[issue21822] KeyboardInterrupt during Thread.join hangs that Thread

[Eryk Sun]

Change by Eryk Sun :


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components: +Extension Modules, Interpreter Core
versions: +Python 3.10, Python 3.9 -Python 3.6, Python 3.7

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[issue21822] KeyboardInterrupt during Thread.join hangs that Thread

[STINNER Victor]

Change by STINNER Victor :


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[issue21822] KeyboardInterrupt during Thread.join hangs that Thread

[Géry]

Géry  added the comment:

I am having the same blocked signal issue on Windows when using Thread.join. 
This program does not print "interrupted" after pressing Ctrl+C:


import threading
import time


def f():
while True:
print("processing")
time.sleep(1)


if __name__ == "__main__":
try:
thread = threading.Thread(target=f)
thread.start()
thread.join()
except KeyboardInterrupt:
print("interrupted")


For reference, 2 years ago Nathaniel Smith gave an interesting explanation here:
https://mail.python.org/pipermail/python-dev/2017-August/148800.html.

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[issue21822] KeyboardInterrupt during Thread.join hangs that Thread

[Steve]

Change by Steve :


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[issue21822] KeyboardInterrupt during Thread.join hangs that Thread

[gaborbernat]

gaborbernat  added the comment:

I think I'm hitting this with subprocesses inside tox (parallel feature), any 
plans to fix this?

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[issue21822] KeyboardInterrupt during Thread.join hangs that Thread

[STINNER Victor]

Change by STINNER Victor :


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[issue21822] KeyboardInterrupt during Thread.join hangs that Thread

[Paul Goins]

Paul Goins  added the comment:

Good point, I forgot about WaitForMultipleObjectsEx; something like that seems 
like it would be much simpler for the first 2 cases.

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[issue21822] KeyboardInterrupt during Thread.join hangs that Thread

[Antoine Pitrou]

Antoine Pitrou  added the comment:

multiprocessing semaphores support Ctrl-C under Windows, so it should be doable 
for regular locks as well (notice the `sigint_event`):
https://github.com/python/cpython/blob/master/Modules/_multiprocessing/semaphore.c#L109-L146

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[issue21822] KeyboardInterrupt during Thread.join hangs that Thread

[Paul Goins]

Paul Goins  added the comment:

Okay, I think I need to abandon my research into this.  This does seem to have 
quite an amount of complexity to it and is probably more than I should be 
taking on at this point in time.

Anyone else who wants to look at this, consider it fair game.

Parting thoughts based on my limited expertise in the area, take them or ignore 
them:

* Semaphore-based AllocNonRecursiveMutex (!_PY_USE_CV_LOCKS): Using 
WaitForSingleObjectEx with alertable set to TRUE may be one path forward, 
however it seems like that would involve tracking all threads performing a 
ctrl-c-interruptible wait and calling QueueUserAPC with a no-op callback for 
each such thread to cause the wait to terminate early.  I don't particularly 
like the need to roll-our-own tracking, but at least off-hand and based on my 
somewhat limited experience in this area, I don't know of a better way.  
Hopefully someone else does.

* CriticalSection/Semaphore-based AllocNonRecursiveMutex (_PY_USE_CV_LOCKS with 
_PY_EMULATED_WIN_CV): I don't know of a way of interrupting the CriticalSection 
lock, but the WaitForSingleObjectEx strategy may be usable on the 
semaphore-based condition variable.

* SRWLock/ConditionVariable-based AllocNonRecursiveMutex (_PY_USE_CV_LOCKS with 
!_PY_EMULATED_WIN_CV): Similarly, I don't know of a way to interrupt the 
SRWLock.  However, similar to WaitForSingleObjectEx, it may be possible to wake 
the condition variables via a Ctrl-C if we add our own tracking for such.  I'm 
not sure if this gets us quite to where we need to be or not.

Hopefully the above notes are of some value.

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[issue21822] KeyboardInterrupt during Thread.join hangs that Thread

[Paul Goins]

Paul Goins  added the comment:

Ahh, thanks for the explanation.  I didn't think about that.  Let's *not* do 
that then. :)

I'll see if I can squeeze in some time to play with the alternatives.

