lostluck commented on code in PR #32029:
URL: https://github.com/apache/beam/pull/32029#discussion_r1697730564
##########
sdks/go/pkg/beam/runners/prism/internal/engine/elementmanager.go:
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@@ -211,6 +211,16 @@ func (em *ElementManager) AddStage(ID string, inputIDs,
outputIDs []string, side
for _, input := range inputIDs {
em.consumers[input] = append(em.consumers[input], ss.ID)
}
+
+ // In very rare cases, we can have a stage without any inputs, such as
a flatten.
+ // In that case, there's nothing that will start the watermark refresh
cycle,
+ // so we must do it here.
+ if len(inputIDs) == 0 {
Review Comment:
Effectively yes, but basically it's due to flatten being special: all Other
Stages must have a single parallel input pCollection. Flattens may have any
number, which apparently includes 0.
In this case, by construction, the flatten has no inputs to begin with.
Impulses and TestStream are also special: Impulses are required to kick off
SDK side processing. TestStream is just weird.
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