Github user StefanRRichter commented on a diff in the pull request:
https://github.com/apache/flink/pull/3005#discussion_r92402545
--- Diff:
flink-streaming-java/src/main/java/org/apache/flink/streaming/api/checkpoint/ListCheckpointed.java
---
@@ -41,6 +41,7 @@
* @param checkpointId The ID of the checkpoint.
* @param timestamp Timestamp of the checkpoint.
* @return The operator state in a list of redistributable, atomic
sub-states.
+ * Should not return null, but empty list instead.
--- End diff --
That is why I wrote "should". It is possible, but not encouraged because i)
`null` is evil ;-) and ii) user will also receive empty list on restore.
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