The resume() method in UndertowEventHandlerAdapterService is missing the synchronized keyword, creating a race condition with concurrent deployment events.

If a deployment event (e.g., onDeploymentStart or onDeploymentStop) occurs concurrently with server resume, the unsynchronized iteration can throw ConcurrentModificationException.

Nevertheless this is quite an uncommon scenario.

Introduced in https://github.com/wildfly/wildfly/commit/99dfd12face64566f0d32540afea15f4c07d3ef2#diff-76725fc7efe1f4432ab7a4e3dde845b2350cef11d5a0b5bebc8adaf68c2ff3a5R197-R199

The solution is to get rid of synchronization and replace by concurrent collection.