patsonluk opened a new pull request, #1438:
URL: https://github.com/apache/solr/pull/1438

   https://issues.apache.org/jira/browse/SOLR-16690
   
   # Description
   
   `ZkStateWriter#enqueueUpdate(ClusterState prevState, List<ZkWriteCommand> 
cmds, ZkWriteCallback callback)` accepts a callback but such callback would not 
be called if either the cmds is empty or a no-op.
   
   However, this could cause issues in some scenarios, for example in 
[Overseer$ClusterStateUpdater#run](https://github.com/apache/solr/blob/main/solr/core/src/java/org/apache/solr/cloud/Overseer.java#L373),
 it calls
   
   ```
   clusterState = processQueueItem(message, clusterState, zkStateWriter, true,
   () ->
   { stateUpdateQueue.remove(processedNodes); processedNodes.clear(); }
   );
   ```
   
   Which it relies on the callback to remove items from the work queue, however 
if the message does not create an empty `List<ZkWriteCommand>` - for example 
"DOWNNODE" for PRS enabled collection, then the callback will be ignored hence 
the `stateUpdateQueue.remove(processedNodes);` would not be run even though it 
has finished processing the message
   
   # Solution
   
   Execute the callback if cmds is empty or noop
   
   
   # Checklist
   
   Please review the following and check all that apply:
   
   - [x] I have reviewed the guidelines for [How to 
Contribute](https://wiki.apache.org/solr/HowToContribute) and my code conforms 
to the standards described there to the best of my ability.
   - [x] I have created a Jira issue and added the issue ID to my pull request 
title.
   - [ ] I have given Solr maintainers 
[access](https://help.github.com/en/articles/allowing-changes-to-a-pull-request-branch-created-from-a-fork)
 to contribute to my PR branch. (optional but recommended)
   - [x] I have developed this patch against the `main` branch.
   - [ ] I have run `./gradlew check`.
   - [ ] I have added tests for my changes.
   - [ ] I have added documentation for the [Reference 
Guide](https://github.com/apache/solr/tree/main/solr/solr-ref-guide)
   


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