这是演示问题https://github.com/medvedev1088/dataflow-flaky-tests的最小项目。每10项测试中就有1项失败，
org.apache.beam.sdk.Pipeline$PipelineExecutionException: java.lang.IllegalArgumentException: Empty PCollection accessed as a singleton view. Consider setting withDefault to provide a default value
at org.apache.beam.runners.direct.DirectRunner$DirectPipelineResult.waitUntilFinish(DirectRunner.java:332)
at org.apache.beam.runners.direct.DirectRunner$DirectPipelineResult.waitUntilFinish(DirectRunner.java:302)
at org.apache.beam.runners.direct.DirectRunner.run(DirectRunner.java:197)
at org.apache.beam.runners.direct.DirectRunner.run(DirectRunner.java:64)
at org.apache.beam.sdk.Pipeline.run(Pipeline.java:313)
at org.apache.beam.sdk.testing.TestPipeline.run(TestPipeline.java:350)
at org.apache.beam.sdk.testing.TestPipeline.run(TestPipeline.java:331)
at com.beam.MyPipelineTest.testTokenTransfersBasic(MyPipelineTest.java:29)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
at org.apache.beam.sdk.testing.TestPipeline$1.evaluate(TestPipeline.java:319)
at org.junit.rules.RunRules.evaluate(RunRules.java:20)
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47)
at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)
at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)
Caused by: java.lang.IllegalArgumentException: Empty PCollection accessed as a singleton view. Consider setting withDefault to provide a default value
at org.apache.beam.sdk.transforms.View$SingletonCombineFn.identity(View.java:378)
at org.apache.beam.sdk.transforms.Combine$BinaryCombineFn.extractOutput(Combine.java:436)
at org.apache.beam.sdk.transforms.Combine$BinaryCombineFn.extractOutput(Combine.java:384)
at org.apache.beam.sdk.transforms.Combine$CombineFn.apply(Combine.java:353)
at org.apache.beam.sdk.transforms.Combine$GroupedValues$1.processElement(Combine.java:2056)
JIRA https://issues.apache.org/jira/browse/BEAM-6086?page=com.atlassian.jira.plugin.system.issuetabpanels%3Aall-tabpanel中的同一问题。与https://jira.apache.org/jira/browse/BEAM-6036
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