Seemingly unnecessary NullPointerExceptions when using Latest.perKey(), Max.perKey()

 

This test
```

@Test
public void shouldNotEncodeNull_LatestPerKey() {
final var actual = pipeline

.apply(Create.of(
ImmutableList.of(

KV.of("a", "not null"),
KV.of("a", "also not null")

)
)
)
.apply(Latest.perKey());

PAssert.that(actual).containsInAnyOrder(KV.of("a", "also not null"));
pipeline.run();
}

```

throws
```

org.apache.beam.sdk.Pipeline$PipelineExecutionException: org.apache.beam.sdk.coders.CoderException:
cannot encode a null String at org.apache.beam.runners.direct.DirectRunner$DirectPipelineResult.waitUntilFinish(DirectRunner.java:373)
at
org.apache.beam.runners.direct.DirectRunner$DirectPipelineResult.waitUntilFinish(DirectRunner.java:341)
at
org.apache.beam.runners.direct.DirectRunner.run(DirectRunner.java:218)
at org.apache.beam.runners.direct.DirectRunner.run(DirectRunner.java:67)
at
org.apache.beam.sdk.Pipeline.run(Pipeline.java:323)
at org.apache.beam.sdk.testing.TestPipeline.run(TestPipeline.java:398)
at
org.apache.beam.sdk.testing.TestPipeline.run(TestPipeline.java:334)
at x.y.z .NullPerKeyTest.shouldNotEncodeNull_LatestPerKey(NullPerKeyTest.java:61)
at
java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at
java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at
java.base/java.lang.reflect.Method.invoke(Method.java:566)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:59)
at
org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:56)
at
org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
at org.apache.beam.sdk.testing.TestPipeline$1.evaluate(TestPipeline.java:322)
at
org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:305)
at org.junit.runners.BlockJUnit4ClassRunner$1.evaluate(BlockJUnit4ClassRunner.java:100)
at
org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:365)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:103)
at
org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:63)
at org.junit.runners.ParentRunner$4.run(ParentRunner.java:330)
at
org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:78)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:328)
at
org.junit.runners.ParentRunner.access$100(ParentRunner.java:65)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:292)
at
org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:305)
at org.junit.runners.ParentRunner.run(ParentRunner.java:412)
at
org.junit.runner.JUnitCore.run(JUnitCore.java:137)
at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:69)
at
com.intellij.rt.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:33)
at com.intellij.rt.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:235)
at
com.intellij.rt.junit.JUnitStarter.main(JUnitStarter.java:54)
Caused by: org.apache.beam.sdk.coders.CoderException:
cannot encode a null String
at org.apache.beam.sdk.coders.StringUtf8Coder.encode(StringUtf8Coder.java:74)
at
org.apache.beam.sdk.coders.StringUtf8Coder.encode(StringUtf8Coder.java:37)
at org.apache.beam.sdk.coders.KvCoder.encode(KvCoder.java:73)
at
org.apache.beam.sdk.coders.KvCoder.encode(KvCoder.java:37)
at org.apache.beam.sdk.util.CoderUtils.encodeToSafeStream(CoderUtils.java:85)
at
org.apache.beam.sdk.util.CoderUtils.encodeToByteArray(CoderUtils.java:69)
at org.apache.beam.sdk.util.CoderUtils.encodeToByteArray(CoderUtils.java:54)
at
org.apache.beam.sdk.util.CoderUtils.clone(CoderUtils.java:144)

```

which is unexpected.

