probe/endpoint: NATMapper missed an edge case

NATMapper can be created with a nil Conntracker if
ConntrackerModulePresent is false, e.g. on Darwin.
Check for that in ApplyNAT.

probe/endpoint/nat.go

@@ -21,8 +21,8 @@ type NATMapper struct {
 	Conntracker
 }
 
-// NewNATMapper is exposed for testing
-func NewNATMapper(ct Conntracker) NATMapper {
+// MakeNATMapper is exposed for testing
+func MakeNATMapper(ct Conntracker) NATMapper {
 	return NATMapper{ct}
 }
 
@@ -50,7 +50,10 @@ func toMapping(f Flow) *endpointMapping {
 // ApplyNAT duplicates Nodes in the endpoint topology of a
 // report, based on the NAT table as returns by natTable.
 func (n NATMapper) ApplyNAT(rpt report.Report, scope string) {
-	n.WalkFlows(func(f Flow) {
+	if n.Conntracker == nil {
+		return
+	}
+	n.Conntracker.WalkFlows(func(f Flow) {
 		var (
 			mapping          = toMapping(f)
 			realEndpointID   = report.MakeEndpointNodeID(scope, mapping.originalIP, strconv.Itoa(mapping.originalPort))

probe/endpoint/nat_test.go

@@ -55,7 +55,7 @@ func TestNat(t *testing.T) {
 			"foo":         "bar",
 		}))
 
-		natmapper := endpoint.NewNATMapper(ct)
+		natmapper := endpoint.MakeNATMapper(ct)
 		natmapper.ApplyNAT(have, "host1")
 		if !reflect.DeepEqual(want, have) {
 			t.Fatal(test.Diff(want, have))
@@ -88,7 +88,7 @@ func TestNat(t *testing.T) {
 			"foo":         "baz",
 		}))
 
-		natmapper := endpoint.NewNATMapper(ct)
+		natmapper := endpoint.MakeNATMapper(ct)
 		natmapper.ApplyNAT(have, "host1")
 		if !reflect.DeepEqual(want, have) {
 			t.Fatal(test.Diff(want, have))

probe/endpoint/reporter.go

@@ -66,7 +66,7 @@ func NewReporter(hostID, hostName string, includeProcesses bool, useConntrack bo
 		if err != nil {
 			log.Printf("Failed to start conntracker for natmapper: %v", err)
 		}
-		natmapper = NewNATMapper(ct)
+		natmapper = MakeNATMapper(ct)
 	}
 	return &Reporter{
 		hostID:           hostID,