BUG: Fix missing "server_close" method on SAMPHubProxy

[embray]

Follow-up to #11167 to try to fix the underlying bug in the astropy.samp tests.

This is affecting the CI on the 4.0.x branch too so it would be helpful to backport it.

I marked this "affects-dev" since it's not really a user-facing bug, but I can still add a changelog if someone thinks it's necessary.

(As an aside, after going through this code, I wonder, now that we're on Python 3.6+ and even dropping 3.6 on the horizon, if it wouldn't make sense to rewrite this code on asyncio, which might make it clearer. Or maybe not, since wrapping one's head around asyncio is not easy either...)

Description

It's possible this bug has existed for a long time and was just hidden.
Another possibility is it appeared at some point during refactoring, but
as far as I can tell it's been around a long time.

The SAMPWebClient.hub attribute is still just a SAMPHubProxy; it
does not make sense for it to have a server_close method because it
does not itself run or wrap a server.

It does however manage a pool of XML-RPC client connections which is
deleted when calling SAMPHubProxy.disconnect(). To be on the safe
side I added a ServerProxyPool.shutdown() method to the proxy pool
class which closes the client connections. This is called when shutting
down the SAMPWebClient server loop. Unlike the normal SAMPClient
class, the SAMPWebClient used only in tests never runs a TCP server
of its own, so calling server_close anywhere in here does not make
sense.

[embray]

github-actions bot added installation and removed testing labels now

but why...?

[pllim]

It added installation because of

astropy/.github/labeler.yml

Line 36 in d971b1c

- setup.*

and removed testing because of what I think is a bug in the Action but Microsoft didn't respond to actions/labeler#112 .

[pllim]

Thanks for looking into this! Tom R should have a look since I think he ported over this code to astropy back in the days.

rewrite this code on asyncio

Not sure. I think this sub-package is stuck in purgatory given #9763 .

[pllim]

Can we just delete the method like this and backport to LTS? This is effectively an API change. Will this break downstream? Does anyone actually use this method?

[embray]

No, it's not documented and does not actually do anything useful. Its mere existence is a bug :)

[embray]

In other words, any attempt to actually call this method will crash.

[pllim]

Ah, okay. Then I guess we can still backport. Thanks for the clarification!

[pllim]

p.s. I removed the "affects dev" label since this actually affects released code. I added another label to indicate you don't need a change log, though given the deletion of server_close, maybe it needs one, not sure; Let's wait and see what Tom R says.

[pllim]

Since the CI pass, I am approving this. But I'll leave it open for a bit for @astrofrog to have a look. If you have not heard back by the time this is really needed to unblock your other work, feel free to merge. Thanks!

[embray]

Since this is a blocker for #11123 I will go ahead and merge, and if @astrofrog has any follow-up suggestions I'll deal with it then.

[eteq]

I moved this to the 4.0.6 milestone to not block release, but @pllim pointed out it's part of the CI work so I re-milestoned it back to 4.0.5 and backported in 3a3f0da

[dhomeier]

I keep seeing (erratic?) failures like this https://github.com/astropy/astropy/pull/11397/checks?check_run_id=2152248020
on TestWebProfile.test_web_profile - is this still related, or another can of worms?

>               warnings.warn(pytest.PytestUnhandledThreadExceptionWarning(msg))
E               pytest.PytestUnhandledThreadExceptionWarning: Exception in thread Thread-275
E               
E               Traceback (most recent call last):
E                 File "/home/runner/work/astropy/astropy/astropy/samp/utils.py", line 62, in __call__
E                   response = function(*args, **kwrds)
E                 File "/opt/hostedtoolcache/Python/3.8.8/x64/lib/python3.8/xmlrpc/client.py", line 1109, in __call__
E                   return self.__send(self.__name, args)
E                 File "/opt/hostedtoolcache/Python/3.8.8/x64/lib/python3.8/xmlrpc/client.py", line 1450, in __request
E                   response = self.__transport.request(
E                 File "/opt/hostedtoolcache/Python/3.8.8/x64/lib/python3.8/xmlrpc/client.py", line 1153, in request
E                   return self.single_request(host, handler, request_body, verbose)
E                 File "/opt/hostedtoolcache/Python/3.8.8/x64/lib/python3.8/xmlrpc/client.py", line 1169, in single_request
E                   return self.parse_response(resp)
E                 File "/opt/hostedtoolcache/Python/3.8.8/x64/lib/python3.8/xmlrpc/client.py", line 1341, in parse_response
E                   return u.close()
E                 File "/opt/hostedtoolcache/Python/3.8.8/x64/lib/python3.8/xmlrpc/client.py", line 655, in close
E                   raise Fault(**self._stack[0])
E               xmlrpc.client.Fault: <Fault 5: 'Private-key 732dec38-88fb-11eb-8e1e-5767653fb47f expired or invalid.'>
E               
E               During handling of the above exception, another exception occurred:
E               
E               Traceback (most recent call last):
E                 File "/opt/hostedtoolcache/Python/3.8.8/x64/lib/python3.8/threading.py", line 932, in _bootstrap_inner
E                   self.run()
E                 File "/opt/hostedtoolcache/Python/3.8.8/x64/lib/python3.8/threading.py", line 870, in run
E                   self._target(*self._args, **self._kwargs)
E                 File "/home/runner/work/astropy/astropy/astropy/samp/tests/web_profile_test_helpers.py", line 163, in _serve_forever
E                   results = self.hub.pull_callbacks(self.get_private_key(), 0)
E                 File "/home/runner/work/astropy/astropy/astropy/samp/tests/web_profile_test_helpers.py", line 90, in pull_callbacks
E                   return self._samp_hub.pullCallbacks(private_key, timeout)
E                 File "/home/runner/work/astropy/astropy/astropy/samp/utils.py", line 64, in __call__
E                   raise SAMPProxyError(exc.faultCode, exc.faultString)
E               astropy.samp.errors.SAMPProxyError: <SAMPProxyError 5: 'Private-key 732dec38-88fb-11eb-8e1e-5767653fb47f expired or invalid.'>