Apply setters empty values checks

functions/go/apply-setters/applysetters/apply_setters.go

@@ -31,6 +31,9 @@ type Setter struct {
 
 // Filter implements Set as a yaml.Filter
 func (as ApplySetters) Filter(nodes []*yaml.RNode) ([]*yaml.RNode, error) {
+	if len(as.Setters) == 0 {
+		return nodes, fmt.Errorf("failed to configure function: input setters list cannot be empty")
+	}
 	for i := range nodes {
 		err := accept(&as, nodes[i])
 		if err != nil {
@@ -91,10 +94,18 @@ func (as *ApplySetters) visitMapping(object *yaml.RNode) error {
 		// get the setter value for the setter name in the comment
 		sv := setterValue(as.Setters, setterPattern)
 
+		if sv == "" {
+			var elements []*yaml.Node
+			elements = append(elements, &yaml.Node{})
+			node.Value.YNode().Content = elements
+			node.Value.YNode().Style = yaml.FoldedStyle
+			return nil
+		}
+
 		// parse the setter value as yaml node
 		rn, err := yaml.Parse(sv)
 		if err != nil {
-			return err
+			return errors.Errorf("input to array setter must be an array of values, but found %q", sv)
 		}
 
 		// the setter value must parse as sequence node

functions/go/apply-setters/applysetters/apply_setters_test.go

@@ -335,6 +335,66 @@ metadata:
   name: my-app-layer
 spec:
   host: my-app-layer.dev.demo.io # kpt-set: my-app-layer.${stage}.${domain}.${tld}
+`,
+		},
+		{
+			name: "error: no input",
+			config: `
+data: {}
+`,
+			input: `apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: nginx-deployment
+spec:
+  replicas: 3
+  template:
+    spec:
+      containers:
+        - name: nginx
+          image: nginx:1.7.9 # kpt-set: ${image}:${tag}
+`,
+			expectedResources: `apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: nginx-deployment
+spec:
+  replicas: 3
+  template:
+    spec:
+      containers:
+        - name: nginx
+          image: nginx:1.7.9 # kpt-set: ${image}:${tag}
+`,
+			errMsg: `failed to configure function: input setters list cannot be empty`,
+		},
+		{
+			name: "set empty values",
+			input: `apiVersion: v1
+kind: Service
+metadata:
+  name: myService # kpt-set: ${app}
+  namespace: foo # kpt-set: ${ns}
+image: nginx:1.7.1 # kpt-set: ${image}:${tag}
+env: # kpt-set: ${env}
+  - foo
+  - bar
+`,
+			config: `
+data:
+  app: ""
+  ns: ~
+  image: ''
+  env: ~
+`,
+			expectedResources: `apiVersion: v1
+kind: Service
+metadata:
+  name: # kpt-set: ${app}
+  namespace: # kpt-set: ${ns}
+image: :1.7.1 # kpt-set: ${image}:${tag}
+env: # kpt-set: ${env}
+  - null
 `,
 		},
 	}