[PR] Make EventCounter thread safe [accumulo]

["DomGarguilo (via GitHub)" <gi...@apache.org>]

DomGarguilo opened a new pull request, #4959:
URL: https://github.com/apache/accumulo/pull/4959

   Fixes #4956
   
   The stack track included in the ticket indicates that a ConcurrentmodificationException occurred in Monitor.EventCounter:
   ```
   java.util.ConcurrentmodificationException
   at java.util.HashMap$HashIterator.nextNode(HashMap.java:1511)
   at java.util.HashMap$HashIterator.next(HashMap.java:1544)
   at java.util.HashMap$HashIterator.next(HashMap.java:1542)
   at org.apache.accumulo.monitor.Monitor$EventCounter.calculateRate(monitor.java:23)
   at org.apache.accumulo.monitor.Monitor.getLookupRate(Monitor.java:977)
   at org.apache.accumulo.monitor.rest.tservers.TabletServerInformation.updateTabletServerInfo(TabletServerinformation.java:162)
   at org.apache.accumulo.monitor.rest.tservers.TabletServerInformation.(TabletServerInformation:101)
   at org.apache.accumulo.monitor.rest.tservers.TabletServer.(TabletServer.java:43)
   at org.apache.accumulo.monitor.rest.tservers.TabletServerResource.getTserverSummary(TabletServerResource.java:88)
   ```
   This PR adds synchronization to several methods in the `EventCounter` class to avoid this from happening again.
   
   Another approach that would probably work would be to make the 3 collections in this class thread safe by making the HashMaps into ConcurrentHashMaps and the HashSet into a `Collections.synchronizedSet(new HashSet<>())`. That would function just fine in the current code as far as I can tell but if we would ever need to iterate over the `synchronizedSet`, we would need to use a `synchronize` block anyways so i figured just making the methods themselves synchronized would future proof and fix the current issue too.


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Re: [PR] Make EventCounter thread safe [accumulo]

["DomGarguilo (via GitHub)" <gi...@apache.org>]

DomGarguilo merged PR #4959:
URL: https://github.com/apache/accumulo/pull/4959


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