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[issue21822] KeyboardInterrupt during Thread.join hangs that Thread

[Antoine Pitrou]

Antoine Pitrou  added the comment:

> I'm guessing this is because of the perceived performance impact?

The problem is polling is pretty detrimental to power saving on modern CPUs.  
There might also be a performance impact that makes things worse :-)

More generally, we really would like locks to be interruptible under Windows, 
as it would be useful in many more situations than joining threads. 
Unfortunately, as you have discovered we have several lock implementations for 
Windows right now.  The SRWLock one is probably difficult to make 
interruptible, but it's never enabled by default.  The Semaphore one looks 
legacy, so could perhaps be removed.  Remains the condition variable-based 
implementation.

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[issue21822] KeyboardInterrupt during Thread.join hangs that Thread

[Paul Goins]

Paul Goins  added the comment:

> I think adding polling to such a widely-used primitive is out of question.

I'm guessing this is because of the perceived performance impact?  (That's the 
main thought I have against polling in this case.)  Or is it something else?

I can certainly look at tweaking the 3 mutex implementations I mentioned 
previously, but I do expect that will be a bit more code; I at least wanted to 
put the "simpler" idea out there.  Fully knowing that "simpler" isn't 
necessarily better.

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[issue21822] KeyboardInterrupt during Thread.join hangs that Thread

[Antoine Pitrou]

Antoine Pitrou  added the comment:

> One way to possibly make this work (although perhaps not as clean as may be 
> desired) would be to add polling logic into the thread_nt.h version of 
> PyThread_acquire_lock_timed.

I think adding polling to such a widely-used primitive is out of question.

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[issue21822] KeyboardInterrupt during Thread.join hangs that Thread

[Paul Goins]

Paul Goins  added the comment:

Focusing on the Windows case specifically...  One way to possibly make this 
work (although perhaps not as clean as may be desired) would be to add polling 
logic into the thread_nt.h version of PyThread_acquire_lock_timed.

That would have the benefit of avoiding the complexity of the various "non 
recursive mutex" implementations (i.e. semaphores vs "emulated" condition 
variables vs native condition variables) and may be less code than setting up 
"alertable" WaitForObjectSignleObjectEx calls (plus whatever else needs to be 
done for the SRW-lock-based code path).

Thoughts or feedback?  (I've not done any mainline Python commits yet so I'm 
totally ready to be completely wrong or misguided here.)

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[issue21822] KeyboardInterrupt during Thread.join hangs that Thread

[Eryk Sun]

Eryk Sun  added the comment:

For POSIX systems, try the following test function several times. For the bug 
to manifest, Thread._wait_for_tstate_lock has to be interrupted in between 
acquiring and releasing the sentinel lock. Maybe it could use a reentrant lock 
in order to avoid this problem.

import os
import signal
import threading
import time

def raise_sigint():
print('raising SIGINT')
time.sleep(0.5)
if os.name == 'nt':
os.kill(0, signal.CTRL_C_EVENT)
else:
os.kill(os.getpid(), signal.SIGINT)
print('finishing')

def test(f=raise_sigint):
global g_sigint
g_sigint = threading.Thread(target=f, name='SIGINT')
g_sigint.start()
print('waiting')
for i in range(100):
try:
if not g_sigint.is_alive():
break
g_sigint.join(0.01)
except KeyboardInterrupt:
print('KeyboardInterrupt ignored')
print('g_sigint is alive:', g_sigint.is_alive())

POSIX-only code normally wouldn't join() with a small timeout in a loop as in 
the above example. This is a workaround for the problem that's demonstrated in 
msg221180, in which a signal doesn't interrupt a wait on the main thread. Other 
than time.sleep(), most waits in Windows CPython have not been updated to 
include the SIGINT Event object when waiting in the main thread. (It's possible 
to fix some cases such as waiting on locks, but only as long as the 
implementation continues to use semaphores and kernel waits instead of native 
condition variables with SRW locks.)

