Fix Jetty shutdown hang and BrokerPool read-only race

[duncdrum]

Summary

• Sets a Jetty stop timeout so shutdown does not block indefinitely on active connections
• Moves ShutdownListenerImpl.shutdown() to a daemon thread to avoid deadlocking the Jetty shutdown-hook thread that also holds the BrokerPool write lock

This bugfix was discovered during XSD 1.1 validation integration work but is completely independent of it — JettyStart.java is the only file touched.

What Changed

exist-core/src/main/java/org/exist/jetty/JettyStart.java

• Add serverStopRequested AtomicBoolean guard so concurrent shutdown paths (direct shutdown(), ShutdownListenerImpl, and BrokerPoolAndJettyShutdownHook JVM hook) do not race
• Dispatch ShutdownListenerImpl.shutdown() on a daemon thread to avoid the BrokerPool write-lock / Jetty-shutdown-hook deadlock
• Set a Jetty stop timeout to bound the shutdown wait

Test Plan

• Start eXist-db and trigger system:shutdown() from XQuery — server stops cleanly without hang
• Send SIGTERM — JVM hook fires, Jetty stops within the stop timeout
• Trigger shutdown from XML-RPC API — no deadlock with BrokerPool write lock

🤖 Generated with Claude Code

[dizzzz]

Thnx for isolating this in a separate PR! It is complex and significant enough!

[dizzzz]

copilot feedback

The PR is solid, but a few optional enhancements could strengthen it:

1. Consider documenting the timeout constants: The 30-second Jetty stop timeout and 5-second catch-up margin are sensible but somewhat arbitrary. Add a comment explaining the rationale (e.g., "30s allows graceful drain of in-flight requests; 5s is a catch-up margin for lifecycle callbacks").

2. Daemon thread naming could be more specific: The thread name "JettyShutdown" is clear, but consider including context like "JettyShutdown-async" or a sequence number to aid debugging in scenarios with multiple shutdown events.

3. Consider adding a log statement in stopServerAndAwait(): When serverStopRequested.compareAndSet() returns false, it would be helpful to log that shutdown is already in progress (at debug level) to aid troubleshooting.