Render empty forwarders for forward zones

A forward zone with no forwarders now renders "forwarders { };", which
BIND accepts and treats as disabling forwarding for the zone.

internal/bind/render.go

@@ -154,9 +154,7 @@ func renderForwardZone(z bindv1alpha1.BindZone, indent string) string {
 	b.WriteString(fmt.Sprintf("%szone \"%s\" {\n", indent, z.Spec.ZoneName))
 	b.WriteString(indent + "    type forward;\n")
 	b.WriteString(indent + "    forward only;\n")
-	if len(z.Spec.Forwarders) > 0 {
-		b.WriteString(fmt.Sprintf("%s    forwarders { %s };\n", indent, terminate(z.Spec.Forwarders)))
-	}
+	b.WriteString(indent + "    " + ForwardersClause(z.Spec.Forwarders) + ";\n")
 	b.WriteString(indent + "};\n")
 	return b.String()
 }
@@ -398,6 +396,14 @@ func terminate(entries []string) string {
 	return strings.Join(parts, " ")
 }
 
+// ForwardersClause renders a zone forwarders clause; empty means "forwarders { }" (no forwarding).
+func ForwardersClause(entries []string) string {
+	if list := terminate(entries); list != "" {
+		return "forwarders { " + list + " }"
+	}
+	return "forwarders { }"
+}
+
 func yesno(b bool) string {
 	if b {
 		return "yes"

internal/bind/render_test.go

@@ -261,3 +261,19 @@ func TestRenderDeterministicWithShuffledForwards(t *testing.T) {
 		t.Fatalf("render must be independent of forward-zone input order:\n--- A ---\n%s\n--- B ---\n%s", pa, pb)
 	}
 }
+
+func TestRenderForwardZoneForwarders(t *testing.T) {
+	for _, tc := range []struct {
+		fwd  []string
+		want string
+	}{
+		{nil, "    forwarders { };\n"},
+		{[]string{"198.18.19.15", "198.18.19.16;"}, "    forwarders { 198.18.19.15; 198.18.19.16; };\n"},
+	} {
+		z := bindv1alpha1.BindZone{Spec: bindv1alpha1.BindZoneSpec{ZoneName: "spamhaus.org", Type: bindv1alpha1.ZoneForward, Forwarders: tc.fwd}}
+		want := "zone \"spamhaus.org\" {\n    type forward;\n    forward only;\n" + tc.want + "};\n"
+		if got := renderForwardZone(z, ""); got != want {
+			t.Errorf("forwarders %v:\ngot  %q\nwant %q", tc.fwd, got, want)
+		}
+	}
+}

internal/controller/bindzone_controller.go

@@ -218,10 +218,7 @@ func (r *BindZoneReconciler) buildZoneConfig(ctx context.Context, zone *bindv1al
 			parts = append(parts, fmt.Sprintf("primaries { %s }", terminateInline(zone.Spec.Primaries)))
 		}
 	case bindv1alpha1.ZoneForward:
-		parts = append(parts, "type forward", "forward only")
-		if len(zone.Spec.Forwarders) > 0 {
-			parts = append(parts, fmt.Sprintf("forwarders { %s }", terminateInline(zone.Spec.Forwarders)))
-		}
+		parts = append(parts, "type forward", "forward only", bind.ForwardersClause(zone.Spec.Forwarders))
 	case bindv1alpha1.ZoneStub:
 		parts = append(parts, "type stub", fmt.Sprintf("file \"%s\"", bind.ZoneFilePath(zone.Spec.ZoneName)))
 		if len(zone.Spec.Primaries) > 0 {

internal/controller/bindzone_notify_test.go

@@ -63,3 +63,23 @@ func TestBuildZoneConfigPrimaryNoNotifyTargets(t *testing.T) {
 		t.Errorf("did not expect notify clauses with no targets: %q", cfg)
 	}
 }
+
+func TestBuildZoneConfigForwardForwarders(t *testing.T) {
+	r := &BindZoneReconciler{}
+	for _, tc := range []struct {
+		fwd  []string
+		want string
+	}{
+		{nil, "{ type forward; forward only; forwarders { }; };"},
+		{[]string{"198.18.19.15"}, "{ type forward; forward only; forwarders { 198.18.19.15; }; };"},
+	} {
+		zone := &bindv1alpha1.BindZone{Spec: bindv1alpha1.BindZoneSpec{ZoneName: "spamhaus.org", Type: bindv1alpha1.ZoneForward, Forwarders: tc.fwd}}
+		cfg, err := r.buildZoneConfig(context.Background(), zone, "", nil, "")
+		if err != nil {
+			t.Fatalf("buildZoneConfig: %v", err)
+		}
+		if cfg != tc.want {
+			t.Errorf("forwarders %v:\ngot  %q\nwant %q", tc.fwd, cfg, tc.want)
+		}
+	}
+}