Attached patch fixes 3 ResourceWarning warnings in Lib/test/test_urllib2_localnet.py.

See also issue #26612 (test_ssl).

Example of warning logged by "./python -X tracemalloc=25  -m test -v test_urllib2_localnet ":
---
/home/haypo/prog/python/default/Lib/test/support/__init__.py:1444: ResourceWarning: unclosed <socket.socket [closed] fd=6, family=AddressFamily.AF_INET, type=SocketKind.SOCK_STREAM, proto=6>
  gc.collect()
Object allocated at (most recent call first):
  File "/home/haypo/prog/python/default/Lib/socket.py", lineno 697
    sock = socket(af, socktype, proto)
  File "/home/haypo/prog/python/default/Lib/http/client.py", lineno 898
    (self.host,self.port), self.timeout, self.source_address)
  File "/home/haypo/prog/python/default/Lib/http/client.py", lineno 926
    self.connect()
  File "/home/haypo/prog/python/default/Lib/http/client.py", lineno 983
    self.send(msg)
  File "/home/haypo/prog/python/default/Lib/http/client.py", lineno 1151
    self._send_output(message_body)
  File "/home/haypo/prog/python/default/Lib/http/client.py", lineno 1200
    self.endheaders(body)
  File "/home/haypo/prog/python/default/Lib/http/client.py", lineno 1155
    self._send_request(method, url, body, headers)
  File "/home/haypo/prog/python/default/Lib/urllib/request.py", lineno 1303
    h.request(req.get_method(), req.selector, req.data, headers)
  File "/home/haypo/prog/python/default/Lib/urllib/request.py", lineno 1331
    return self.do_open(http.client.HTTPConnection, req)
  File "/home/haypo/prog/python/default/Lib/urllib/request.py", lineno 503
    result = func(*args)
  File "/home/haypo/prog/python/default/Lib/urllib/request.py", lineno 543
    '_open', req)
  File "/home/haypo/prog/python/default/Lib/urllib/request.py", lineno 525
    response = self._open(req, data)
  File "/home/haypo/prog/python/default/Lib/urllib/request.py", lineno 222
    return opener.open(url, data, timeout)
  File "/home/haypo/prog/python/default/Lib/test/test_urllib2_localnet.py", lineno 595
    urllib.request.urlopen(req)
  (...)
---

This patch looks okay to me. I left one review suggestion.

Focussing on test_sending_headers(), the ResourceWarning seems to be only shown since revision 46329eec5515 (Issue 26590). In simpler cases, the warning would be bypassed due to Issue 19829. But in this cases it seems there is a garbage cycle which may be involved in the new warning.

New changeset 54d7e9919876 by Victor Stinner in branch 'default':
Closes #26620: Fix ResourceWarning in test_urllib2_localnet
https://hg.python.org/cpython/rev/54d7e9919876

Martin wrote:
> This patch looks okay to me. I left one review suggestion.

Thanks for the review, I changed this code.

While testing one more time my patch with CTRL+c, I noticed that sometimes the servers are not stopped properly and os.environ is not restored.

I made some extra changes to try to restore os.environ and try to close the server in more cases.

It's not perfect. Since unittest.TestCase.doCleanups() doesn't handle KeyboardInterrupt, there is no warranty that cleanup functions are always executed.

> Focussing on test_sending_headers(), the ResourceWarning seems to be only shown since revision 46329eec5515 (Issue 26590). In simpler cases, the warning would be bypassed due to Issue 19829. But in this cases it seems there is a garbage cycle which may be involved in the new warning.

It's possible that Python 3.5 failed to log ResourceWarning in some cases, Python 3.6 should be better on this part.

This handling of KeyboardInterrupt seems like a bug with unittest IMO. But personally I don’t spend too much thought or code handling KeyboardInterrupt in tests, for similar reasons to not doing special handling of test failures in Issue 26612.

New changeset d76fccbad014 by Martin Panter in branch '3.5':
Issue #26620: Fix ResourceWarning in test_urllib2_localnet
https://hg.python.org/cpython/rev/d76fccbad014