Unexpected empty state address window set for ACTIVE window

[Ke Wu <ke...@gmail.com>]

Hello,

We recently encountered an weird exception:

Caused by: java.lang.IllegalStateException: Unexpected empty state address window set for ACTIVE window [2021-02-10T17:25:10.715Z..2021-02-10T20:25:10.715Z)
	at org.apache.beam.vendor.guava.v26_0_jre.com.google.common.base.Preconditions.checkState(Preconditions.java:588) ~[beam-vendor-guava-26_0-jre-0.1.jar:?]
	at org.apache.beam.runners.core.MergingActiveWindowSet.checkInvariants(MergingActiveWindowSet.java:329) ~[beam-runners-core-java-2.20.7.9.jar:?]
	at org.apache.beam.runners.core.MergingActiveWindowSet.persist(MergingActiveWindowSet.java:88) ~[beam-runners-core-java-2.20.7.9.jar:?]
	at org.apache.beam.runners.core.ReduceFnRunner.persist(ReduceFnRunner.java:380) ~[beam-runners-core-java-2.20.7.9.jar:?]
	at org.apache.beam.runners.core.GroupAlsoByWindowViaWindowSetNewDoFn.processElement(GroupAlsoByWindowViaWindowSetNewDoFn.java:138) ~[beam-runners-core-java-2.20.7.9.jar:?]

which suggests there is an empty window in ReduceFnRunner#activeWindows

After some more digging and we found there could be two possibilities that could result in such state:

1. ReduceFnRunner#onTimers is clearing entry value, causing an empty value entry in ReduceFnRunner#activeWindows

2. ReduceFnRunner#activeWindows initialized with empty value in MergingActiveWindowSet#emptyIfNull, then empty iterable windowed values is passed in the subsequent ReduceFnRunner#processElements, where cleanupTemporaryWindows() is not invoked at all before ReduceFnRunner#persist

Questions are, are #1 and/or #2 possible to happen? If Yes, under what circumstances?

Best,
Ke