This was uncovered invesigating:
"Server startup is delayed if recovery starts before all resource plugins register"

If the startup of JBoss is sufficiently slow, it is possible for a recovery run to already be in progress when a resource plugin is added. When this occurs, the startup will block until the recovery run has finished.

It would be much better if starting a recovery run did not block AS startup.

An example of this occurring on EAP 5 is:
"main" prio=10 tid=0x00002aac680cc000 nid=0x5e67 waiting for monitor entry [0x00000000412cb000]
java.lang.Thread.State: BLOCKED (on object monitor)
at com.arjuna.ats.internal.jta.recovery.arjunacore.XARecoveryModule.addXAResourceRecoveryHelper(XARecoveryModule.java:85)

• waiting to lock <0x00002aab3e8b9d68> (a java.util.LinkedList)
  at com.arjuna.ats.jbossatx.jta.TransactionManagerService.addXAResourceRecovery(TransactionManagerService.java:695)
  at org.jboss.resource.connectionmanager.ManagedConnectionFactoryDeployment.startService(ManagedConnectionFactoryDeployment.java:500)
  at org.jboss.system.ServiceMBeanSupport.jbossInternalStart(ServiceMBeanSupport.java:376)
  at org.jboss.system.ServiceMBeanSupport.jbossInternalLifecycle(ServiceMBeanSupport.java:322)
  at org.jboss.system.ServiceDynamicMBeanSupport.invoke(ServiceDynamicMBeanSupport.java:124)
  at org.jboss.mx.server.RawDynamicInvoker.invoke(RawDynamicInvoker.java:164)
  at org.jboss.mx.server.MBeanServerImpl.invoke(MBeanServerImpl.java:668)
  at org.jboss.system.server.jmx.LazyMBeanServer.invoke(LazyMBeanServer.java:283)
  at org.jboss.system.microcontainer.ServiceProxy.invoke(ServiceProxy.java:189)
  at $Proxy43.start(Unknown Source)
  at org.jboss.system.microcontainer.StartStopLifecycleAction.installAction(StartStopLifecycleAction.java:42)
  at org.jboss.system.microcontainer.StartStopLifecycleAction.installAction(StartStopLifecycleAction.java:37)
  ...

"Thread-21" prio=10 tid=0x00002aacdd1f7000 nid=0x5ee3 runnable [0x000000004416a000]
java.lang.Thread.State: RUNNABLE
...
at org.jboss.resource.adapter.jdbc.xa.XAManagedConnection.recover(XAManagedConnection.java:294)
at com.arjuna.ats.internal.jta.recovery.arjunacore.XARecoveryModule.xaRecovery(XARecoveryModule.java:773)
at com.arjuna.ats.internal.jta.recovery.arjunacore.XARecoveryModule.resourceInitiatedRecoveryForRecoveryHelpers(XARecoveryModule.java:712)

• locked <0x00002aab3e8b9d68> (a java.util.LinkedList)
  at com.arjuna.ats.internal.jta.recovery.arjunacore.XARecoveryModule.periodicWorkSecondPass(XARecoveryModule.java:201)
  at com.arjuna.ats.internal.arjuna.recovery.PeriodicRecovery.doWorkInternal(PeriodicRecovery.java:799)
  at com.arjuna.ats.internal.arjuna.recovery.PeriodicRecovery.run(PeriodicRecovery.java:412)