horizonzy opened a new issue, #3362:
URL: https://github.com/apache/bookkeeper/issues/3362

   **BUG REPORT**
   
   There are two methods in AbstractZkLedgerManager.
   1. **registerLedgerMetadataListener** 
   
https://github.com/apache/bookkeeper/blob/677ccec3eb84f5be1b3556537871e14eb5e8359c/bookkeeper-server/src/main/java/org/apache/bookkeeper/meta/AbstractZkLedgerManager.java#L389-L409
   
   2. **unregisterLedgerMetadataListener**
   
https://github.com/apache/bookkeeper/blob/677ccec3eb84f5be1b3556537871e14eb5e8359c/bookkeeper-server/src/main/java/org/apache/bookkeeper/meta/AbstractZkLedgerManager.java#L412-L426
   
   
   Thread 1: doing unregisterLedgerMetadataListener, it unregister ledger 1.
   It get listenerSet and remove watcher, the listenerSet is empty, then remove 
key ledger 1.
   
   Thread 2: doing registerLedgerMetadataListener, it get same listenerSet, at 
the same time, thread 1 not remove ledger 1 from listenerSet yet. Thread 2 will 
add listener to this listenerSet which will be remove from listeners.
   
   Final Result: 
   AbstractZkLedgerManager#listeners is empty. In fact, it should hold thread 2 
listener.
   
   The pr come later.
   
   
   


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