regrtest: change -R/--huntrleaks rule to decide if a test leaks references

[vstinner]

BPO	30776
Nosy	@pitrou, @vstinner, @serhiy-storchaka
PRs	bpo-30776: reduce regrtest -R false positives #2422 [3.6] bpo-30523, bpo-30764, bpo-30776: Sync regrtest from master #2441 [3.5] bpo-30523, bpo-30764, bpo-30776: Sync regrtest from master #2442 [2.7] bpo-30523, bpo-30764, bpo-30776: Sync regrtest from master #2444 bpo-30776: regrtest: reduce memleak false positive #2484 [3.6] bpo-29512, bpo-30776: Backport regrtest enhancements from master to 3.6 #2513 [3.5] bpo-29512, bpo-30764, bpo-30776: Backport regrtest enhancements from 3.6 to 3.5 #2540

^{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/vstinner'
closed_at = <Date 2018-09-19.23:10:15.484>
created_at = <Date 2017-06-26.21:49:23.239>
labels = ['3.7', 'tests']
title = 'regrtest: change -R/--huntrleaks rule to decide if a test leaks references'
updated_at = <Date 2018-09-19.23:10:15.483>
user = 'https://github.com/vstinner'

bugs.python.org fields:

activity = <Date 2018-09-19.23:10:15.483>
actor = 'vstinner'
assignee = 'vstinner'
closed = True
closed_date = <Date 2018-09-19.23:10:15.484>
closer = 'vstinner'
components = ['Tests']
creation = <Date 2017-06-26.21:49:23.239>
creator = 'vstinner'
dependencies = []
files = []
hgrepos = []
issue_num = 30776
keywords = []
message_count = 16.0
messages = ['296950', '296952', '296977', '296978', '297034', '297041', '297044', '297085', '297262', '297420', '297565', '301066', '301071', '301078', '301080', '301102']
nosy_count = 3.0
nosy_names = ['pitrou', 'vstinner', 'serhiy.storchaka']
pr_nums = ['2422', '2441', '2442', '2444', '2484', '2513', '2540']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue30776'
versions = ['Python 2.7', 'Python 3.5', 'Python 3.6', 'Python 3.7']

[vstinner]

Currently, regrtest considers that a test file leaks if at least one run increased or decreased the global reference counter. The problem is that in practice, Python is full of singletons, caches, and other jokes of the garbage collector, etc.

To reduce false alarms and focus more on real bugs, I suggest to change the reference difference checker from:

 def check_rc_deltas(deltas):
 return any(deltas)

to:

 def check_rc_deltas(deltas):
 return all(delta>=1 for delta in deltas)

It would allow to ignore false positives like:

• [3, 0, 0]
• [0, 1, 0]
• [8, -8, 1]

[vstinner]

I had to write this change to be able to bisect bpo-30775 reference leak. Without this change, my bisection tool picked the wrong tests and failed to find the tests which leaked. For example, it took a subtest of tests which was considered as leaking because the reference differences was [3, 0, 0]. This pattern more looks like a late initialization than a real leak. In the case of the bpo-30775, a leak was closer to [222, 222, 225]: much more references than just 3 :-) Many leaks for all runs :-)

[vstinner]

Real-world examples of false positives from the x86 Gentoo Refleaks 2.7 buildbot:

http://buildbot.python.org/all/builders/x86%20Gentoo%20Refleaks%202.7/builds/32/steps/test/logs/stdio

test_nntplib leaked [0, 85, 0] references, sum=85
test_multiprocessing leaked [0, 0, -35] references, sum=-35

[vstinner]

New changeset 48b5c42 by Victor Stinner in branch 'master':
bpo-30776: reduce regrtest -R false positives (bpo-2422)
48b5c42

[vstinner]

New changeset 35d2ca2 by Victor Stinner in branch '3.6':
[3.6] bpo-30523, bpo-30764, bpo-30776: Sync regrtest from master (bpo-2441)
35d2ca2

[vstinner]

New changeset de1850b by Victor Stinner in branch '3.5':
[3.5] bpo-30523, bpo-30764, bpo-30776: Sync regrtest from master (bpo-2442)
de1850b

[vstinner]

New changeset fea98bf by Victor Stinner in branch '2.7':
[2.7] bpo-30523, bpo-30764, bpo-30776: Sync regrtest from master (bpo-2444)
fea98bf

[vstinner]

It seems like the change works well since I didn't see any more false alarms on the Refleaks buildbots. I close the issue. The change has been applied to 2.7, 3.5, 3.6 and master branches.

[vstinner]

New changeset beeca6e by Victor Stinner in branch 'master':
bpo-30776: regrtest: reduce memleak false positive (bpo-2484)
beeca6e

[vstinner]

New changeset a3ca94d by Victor Stinner in branch '3.6':
[3.6] bpo-29512, bpo-30776: Backport regrtest enhancements from master to 3.6 (bpo-2513)
a3ca94d

[vstinner]

New changeset 0b12107 by Victor Stinner in branch '3.5':
[3.5] bpo-29512, bpo-30764, bpo-30776: Backport regrtest enhancements from 3.6 to 3.5 (bpo-2540)
0b12107