Additionally  this test
```

@Test
public void shouldNotEncodeNull_MaxPerKey() {

final var actual = pipeline

.apply(TestStream.create(KvCoder.of(StringUtf8Coder.of(), VarIntCoder.of()))

.addElements(
TimestampedValue.of(KV.of("a", 2), Instant.ofEpochMilli(42000L)),

TimestampedValue.of(KV.of("a", 1), Instant.ofEpochMilli(960000L))

)
.advanceWatermarkToInfinity()
)
.apply(Window

.>into(new GlobalWindows())
.triggering(Repeatedly.forever(AfterPane.elementCountAtLeast(1)))

.discardingFiredPanes())
.apply(Max.perKey());

PAssert.that(actual).containsInAnyOrder(KV.of("a",
2));
pipeline.run();
}

```

throws
```

org.apache.beam.sdk.Pipeline$PipelineExecutionException: java.lang.RuntimeException: org.apache.beam.sdk.coders.CoderException:
cannot encode a null Integer at org.apache.beam.runners.direct.DirectRunner$DirectPipelineResult.waitUntilFinish(DirectRunner.java:373)
at
org.apache.beam.runners.direct.DirectRunner$DirectPipelineResult.waitUntilFinish(DirectRunner.java:341)
at
org.apache.beam.runners.direct.DirectRunner.run(DirectRunner.java:218)
at org.apache.beam.runners.direct.DirectRunner.run(DirectRunner.java:67)
at
org.apache.beam.sdk.Pipeline.run(Pipeline.java:323)
at org.apache.beam.sdk.testing.TestPipeline.run(TestPipeline.java:398)
at
org.apache.beam.sdk.testing.TestPipeline.run(TestPipeline.java:334)
at co.x.y.NullPerKeyTest.shouldNotEncodeNull_MaxPerKey(NullPerKeyTest.java:45)
at
java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at
java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at
java.base/java.lang.reflect.Method.invoke(Method.java:566)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:59)
at
org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:56)
at
org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
at org.apache.beam.sdk.testing.TestPipeline$1.evaluate(TestPipeline.java:322)
at
org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:305)
at org.junit.runners.BlockJUnit4ClassRunner$1.evaluate(BlockJUnit4ClassRunner.java:100)
at
org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:365)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:103)
at
org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:63)
at org.junit.runners.ParentRunner$4.run(ParentRunner.java:330)
at
org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:78)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:328)
at
org.junit.runners.ParentRunner.access$100(ParentRunner.java:65)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:292)
at
org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:305)
at org.junit.runners.ParentRunner.run(ParentRunner.java:412)
at
org.junit.runner.JUnitCore.run(JUnitCore.java:137)
at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:69)
at
com.intellij.rt.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:33)
at com.intellij.rt.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:235)
at
com.intellij.rt.junit.JUnitStarter.main(JUnitStarter.java:54)
Caused by: java.lang.RuntimeException:
org.apache.beam.sdk.coders.CoderException: cannot encode a null Integer
at org.apache.beam.runners.direct.ImmutabilityCheckingBundleFactory$ImmutabilityEnforcingBundle.add(ImmutabilityCheckingBundleFactory.java:117)
at
org.apache.beam.runners.direct.ParDoEvaluator$BundleOutputManager.output(ParDoEvaluator.java:305)
at
org.apache.beam.repackaged.direct_java.runners.core.SimpleDoFnRunner.outputWindowedValue(SimpleDoFnRunner.java:272)
at
org.apache.beam.repackaged.direct_java.runners.core.SimpleDoFnRunner.access$900(SimpleDoFnRunner.java:84)
at
org.apache.beam.repackaged.direct_java.runners.core.SimpleDoFnRunner$DoFnProcessContext.output(SimpleDoFnRunner.java:418)
at
