Fix: Calling close() twice hangs multi-isolate servers

[coderabbitai]

Problem

Calling close() twice on a multi-isolate Relic server causes the application to hang. This was discovered during testing of graceful shutdown behavior.

Current Behavior

When server.close() is called twice on a multi-isolate server configuration, the second call causes the server to hang indefinitely instead of returning immediately or throwing an appropriate error.

Expected Behavior

• The second call to close() should either:
  • Return immediately (idempotent behavior)
  • Throw a clear error indicating the server is already closed
  • Complete without hanging

Context

This issue was identified in PR #292 during graceful shutdown testing. A workaround was implemented using a serverClosed flag in tests to prevent calling close() twice in tearDown.

Related

• PR: test: Add test to verify graceful shutdown of in-flight requests #292
• Reported by: @nielsenko

[nielsenko]

Close should be idempotent.