise_exception
raise TimeExpiredException()
StopWatch._input_timeout.<locals>.TimeExpiredException:
FINALLY
real 0m25,035s
user 0m0,025s
sys 0m0,011s
reliant [/tmp/heisenbug]: rm -fr __pycache__/ && time ./stopwatch
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Exception ignored in tp_clear of HelpFormatter:
Traceback (most recent call last):
File "/tmp/heisenbug/./stopwatch", line 69, in raise_exception
raise TimeExpiredException()
StopWatch._input_timeout.<locals>.TimeExpiredException:
FINALLY
real 0m25,035s
user 0m0,025s
sys 0m0,011s
reliant [/tmp/heisenbug]: rm -fr __pycache__/ && time ./stopwatch
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Exception ignored in tp_clear of HelpFormatter:
Traceback (most recent call last):
File "/tmp/heisenbug/./stopwatch", line 69, in raise_exception
raise TimeExpiredException()
StopWatch._input_timeout.<locals>.TimeExpiredException:
FINALLY
real 0m25,035s
user 0m0,025s
sys 0m0,010s
And here some runs as user:
reliant joe [/tmp/heisenbug]: rm -fr __pycache__ && time ./stopwatch
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 Exception ignored in tp_clear of HelpFormatter:
Traceback (most recent call last):
File "/tmp/heisenbug/./stopwatch", line 69, in raise_exception
raise TimeExpiredException()
StopWatch._input_timeout.<locals>.TimeExpiredException:
FINALLY
real 0m29,038s
user 0m0,029s
sys 0m0,010s
reliant joe [/tmp/heisenbug]: rm -fr __pycache__ && time ./stopwatch
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 Exception ignored in tp_clear of HelpFormatter:
Traceback (most recent call last):
File "/tmp/heisenbug/./stopwatch", line 69, in raise_exception
raise TimeExpiredException()
StopWatch._input_timeout.<locals>.TimeExpiredException:
FINALLY
real 0m29,039s
user 0m0,027s
sys 0m0,012s
reliant joe [/tmp/heisenbug]: rm -fr __pycache__ && time ./stopwatch
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 Exception ignored in tp_clear of HelpFormatter:
Traceback (most recent call last):
File "/tmp/heisenbug/./stopwatch", line 69, in raise_exception
raise TimeExpiredException()
StopWatch._input_timeout.<locals>.TimeExpiredException:
FINALLY
real 0m29,038s
user 0m0,029s
sys 0m0,009s
I'm running on 6.14.0-15-generic #15-Ubuntu. My CPU is an AMD Ryzen Threadripper PRO 3945WX 12-Cores. I'm using the Ubuntu-provided Python package (Ubuntu calls it 3.13.3-1):
Python 3.13.3 (main, Apr 8 2025, 19:55:40) [GCC 14.2.0] on linux
I cannot reproduce this issue on Debian with Python 3.11/ARM nor on another Ubuntu with Python 3.12/x86_64.
I cannot attach files (the reproducer) to this bug, so here it is as base64:
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You can simply unpack it by doing:
$ openssl base64 -d | tar xvz
And pasting the above base64 blob in. Remember you have to clear __pycache__ to trigger the bug. I'm including the bytecode in case it contains something interesting.
Please someone tell me this is reproducible and I'm not completely losing my marbles.
CPython versions tested on:
3.13
Operating systems tested on:
Linux
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