[JENKINS-63520] Sequence the startup and receiving to avoid a deadlock.

[jeffret-b]

See JENKINS-63520.

Some users have reported a rare thread deadlock when starting an inbound TCP agent. There are no known reports of this occurring at other times. The reporter included a stack track showing information about the deadlock. It occurs when the agent ProtocolStack is still initializing and a message is received on the networking layer.

Personally, I've never observed the problem and have no way of testing it. I've tested normal behavior and this fix has worked fine.

This fix delays the processing of any received messages until after the ProtocolStack has fully initialized.

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[jeffret-b]

Looks like we lost the agent mid-build. Let's try again.

[jvz]

Excellent!