[#31462] [prism] Correctly chose earliest or latest in pane timestamps for aggregations.

sdks/go/pkg/beam/runners/prism/internal/handlerunner.go

@@ -291,20 +291,32 @@ func windowingStrategy(comps *pipepb.Components, tid string) *pipepb.WindowingSt
 
 // gbkBytes re-encodes gbk inputs in a gbk result.
 func gbkBytes(ws *pipepb.WindowingStrategy, wc, kc, vc *pipepb.Coder, toAggregate [][]byte, coders map[string]*pipepb.Coder, watermark mtime.Time) []byte {
-	var outputTime func(typex.Window, mtime.Time) mtime.Time
+	// Pick how the timestamp of the aggregated output is computed.
+	var outputTime func(typex.Window, mtime.Time, mtime.Time) mtime.Time
 	switch ws.GetOutputTime() {
 	case pipepb.OutputTime_END_OF_WINDOW:
-		outputTime = func(w typex.Window, et mtime.Time) mtime.Time {
+		outputTime = func(w typex.Window, _, _ mtime.Time) mtime.Time {
 			return w.MaxTimestamp()
 		}
-	case pipepb.OutputTime_EARLIEST_IN_PANE, pipepb.OutputTime_LATEST_IN_PANE:
-		outputTime = func(w typex.Window, et mtime.Time) mtime.Time {
-			return et
+	case pipepb.OutputTime_EARLIEST_IN_PANE:
+		outputTime = func(_ typex.Window, cur, et mtime.Time) mtime.Time {
+			if et < cur {
+				return et
+			}
+			return cur
+		}
+	case pipepb.OutputTime_LATEST_IN_PANE:
+		outputTime = func(_ typex.Window, cur, et mtime.Time) mtime.Time {
+			if et > cur {
+				return et
+			}
+			return cur
 		}
 	default:
 		// TODO need to correct session logic if output time is different.
 		panic(fmt.Sprintf("unsupported OutputTime behavior: %v", ws.GetOutputTime()))
 	}
+
 	wDec, wEnc := makeWindowCoders(wc)
 
 	type keyTime struct {
@@ -340,14 +352,18 @@ func gbkBytes(ws *pipepb.WindowingStrategy, wc, kc, vc *pipepb.Coder, toAggregat
 			key := string(keyByt)
 			value := vd(buf)
 			for _, w := range ws {
-				ft := outputTime(w, tm)
 				wk, ok := windows[w]
 				if !ok {
 					wk = make(map[string]keyTime)
 					windows[w] = wk
 				}
-				kt := wk[key]
-				kt.time = ft
+				kt, ok := wk[key]
+				if !ok {
+					// If the window+key map doesn't have a value, inititialize time with the element time.
+					// This allows earliest or latest to work properly in the outputTime function's first use.
+					kt.time = tm
+				}
+				kt.time = outputTime(w, kt.time, tm)
 				kt.key = keyByt
 				kt.w = w
 				kt.values = append(kt.values, value)