2216 notify relay when finished

[cakekoa]

What does this PR do?

Fixes part of #2216.

Sends the notification upstream to tell the agent's relay that it is disconnecting.

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Testing Checklist

• Added relevant unit tests?
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[mssalvatore]

Are there other errors that could be raised? ATM, _close_tunnel() doesn't catch any exceptions. If notify_disconnect() raises an exception, than self._singleton.unlock() will never be called, which would prevent new agents from being spawned on the machine.

I think it's fine if this function catches all exceptions and logs them. Either that or _close_tunne() should catch and log. Either way is fine with me, as long as we're confident that self._singleton.unlock() will always get called.

[mssalvatore]

Update: I pushed 8858bf9 to ensure that the "singleton" is always unlocked. We should still scrutinize this to make sure we're happy with how errors are being caught. If the cleanup routine changes in the future, uncaught errors here could cause issues.

[cakekoa]

Yeah, I looked into this but the socket documentation is pretty vague. From what I can surmise, all socket errors derive from OSError. I opted to allow the function to raise errors to give the caller the option to decide what to do if the disconnect message fails to send. If we're happy not handling that and just logging it, I can switch to logging it here.

[mssalvatore]

Let's handle it in _close_tunnel(). Then the caller still decides and it's handled.

[cakekoa]

This is now updated after rebasing onto the latest 2216-tcp-relay