[IMPROVED] Add test veryfying if mirror can be updated to regular stream (#1934)

Signed-off-by: Piotr Piotrowski <piotr@synadia.com>

jetstream/test/jetstream_test.go

@@ -2403,3 +2403,78 @@ func TestCreateOrUpdateStreamCrossDomains(t *testing.T) {
 		t.Fatalf("Expected subject to be updated to 'bar', got %q", info2.Config.Subjects[0])
 	}
 }
+
+func TestPromoteMirrorToStream(t *testing.T) {
+	srv := RunBasicJetStreamServer()
+	defer shutdownJSServerAndRemoveStorage(t, srv)
+	nc, err := nats.Connect(srv.ClientURL())
+	if err != nil {
+		t.Fatalf("Unexpected error: %v", err)
+	}
+	defer nc.Close()
+
+	js, err := jetstream.New(nc)
+	if err != nil {
+		t.Fatalf("Unexpected error: %v", err)
+	}
+
+	ctx := context.Background()
+	// Create source stream
+	_, err = js.CreateStream(ctx, jetstream.StreamConfig{
+		Name:     "SOURCE",
+		Subjects: []string{"foo"},
+	})
+	if err != nil {
+		t.Fatalf("Unexpected error: %v", err)
+	}
+
+	// Publish some messages to source
+	for range 10 {
+		if _, err := js.Publish(ctx, "foo", []byte("hello")); err != nil {
+			t.Fatalf("Unexpected error: %v", err)
+		}
+	}
+
+	// Create mirror stream
+	mirror, err := js.CreateStream(ctx, jetstream.StreamConfig{
+		Name:   "MIRROR",
+		Mirror: &jetstream.StreamSource{Name: "SOURCE"},
+	})
+	if err != nil {
+		t.Fatalf("Unexpected error: %v", err)
+	}
+
+	// Wait for mirror to catch up
+	checkFor(t, 5*time.Second, 100*time.Millisecond, func() error {
+		info, err := mirror.Info(ctx)
+		if err != nil {
+			return err
+		}
+		if info.State.Msgs != 10 {
+			return fmt.Errorf("not caught up")
+		}
+		return nil
+	})
+
+	// Remove mirror configuration
+	updatedMirror, err := js.UpdateStream(ctx, jetstream.StreamConfig{
+		Name:     "MIRROR",
+		Subjects: []string{"bar"},
+	})
+	if err != nil {
+		t.Fatalf("Unexpected error: %v", err)
+	}
+
+	info, err := updatedMirror.Info(ctx)
+	if err != nil {
+		t.Fatalf("Unexpected error: %v", err)
+	}
+
+	if info.Config.Mirror != nil {
+		t.Fatal("Expected mirror to be nil after promote")
+	}
+
+	if info.Config.Subjects[0] != "bar" {
+		t.Fatalf("Expected subject to be 'bar', got %q", info.Config.Subjects[0])
+	}
+}