cmd/list: Add type assertions and error reporter

Every time Podman changes their JSON API Toolbox breaks horribly. That
is caused by the complete lack of type guards. With this the verbosity
of the code goes up a bit but it should be much more resistant to API
changes.

If a field is of a not-expected type, the field is set to "<error>" and
an error message is printed after the rest of the table is printed into
the console.

src/cmd/list.go

@@ -184,6 +184,7 @@ func listImages() ([]map[string]interface{}, error) {
 
 func listOutput(images, containers []map[string]interface{}) {
 	if len(images) != 0 {
+		var err error = nil
 		writer := tabwriter.NewWriter(os.Stdout, 0, 0, 2, ' ', 0)
 		fmt.Fprintf(writer, "%s\t%s\t%s\n", "IMAGE ID", "IMAGE NAME", "CREATED")
 
@@ -202,21 +203,47 @@ func listOutput(images, containers []map[string]interface{}) {
 			createdKey = "created"
 		}
 
+		var id, name, created string
 		for _, image := range images {
-			id := utils.ShortID(image[idKey].(string))
-			name := image[nameKey].([]interface{})[0].(string)
-			created := image[createdKey].(string)
+			switch value := image[idKey].(type) {
+			case string:
+				id = utils.ShortID(value)
+			default:
+				id = "<error>"
+				err = fmt.Errorf("expected type of field %s is string, found %T", idKey, image[idKey])
+			}
+
+			switch value := image[nameKey].(type) {
+			case []interface{}:
+				name = value[0].(string)
+			default:
+				name = "<error>"
+				err = fmt.Errorf("expected type of field %s is []interface{}, found %T", idKey, image[nameKey])
+			}
+
+			switch value := image[createdKey].(type) {
+			case string:
+				created = value
+			default:
+				created = "<error>"
+				err = fmt.Errorf("expected type of field %s is string, found %T", idKey, image[createdKey])
+			}
+
 			fmt.Fprintf(writer, "%s\t%s\t%s\n", id, name, created)
 		}
 
 		writer.Flush()
+		if err != nil {
+			logrus.Errorf("There was an error while listing toolbox images: %v", err)
+		}
 	}
 
 	if len(images) != 0 && len(containers) != 0 {
 		fmt.Println()
 	}
 
 	if len(containers) != 0 {
+		var err error = nil
 		writer := tabwriter.NewWriter(os.Stdout, 0, 0, 2, ' ', 0)
 		fmt.Fprintf(writer,
 			"%s\t%s\t%s\t%s\t%s\n",
@@ -226,7 +253,7 @@ func listOutput(images, containers []map[string]interface{}) {
 			"STATUS",
 			"IMAGE NAME")
 
-		var idKey, createdKey, statusKey string
+		var idKey, namesKey, createdKey, statusKey, imageKey string
 		if podman.CheckVersion("2.0.0") {
 			idKey = "Id"
 			createdKey = "CreatedAt"
@@ -236,25 +263,59 @@ func listOutput(images, containers []map[string]interface{}) {
 			createdKey = "Created"
 			statusKey = "Status"
 		}
+		namesKey = "Names"
+		imageKey = "Image"
 
+		var id, name, created, status, imageName string
 		for _, container := range containers {
-			id := utils.ShortID(container[idKey].(string))
+			switch value := container[idKey].(type) {
+			case string:
+				id = utils.ShortID(value)
+			default:
+				id = "<error>"
+				err = fmt.Errorf("expected type of field %s is string, found %T", idKey, container[createdKey])
+			}
 
-			var nameString string
-			switch name := container["Names"].(type) {
+			switch value := container[namesKey].(type) {
 			case string:
-				nameString = name
+				name = value
 			case []interface{}:
-				nameString = name[0].(string)
+				name = value[0].(string)
+			default:
+				name = "<error>"
+				err = fmt.Errorf("expected type of field %s is string or []interface{}, found %T", idKey, container[namesKey])
+			}
+
+			switch value := container[createdKey].(type) {
+			case string:
+				created = value
+			default:
+				created = "<error>"
+				err = fmt.Errorf("expected type of field %s is string, found %T", idKey, container[createdKey])
 			}
 
-			created := container[createdKey].(string)
-			status := container[statusKey].(string)
-			imageName := container["Image"].(string)
+			switch value := container[statusKey].(type) {
+			case string:
+				status = value
+			default:
+				status = "<error>"
+				err = fmt.Errorf("expected type of field %s is string, found %T", idKey, container[statusKey])
+			}
 
-			fmt.Fprintf(writer, "%s\t%s\t%s\t%s\t%s\n", id, nameString, created, status, imageName)
+			switch value := container[imageKey].(type) {
+			case string:
+				imageName = value
+			default:
+				imageName = "<error>"
+				err = fmt.Errorf("expected type of field %s is string, found %T", idKey, container[imageKey])
+			}
+
+			fmt.Fprintf(writer, "%s\t%s\t%s\t%s\t%s\n", id, name, created, status, imageName)
 		}
 
 		writer.Flush()
+		if err != nil {
+			logrus.Errorf("There was an error while listing toolbox containers: %v", err)
+		}
 	}
 }