fix: unknown ostype when getting an index should probably create an error

pkg/armhelpers/utils/util.go

@@ -155,15 +155,18 @@ func GetVMNameIndex(osType compute.OperatingSystemTypes, vmName string) (int, er
 			log.Errorln(err)
 			return 0, err
 		}
-	} else if osType == compute.Windows {
+		return agentIndex, nil
+	}
+	if osType == compute.Windows {
 		_, _, _, agentIndex, err = WindowsVMNameParts(vmName)
 		if err != nil {
 			log.Errorln(err)
 			return 0, err
 		}
+		return agentIndex, nil
 	}
 
-	return agentIndex, nil
+	return 0, errors.Errorf("Unknown os type. Can't determine vm index")
 }
 
 // GetK8sVMName reconstructs the VM name

pkg/armhelpers/utils/util_test.go

@@ -133,6 +133,14 @@ func Test_WindowsVMNameParts(t *testing.T) {
 	}
 }
 
+func Test_GetVMNameIndexUnknown(t *testing.T) {
+	var ostype compute.OperatingSystemTypes
+	_, err := GetVMNameIndex(ostype, "k8s-agentpool1-38988164-65")
+	if err == nil {
+		t.Fatalf("unexpected error but got none")
+	}
+}
+
 func Test_GetVMNameIndexLinux(t *testing.T) {
 	expectedAgentIndex := 65