Fix windowed singleton side inputs

[jto]

See #29547 for context

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[Abacn]

Run Flink ValidatesRunner

[Abacn]

Run Flink ValidatesRunner Java 11

[Abacn]

Thanks for the quick fix! The failure was in BatchWithDataStream mode while this fix code path is generic. Could this affect the default (withdatastream not enabled or streaming) ? Relatedly, would you mind explain a little bit how the fix works?

[jto]

I shamelessly copied the implementation from the DirectRunner:

beam/runners/direct-java/src/main/java/org/apache/beam/runners/direct/DirectWriteViewVisitor.java

Lines 94 to 100 in f684204

	if (view.getViewFn() instanceof PCollectionViews.IsSingletonView) {
	iterable =
	input
	.apply(
	MapElements.into(TypeDescriptors.iterables(input.getTypeDescriptor()))
	.via(Lists::newArrayList))
	.setCoder(IterableCoder.of(input.getCoder()));

.

The DataflowRunner also does something very similar:

beam/runners/google-cloud-dataflow-java/src/main/java/org/apache/beam/runners/dataflow/StreamingViewOverrides.java

Lines 70 to 84 in f684204

	if (view.getViewFn() instanceof PCollectionViews.IsSingletonView) {
	elements =
	input.apply(
	ParDo.of(
	new DoFn<ElemT, List<ElemT>>() {
	@DoFn.ProcessElement
	public void process(@Element ElemT elemT, OutputReceiver<List<ElemT>> o) {
	List<ElemT> elements = Lists.newArrayListWithExpectedSize(1);
	elements.add(elemT);
	o.output(elements);
	}
	}));
	} else {
	elements = input.apply(Combine.globally(new Concatenate<ElemT>()).withoutDefaults());
	}

There's an explanation of how that works in the DF fix: #25940

[Abacn]

Thanks!