IDLE: in shell, time.sleep ignores _thread.interrupt_main()

[Mark]

BPO	29926
Nosy	@terryjreedy, @pitrou, @njsmith, @vadmium, @eryksun, @mlouielu
PRs	gh-74112: IDLE: Fix blocking function ignore SIGINT #2466
Files	int-unix.patch

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/terryjreedy'
closed_at = None
created_at = <Date 2017-03-27.22:51:11.745>
labels = ['expert-IDLE', 'type-bug', '3.9']
title = 'IDLE: in shell, time.sleep ignores _thread.interrupt_main()'
updated_at = <Date 2019-09-19.18:42:59.551>
user = 'https://bugs.python.org/Mark'

bugs.python.org fields:

activity = <Date 2019-09-19.18:42:59.551>
actor = 'terry.reedy'
assignee = 'terry.reedy'
closed = False
closed_date = None
closer = None
components = ['IDLE']
creation = <Date 2017-03-27.22:51:11.745>
creator = 'Mark'
dependencies = []
files = ['46771']
hgrepos = []
issue_num = 29926
keywords = ['patch']
message_count = 31.0
messages = ['290663', '290675', '290677', '290679', '290715', '290728', '290743', '290744', '290749', '290751', '290761', '291049', '291052', '291054', '297157', '297163', '297176', '297241', '297242', '297286', '297288', '297291', '297307', '297323', '297330', '297334', '297345', '297347', '297349', '297352', '297477']
nosy_count = 7.0
nosy_names = ['terry.reedy', 'pitrou', 'njs', 'martin.panter', 'Mark', 'eryksun', 'louielu']
pr_nums = ['2466']
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue29926'
versions = ['Python 3.9']

[Mark]

Consider the following code, typed interactively:

>> import time
>> time.sleep(1e6)

This will sleep for a bit over one and a half weeks. If this was typed in error, you may want to interrupt it. If using the command line, this is easy: just use Ctrl-C. If using IDLE, Ctrl-C has no effect. One could attempt to restart the shell with Ctrl-F6, which seems to work, but in fact the process remains in the background, hung until the timeout expires. There are two obvious workarounds: one is to sleep in a separate thread, so as to avoid blocking the main thread, and the other is to use a loop with smaller sleep increments:

for ii in range(1e5): sleep(10)

Now it only takes 10 seconds to interrupt a sleep. But these are both clumsy workarounds. They're so clumsy that I think I'm not going to use IDLE for this particular program and just use python -I. Would be nice if this were fixed.

[terryjreedy]

I verified on Win 10 with 3.6.1. What system are you running on?

An automatic unittest test would be nice, but I cannot imagine how to write one. Even a human-verified htest (IDLE private term) would be good. Maybe I should write a live-interaction test script.

^C is bound to pyshell.PyShell.cancel_callback. When code is executing, this calls pyshell.ModifiedInterpreter.interrupt_subprocess. In a new thread, this starts pyshell.ModifiedInterpreter.__request_interrupt. I verified the calls thus far, with debug prints, while the user execution thread is sleeping. __request_interrupt sends vua rpc ('exec', 'interrupt_the_server').

'interrupt_the_server' refers to a method of run.Executor:
if interruptable:
_thread.interrupt_main()

Here I am unsure which thread this executes in and when. Interrupting "while True: a=1" is no problem. Does interrup_main not work while the main thread is sleeping, or does the above not get executed?

[terryjreedy]

Scratch my confusion. I added "print('after')" immediately after _thread.interrupt_main() in idlelib.run.Executor.interrupt_the_server() and 'after' is printed in Shell right after I hit ^C but a time.sleep(10) is not immediately interrupted and 'KeyboardInterrupt' does not show for nearly 10 seconds. I don't know if this is a bug or unavoidable limitation not mentioned in the _thread doc. I might be system dependent.
https://docs.python.org/3/library/_thread.html#_thread.interrupt_main

[eryksun]

It's simple to fix this in Python 3 on Windows. PyErr_SetInterrupt in Modules/signalmodule.c needs the following addition:

    #ifdef MS_WINDOWS
        SetEvent(sigint_event);
    #endif

In the main thread on Windows, time.sleep() waits on this event.

On Unix, time.sleep() uses select(). We could interrupt it by signaling the process, or explicitly the main thread, via kill() or pthread_kill(). See bpo-21895 (#66094) for a related discussion.

[terryjreedy]

Eryk, would the addition go before or after "trip_signal(SIGINT);"

I posted "Issue with _thread.interrupt_main (29926)" on pydev list. Steven D'Aprano verified delayed response on *nix.

Martin Panter said
"Looking at the implementation, _thread.interrupt_main just calls PyErr_SetInterrupt. It doesn’t appear to send a signal. I played with “strace” and couldn’t see any evidence of a signal. I guess it just sets a flag that will be polled. To actually interrupt the “sleep” call, you might need to use “pthread_kill” or similar (at least on Unix)."