Join started threads in tests

[serhiy-storchaka]

BPO	23799
Nosy	@pitrou, @vstinner, @ezio-melotti, @voidspace, @serhiy-storchaka
Files	test_support_start_threads.patch test_support_start_threads_2.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/serhiy-storchaka'
closed_at = <Date 2015-04-04.06:53:34.628>
created_at = <Date 2015-03-28.20:10:30.324>
labels = ['type-feature', 'tests']
title = 'Join started threads in tests'
updated_at = <Date 2015-04-04.06:53:34.627>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2015-04-04.06:53:34.627>
actor = 'serhiy.storchaka'
assignee = 'serhiy.storchaka'
closed = True
closed_date = <Date 2015-04-04.06:53:34.628>
closer = 'serhiy.storchaka'
components = ['Tests']
creation = <Date 2015-03-28.20:10:30.324>
creator = 'serhiy.storchaka'
dependencies = []
files = ['38720', '38738']
hgrepos = []
issue_num = 23799
keywords = ['patch']
message_count = 4.0
messages = ['239465', '239583', '239601', '239780']
nosy_count = 6.0
nosy_names = ['pitrou', 'vstinner', 'ezio.melotti', 'michael.foord', 'python-dev', 'serhiy.storchaka']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue23799'
versions = ['Python 2.7', 'Python 3.4', 'Python 3.5']

[serhiy-storchaka]

When a test starts many threads in a loop, there is a chance that the starting thread will fail due to lack of memory. In this case all started threads left dangling and make MemotyError even more probably. They also can provoke dim error messages in this and in following tests. Proposed patch adds new helper function test.support.start_threads() that starts threads and then join started threads.

Some tests even hang without this patch.

[vstinner]

In this case all started threads left dangling and make MemotyError even more probably

Oh, I didn't know that. Good catch.

test_support_start_threads.patch looks good to me and it looks like a nice enhancement.

But would it be possible to emit a warning if a thread hangs?

For example, try to join with a timeout of 60 seconds, and then display a warning, and retry with a timeout of 120 seconds, then display a second warning, etc.

Maybe we may even give up after a timeout (ex: 5 minutes, or 15 minutes, it's up to you). An exception must be raised in this case. You can use faulthandler.dump_traceback() to display the traceback of all threads in this case.

I hope that my concern is more theorical, but it would reduce the duration of tests when the test suite is stuck (which is quite common), and so being able to test more changesets per day.

--

I noticed that under low memory condition, some tests hang because Python doesn't handle notify the "parent thread" that the "child thread" fails to start. So the parent thread is stuck and nothing happens. Well, I'm not sure that it's related to this issue. At least, test_support_start_threads.patch doesn't help.

[serhiy-storchaka]

Something like this?

[python-dev]

New changeset f8358f6e50e7 by Serhiy Storchaka in branch '2.7':
Issue bpo-23799: Added test.test_support.start_threads() for running and
https://hg.python.org/cpython/rev/f8358f6e50e7

New changeset 8987218adc99 by Serhiy Storchaka in branch '3.4':
Issue bpo-23799: Added test.support.start_threads() for running and cleaning up
https://hg.python.org/cpython/rev/8987218adc99

New changeset a2df4baa112b by Serhiy Storchaka in branch 'default':
Issue bpo-23799: Added test.support.start_threads() for running and cleaning up
https://hg.python.org/cpython/rev/a2df4baa112b