ConcurrentModificationException iterating over partitions

[johnny-costanzo]

Bug Report

I'm calling the readonly method on an async commands instance to select some nodes and hit a ConcurrentModificationException. It looked like a topology refresh happened at the same time and modified the partitions. Looking into the code, Partitions.getPartitions offers up access to its internal list of partitions (which is otherwise synchronized on in all internal methods), but client code that calls getPartitions is not protected. The StaticNodeSelection constructor calls this method and iterates over the list can potentially result in this issue when the list changes. Consider using a CopyOnWriteArrayList or some other mechanism so that clients of the getPartitions call can safely iterate over the list.

Current Behavior

Stack trace

java.util.ConcurrentModificationException
        at java.base/java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1660)
        at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:484)
        at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:474)
        at java.base/java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:913)
        at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
        at java.base/java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:578)
        at io.lettuce.core.cluster.StaticNodeSelection.<init>(StaticNodeSelection.java:51)
        at io.lettuce.core.cluster.RedisAdvancedClusterAsyncCommandsImpl.nodes(RedisAdvancedClusterAsyncCommandsImpl.java:542)
        at io.lettuce.core.cluster.RedisAdvancedClusterAsyncCommandsImpl.readonly(RedisAdvancedClusterAsyncCommandsImpl.java:522)

Environment

• Lettuce version(s): 5.2.0.RELEASE

[johnny-costanzo]

or possibly this use case should have been querying from Partitions internal nodeReadView instead of calling getPartitions() since it doesn't look like that collection mutates

[mp911de]

Thanks for the report. The described approach is exactly what we need to do here. StaticNodeSelection (and a couple other places) use the mutable collection while they should consume the read-only one that is not mutated once returned from the Partitions object.

[mp911de]

That's fixed now.