org.apache.beam.repackaged.direct_java.runners.core.SimpleDoFnRunner$DoFnProcessContext.output(SimpleDoFnRunner.java:406)
at
org.apache.beam.sdk.transforms.Combine$GroupedValues$1.processElement(Combine.java:2180)
Caused by:
org.apache.beam.sdk.coders.CoderException: cannot encode a null Integer
at org.apache.beam.sdk.coders.VarIntCoder.encode(VarIntCoder.java:49)
at
org.apache.beam.sdk.coders.VarIntCoder.encode(VarIntCoder.java:33)
at org.apache.beam.sdk.coders.Coder.encode(Coder.java:136)
at
org.apache.beam.sdk.coders.KvCoder.encode(KvCoder.java:73)
at org.apache.beam.sdk.coders.KvCoder.encode(KvCoder.java:37)
at
org.apache.beam.sdk.util.CoderUtils.encodeToSafeStream(CoderUtils.java:85)
at org.apache.beam.sdk.util.CoderUtils.encodeToByteArray(CoderUtils.java:69)
at
org.apache.beam.sdk.util.CoderUtils.encodeToByteArray(CoderUtils.java:54)
at org.apache.beam.sdk.util.CoderUtils.clone(CoderUtils.java:144)
at
org.apache.beam.sdk.util.MutationDetectors$CodedValueMutationDetector.(MutationDetectors.java:118)
at
org.apache.beam.sdk.util.MutationDetectors.forValueWithCoder(MutationDetectors.java:49)
at org.apache.beam.runners.direct.ImmutabilityCheckingBundleFactory$ImmutabilityEnforcingBundle.add(ImmutabilityCheckingBundleFactory.java:115)
at
org.apache.beam.runners.direct.ParDoEvaluator$BundleOutputManager.output(ParDoEvaluator.java:305)
at
org.apache.beam.repackaged.direct_java.runners.core.SimpleDoFnRunner.outputWindowedValue(SimpleDoFnRunner.java:272)
at
org.apache.beam.repackaged.direct_java.runners.core.SimpleDoFnRunner.access$900(SimpleDoFnRunner.java:84)
at
org.apache.beam.repackaged.direct_java.runners.core.SimpleDoFnRunner$DoFnProcessContext.output(SimpleDoFnRunner.java:418)
at
org.apache.beam.repackaged.direct_java.runners.core.SimpleDoFnRunner$DoFnProcessContext.output(SimpleDoFnRunner.java:406)
at
org.apache.beam.sdk.transforms.Combine$GroupedValues$1.processElement(Combine.java:2180)
at org.apache.beam.sdk.transforms.Combine$GroupedValues$1$DoFnInvoker.invokeProcessElement(Unknown
Source)
at org.apache.beam.repackaged.direct_java.runners.core.SimpleDoFnRunner.invokeProcessElement(SimpleDoFnRunner.java:232)
at
org.apache.beam.repackaged.direct_java.runners.core.SimpleDoFnRunner.processElement(SimpleDoFnRunner.java:191)
at
org.apache.beam.repackaged.direct_java.runners.core.SimplePushbackSideInputDoFnRunner.processElementInReadyWindows(SimplePushbackSideInputDoFnRunner.java:79)
at
org.apache.beam.runners.direct.ParDoEvaluator.processElement(ParDoEvaluator.java:244)
at org.apache.beam.runners.direct.DoFnLifecycleManagerRemovingTransformEvaluator.processElement(DoFnLifecycleManagerRemovingTransformEvaluator.java:54)
at
org.apache.beam.runners.direct.DirectTransformExecutor.processElements(DirectTransformExecutor.java:165)
at
org.apache.beam.runners.direct.DirectTransformExecutor.run(DirectTransformExecutor.java:129)
at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515)
at
java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
at
java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
at java.base/java.lang.Thread.run(Thread.java:829)
```

unless you change `discardingFiredPanes()` to `accumulatingFiredPanes()`, which is also unexpected.

Imported from Jira [BEAM-12924](https://issues.apache.org/jira/browse/BEAM-12924). Original Jira may contain additional context.
Reported by: joar.

Start by reproducing the two NullPerKeyTest cases for Latest.perKey() and Max.perKey(), including the windowing and pane settings shown. Inspect Combine.java's GroupedValues path and the per-key transforms; done means both pipelines complete without null-coder errors and their existing assertions pass.

Written by the indexing model from the issue text.