Check for signals during regular expression matches

[joshhoyt]

BPO	846388
Nosy	@gvanrossum, @loewis, @facundobatista
Files	_sre.c.patch: Allow signal handlers to run during _sre searches patch patch.speedup sre_exception.diff

^{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/facundobatista'
closed_at = <Date 2008-01-09.14:22:46.703>
created_at = <Date 2003-11-21.06:29:21.000>
labels = ['interpreter-core']
title = 'Check for signals during regular expression matches'
updated_at = <Date 2008-02-05.04:13:26.208>
user = 'https://bugs.python.org/joshhoyt'

bugs.python.org fields:

activity = <Date 2008-02-05.04:13:26.208>
actor = 'gvanrossum'
assignee = 'facundobatista'
closed = True
closed_date = <Date 2008-01-09.14:22:46.703>
closer = 'facundobatista'
components = ['Interpreter Core']
creation = <Date 2003-11-21.06:29:21.000>
creator = 'joshhoyt'
dependencies = []
files = ['5695', '8679', '8680', '9064']
hgrepos = []
issue_num = 846388
keywords = ['patch']
message_count = 12.0
messages = ['44910', '44911', '44912', '57056', '57067', '57068', '59246', '59247', '59267', '59600', '61925', '62062']
nosy_count = 6.0
nosy_names = ['gvanrossum', 'loewis', 'effbot', 'facundobatista', 'joshhoyt', 'schmir']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue846388'
versions = ['Python 2.4']

[joshhoyt]

This patch adds a call to PyErr_CheckSignals to
SRE_MATCH so that signal handlers can be invoked during
long regular expression matches. It also adds a new
error return value indicating that an exception
occurred in a signal handler during the match, allowing
exceptions in the signal handler to propagate up to the
main loop.

Rationale:

Regular expressions can run away inside of the C code.
There is no way for Python code to stop the C code from
running away, so we attempted to use setrlimit to
interrupt the process when the CPU usage exceeded a limit.

When the signal was received, the signal function was
triggered. The sre code does not allow the main loop to
run, so the triggered handlers were not called until
the regular expression finished, if ever. This
behaviour makes the interruption by the signal useless
for the purposes of constraining the running time of
regular expression matches.

I am unsure whether the PyErr_CheckSignals is
lightweight enough to be called inside of the for loop
in SRE_MATCH, so I took the conservative approach and
only checked on recursion or match invocation. I
believe that the performance hit from this check would
not be prohibitive inside of the loop, since
PyErr_CheckSignals does very little work unless there
is a signal to handle.

[loewis]

Logged In: YES
user_id=21627

Can you give an example for a SRE matching that is so slow
that the user may press Ctrl-C?

[loewis]

Logged In: YES
user_id=21627

Fredrik, what do you think about this patch?

[schmir]

here is an example (from http://swtch.com/~rsc/regexp/regexp1.html)

python -c 'import re; num=25; r=re.compile("a?"*num+"a"*num);
r.match("a"*num)'

At work I have seen a real world case of a regular expression which ran
for minutes rendering the application unresponsive. We would have been
glad to be able to interrupt the regular expression engine and getting a
traceback.

[schmir]

I'm attaching a working patch against 2.5.1 and a short test program.

#! /usr/bin/env python

import signal
import re
import time


def main():
    num=28 # need more than 60s on a 2.4Ghz core 2
    r=re.compile("a?"*num+"a"*num)

    signal.signal(signal.SIGALRM, signal.default_int_handler)
    signal.alarm(1)
    stime = time.time()
    try:
        r.match("a"*num)
    except KeyboardInterrupt:
        assert time.time()-stime<3
    else:
        raise RuntimeError("no keyboard interrupt")

if __name__=='__main__':
    main()

[schmir]

hm. just noticed that calling PyErr_CheckSignals slows down regular
expression matching considerably (50%).

I'm adding another patch, which only checks every 4096th iteration for
signals.

[facundobatista]

Couldn't apply cleanly the patch, as it appears to be a diff in other
format.

Anyway, applied it by hand, and now I attach the correct svn diff.

The test cases run ok with this change, and the problem is solved.

Regarding the delay introduced, I tested it with:

  $ ./python timeit.py -s "import re;r=re.compile('a?a?a?a?a?aaaaa')"
"r.match('aaaaa')"

Trunk:
100000 loops, best of 3: 5.4 usec per loop
100000 loops, best of 3: 5.32 usec per loop
100000 loops, best of 3: 5.41 usec per loop

Patch applied:
100000 loops, best of 3: 7.28 usec per loop
100000 loops, best of 3: 6.79 usec per loop
100000 loops, best of 3: 7.00 usec per loop

I don't like that. Anyway, I do NOT trust for timing the system where
I'm making the timing, so you may get different results.

Suggestions?