[#32003][prism] Support empty transform input sets, such as for flattens.

sdks/go/pkg/beam/runners/prism/internal/engine/elementmanager.go

@@ -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 {
+		refreshes := singleSet(ss.ID)
+		em.addToTestStreamImpulseSet(refreshes)
+		em.addRefreshes(refreshes)
+	}
+
 	for _, side := range ss.sides {
 		// Note that we use the StageID as the global ID in the value since we need
 		// to be able to look up the consuming stage, from the global PCollectionID.
@@ -265,14 +275,19 @@ func (em *ElementManager) Impulse(stageID string) {
 	}
 	refreshes := stage.updateWatermarks(em)
 
-	// Since impulses are synthetic, we need to simulate them properly
-	// if a pipeline is only test stream driven.
+	em.addToTestStreamImpulseSet(refreshes)
+	em.addRefreshes(refreshes)
+}
+
+// addToTestStreamImpulseSet adds to the set of stages to refresh on pipeline start.
+// We keep this separate since impulses are synthetic. In a test stream driven pipeline
+// these will need to be stimulated separately, to ensure the test stream has progressed.
+func (em *ElementManager) addToTestStreamImpulseSet(refreshes set[string]) {
 	if em.impulses == nil {
 		em.impulses = refreshes
 	} else {
 		em.impulses.merge(refreshes)
 	}
-	em.addRefreshes(refreshes)
 }
 
 type RunBundle struct {
@@ -470,7 +485,19 @@ func (em *ElementManager) checkForQuiescence(advanced set[string]) error {
 		outW := ss.OutputWatermark()
 		upPCol, upW := ss.UpstreamWatermark()
 		upS := em.pcolParents[upPCol]
+		if upS == "" {
+			upS = "IMPULSE  " // (extra spaces to allow print to align better.)
+		}
 		stageState = append(stageState, fmt.Sprintln(id, "watermark in", inW, "out", outW, "upstream", upW, "from", upS, "pending", ss.pending, "byKey", ss.pendingByKeys, "inprogressKeys", ss.inprogressKeys, "byBundle", ss.inprogressKeysByBundle, "holds", ss.watermarkHolds.heap, "holdCounts", ss.watermarkHolds.counts, "holdsInBundle", ss.inprogressHoldsByBundle, "pttEvents", ss.processingTimeTimers.toFire))
+
+		var outputConsumers, sideConsumers []string
+		for _, col := range ss.outputIDs {
+			outputConsumers = append(outputConsumers, em.consumers[col]...)
+			for _, l := range em.sideConsumers[col] {
+				sideConsumers = append(sideConsumers, l.Global)
+			}
+		}
+		stageState = append(stageState, fmt.Sprintf("\tsideInputs: %v outputCols: %v outputConsumers: %v sideConsumers: %v\n", ss.sides, ss.outputIDs, outputConsumers, sideConsumers))
 	}
 	return errors.Errorf("nothing in progress and no refreshes with non zero pending elements: %v\n%v", v, strings.Join(stageState, ""))
 }
@@ -721,13 +748,13 @@ func (em *ElementManager) PersistBundle(rb RunBundle, col2Coders map[string]PCol
 			}
 		}
 		consumers := em.consumers[output]
-		slog.Debug("PersistBundle: bundle has downstream consumers.", "bundle", rb, slog.Int("newPending", len(newPending)), "consumers", consumers)
+		sideConsumers := em.sideConsumers[output]
+		slog.Debug("PersistBundle: bundle has downstream consumers.", "bundle", rb, slog.Int("newPending", len(newPending)), "consumers", consumers, "sideConsumers", sideConsumers)
 		for _, sID := range consumers {
 			consumer := em.stages[sID]
 			count := consumer.AddPending(newPending)
 			em.addPending(count)
 		}
-		sideConsumers := em.sideConsumers[output]
 		for _, link := range sideConsumers {
 			consumer := em.stages[link.Global]
 			consumer.AddPendingSide(newPending, link.Transform, link.Local)