test_ssl: reference cycle between ThreadedEchoServer and its ConnectionHandler

[vstinner]

BPO	31323
Nosy	@vstinner
PRs	bpo-31323: Fix reference leak in test_ssl #3263 [3.6] bpo-31323: Fix reference leak in test_ssl (GH-3263) #3538

^{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 = None
closed_at = <Date 2017-09-13.20:56:41.797>
created_at = <Date 2017-09-01.14:58:24.937>
labels = ['3.7', 'tests', 'performance']
title = 'test_ssl: reference cycle between ThreadedEchoServer and its ConnectionHandler'
updated_at = <Date 2017-09-13.20:56:41.796>
user = 'https://github.com/vstinner'

bugs.python.org fields:

activity = <Date 2017-09-13.20:56:41.796>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2017-09-13.20:56:41.797>
closer = 'vstinner'
components = ['Tests']
creation = <Date 2017-09-01.14:58:24.937>
creator = 'vstinner'
dependencies = []
files = []
hgrepos = []
issue_num = 31323
keywords = ['patch']
message_count = 4.0
messages = ['301116', '301156', '302044', '302054']
nosy_count = 1.0
nosy_names = ['vstinner']
pr_nums = ['3263', '3538']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'resource usage'
url = 'https://bugs.python.org/issue31323'
versions = ['Python 3.7']

[vstinner]

While testing bpo-31234 fix, I noticed that some test_ssl tests using ThreadedEchoServer create reference cycles with ConnectionHandler: ConnectionHandler fills ThreadedEchoServer.conn_errors with exceptions, but exceptions keep an alive traceback object which contains frame locals.

I tried to use traceback.clear_frames(), but this function is broken: see bpo-31321.

Attached PR works around the issue by storing errors are text rather than objects.

The PR prevents warnings about dangling threads.

[vstinner]

New changeset 8687101 by Victor Stinner in branch 'master':
bpo-31323: Fix reference leak in test_ssl (bpo-3263)
8687101

[vstinner]

I don't think that Python 2.7 is affected since Exception has no __traceback__ attribute in Python 2.7.

[vstinner]

New changeset 1b00bdd by Victor Stinner (Miss Islington (bot)) in branch '3.6':
[3.6] bpo-31323: Fix reference leak in test_ssl (GH-3263) (bpo-3538)
1b00bdd