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[issue21822] KeyboardInterrupt during Thread.join hangs that Thread

[Paul Goins]

Paul Goins  added the comment:

Not sure if I'll do the full fix (need to check w/ my employer), but I'm doing 
some investigation.  Here's what I know so far:

At the Python level, the KeyboardInterrupt is being raised within 
_wait_for_tstate_lock, on "elif lock.acquire(block, timeout)".

Going into the C code, it looks like this goes through 
lock_PyThread_acquire_lock -> acquire_timed -> PyThread_acquire_lock_timed.  
acquire_timed .  lock_PyThread_acquire_lock will abort the lock if it receives 
PY_LOCK_INTR from acquire_timed.

My best guess right now is that PyThread_acquire_lock_timed never returns 
PY_LOCK_INTR.  Indeed, I see this comment at the top of the NT version of that 
function:

/* Fow now, intr_flag does nothing on Windows, and lock acquires are
 * uninterruptible.  */

And indeed, the thread_pthread.h implementations both have a path for returning 
PY_LOCK_INTR, while the thread_nt.h version does not.

...And that's where I am thus far.

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[issue21822] KeyboardInterrupt during Thread.join hangs that Thread

[Antoine Pitrou]

Antoine Pitrou  added the comment:

Thanks for the update.  Since I still can't reproduce on Linux, perhaps you (or 
one of our resident Windows experts) can try to propose a patch fixing the 
issue?

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components: +Windows
nosy: +paul.moore, steve.dower, tim.golden, zach.ware
versions: +Python 3.7, Python 3.8 -Python 3.4, Python 3.5

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[issue21822] KeyboardInterrupt during Thread.join hangs that Thread

[Paul Goins]

Paul Goins  added the comment:

I've confirmed this issue on Windows.  Attached is an SSCCE which seems to 
reliably reproduce the issue in a Windows environment.

Long story short: if a KeyboardInterrupt occurs during Thread.join(), there's a 
good chance that Thread._is_stopped never gets set.

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nosy: +pdgoins
versions: +Python 3.6
Added file: 
https://bugs.python.org/file47549/sscce_thread_join_fails_on_keyboardinterrupt.py

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[issue21822] KeyboardInterrupt during Thread.join hangs that Thread

[STINNER Victor]

Changes by STINNER Victor victor.stin...@gmail.com:


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[issue21822] KeyboardInterrupt during Thread.join hangs that Thread

[Antoine Pitrou]

Changes by Antoine Pitrou pit...@free.fr:


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[issue21822] KeyboardInterrupt during Thread.join hangs that Thread

[Antoine Pitrou]

Antoine Pitrou added the comment:

This works for me under Linux.

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[issue21822] KeyboardInterrupt during Thread.join hangs that Thread

[Steve]

New submission from Steve:

I am attempting to join a thread after a previous join was interrupted by 
Ctrl-C.

I did not find any warning for this case in threading module docs, so I assume 
this is legal.

The full test script is attached, but the essential code is:

def work(t):
sleep(t)

twork = 3; twait = 4
t = Thread(target=work, args=(twork,))
try:
t.start()
t.join(twait)  # here I do Ctrl-C
except KeyboardInterrupt:
pass
t.join()  # this hangs if twork  twait


I can observe the following reproduce sequence:

1. start another thread that sleeps (or works) for T seconds
2. join that thread with timeout longer than T
3. before thread finishes and join returns, hit Ctrl-C to raise 
KeyboardInterrupt (KI)
4. after T seconds, thread finishes its (Python) code and KI is raised
5. Process Explorer confirms that thread really terminates (looked at .ident())
6. thread still reports .is_alive() == True
7. second attempt to join that thread hangs indefinitely

I tried replacing try-except clause with custom signal handler for SIGINT, as 
shown in the script. If the handler does not raise an exception, the thread can 
be normally joined. If it does, however, the behavior is the same as with 
default handler.

My _guess_ is that the exception prevents some finishing code that puts Thread 
into proper stopped state after its target completes.

Running Python 3.4.0 on Windows 7 x64

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components: Library (Lib)
files: join.py
messages: 221180
nosy: tupl
priority: normal
severity: normal
status: open
title: KeyboardInterrupt during Thread.join hangs that Thread
versions: Python 3.4
Added file: http://bugs.python.org/file35715/join.py

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