Make the merges consumer before the events consumer narrows, from the old one's ack floor, so no merge is lost to the change (issue 973)

internal/broker/controller_buckets_test.go

@@ -2,6 +2,7 @@ package broker
 
 import (
 	"context"
+	"github.com/nats-io/nats.go"
 	"slices"
 	"strings"
 	"testing"
@@ -368,3 +369,33 @@ func TestAResetOfTheEventsConsumerLeavesTheMergesAndTheirConsumerAlone(t *testin
 		t.Fatalf("after the reset the merges consumer holds %+v (%v); want its merge left alone", merges, err)
 	}
 }
+
+// **Delivering this change loses no merge** (review of issue 973): a merge published under the old filter of the
+// `controller` consumer and never acknowledged by it is delivered by the merges consumer once the assertion has
+// made it, because it starts where `controller` has acknowledged up to, not from now.
+func TestAMergeTheOldConsumerHadNotAcknowledgedIsTheMergesConsumers(t *testing.T) {
+	js, err := Dial(testbus.URL(t))
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer js.Close()
+	if err := AssertMeshStreams(js); err != nil {
+		t.Fatal(err)
+	}
+	old := &nats.ConsumerConfig{Durable: ControllerName, AckPolicy: nats.AckExplicitPolicy, AckWait: 30 * time.Second,
+		MaxDeliver: 5, MaxAckPending: 1, DeliverSubject: "_DELIVER.controller.EVENTS", DeliverPolicy: nats.DeliverNewPolicy,
+		FilterSubjects: append(append([]string(nil), ControllerFollows...), AgainFilter(ControllerName))}
+	if _, err := js.Context().AddConsumer(EventsStream, old); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := js.Context().Publish(ControllerFollows[3], []byte(`{"commit":"c1"}`)); err != nil {
+		t.Fatal(err)
+	}
+	if err := AssertMeshConsumers(js); err != nil {
+		t.Fatal(err)
+	}
+	merges, err := js.Context().ConsumerInfo(EventsStream, MergesConsumerName)
+	if err != nil || merges.NumPending != 1 {
+		t.Fatalf("the merges consumer holds %v (%v); want the merge the old consumer never acknowledged", merges, err)
+	}
+}