`ConcurrentModificationException` in `BulkOperation`

[piergm]

Elasticsearch Version

9.2.0

Installed Plugins

No response

Java Version

bundled

OS Version

---

Problem Description

This error occurs:

java.util.ConcurrentModificationException
	at java.base/java.util.ArrayList$Itr.checkForComodification(ArrayList.java:1096)
	at java.base/java.util.ArrayList$Itr.next(ArrayList.java:1050)
	at org.elasticsearch.server@9.2.0/org.elasticsearch.common.collect.Iterators$EnumeratingIterator.next(Iterators.java:416)
	at org.elasticsearch.server@9.2.0/org.elasticsearch.action.bulk.BulkOperation.groupRequestsByShards(BulkOperation.java:303)
	at org.elasticsearch.server@9.2.0/org.elasticsearch.action.bulk.BulkOperation.groupBulkRequestsByShards(BulkOperation.java:277)
	at org.elasticsearch.server@9.2.0/org.elasticsearch.action.bulk.BulkOperation.doRun(BulkOperation.java:194)
	at org.elasticsearch.server@9.2.0/org.elasticsearch.common.util.concurrent.AbstractRunnable.run(AbstractRunnable.java:27)
	at org.elasticsearch.server@9.2.0/org.elasticsearch.action.bulk.TransportBulkAction.executeBulk(TransportBulkAction.java:621)
	at org.elasticsearch.server@9.2.0/org.elasticsearch.action.bulk.TransportBulkAction.createMissingIndicesAndIndexData(TransportBulkAction.java:382)
	at org.elasticsearch.server@9.2.0/org.elasticsearch.action.bulk.TransportBulkAction.doInternalExecute(TransportBulkAction.java:239)
	at org.elasticsearch.server@9.2.0/org.elasticsearch.action.bulk.TransportAbstractBulkAction.applyPipelinesAndDoInternalExecute(TransportAbstractBulkAction.java:404)
	at org.elasticsearch.server@9.2.0/org.elasticsearch.action.bulk.TransportAbstractBulkAction$2.doRun(TransportAbstractBulkAction.java:186)
	at org.elasticsearch.server@9.2.0/org.elasticsearch.common.util.concurrent.ThreadContext$ContextPreservingAbstractRunnable.doRun(ThreadContext.java:1044)
	at org.elasticsearch.server@9.2.0/org.elasticsearch.common.util.concurrent.AbstractRunnable.run(AbstractRunnable.java:27)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1095)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:619)
	at java.base/java.lang.Thread.run(Thread.java:1447)

Steps to Reproduce

unclear

Logs (if relevant)

No response

[elasticsearchmachine]

Pinging @elastic/es-distributed-indexing (Team:Distributed Indexing)