Fix format bug with empty string and null

cft/diff/compare.go

@@ -14,6 +14,12 @@ func New(a, b cft.Template) Diff {
 
 func compareValues(old, new interface{}) Diff {
 	if reflect.TypeOf(old) != reflect.TypeOf(new) {
+
+		// In YAML there is no difference between "" and null
+		if old == "" && new == nil {
+			return value{new, Unchanged}
+		}
+
 		return value{new, Changed}
 	}
 

cft/format/format.go

@@ -33,8 +33,6 @@ func CheckMultilineBegin(s string) bool {
 func String(t cft.Template, opt Options) string {
 	node := t.Node
 
-	// config.Debugf("%v", rnode.ToSJson(node))
-
 	buf := strings.Builder{}
 	enc := yaml.NewEncoder(&buf)
 	enc.SetIndent(2)

cft/format/format_test.go

@@ -717,3 +717,46 @@ Parameters:
 	}
 
 }
+
+func TestFnGetAZs(t *testing.T) {
+	input := `
+Resources:
+  PrivateSubnet1Subnet:
+    Type: AWS::EC2::Subnet
+    Properties:
+      AvailabilityZone: !Select
+        - 0
+        - !GetAZs ""
+`
+
+	// Parse the template
+	source, err := parse.String(string(input))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	output := format.String(source, format.Options{
+		JSON:     false,
+		Unsorted: true,
+	})
+
+	// Verify the output is valid
+	if err = parse.Verify(source, output); err != nil {
+		t.Fatal(err)
+	}
+
+	expected := `
+Resources:
+  PrivateSubnet1Subnet:
+    Type: AWS::EC2::Subnet
+    Properties:
+      AvailabilityZone: !Select
+        - 0
+        - !GetAZs 
+`
+
+	if strings.TrimSpace(output) != strings.TrimSpace(expected) {
+		t.Fatalf("Got:\n%s\n\nExpected:\n%s", output, expected)
+	}
+
+}

cft/format/transform.go

@@ -28,6 +28,7 @@ func formatNode(n *yaml.Node) *yaml.Node {
 	if n.Kind == yaml.MappingNode {
 		// Does it have just one key/value pair?
 		if len(n.Content) == 2 {
+
 			// Is the key relevant?
 			for tag, funcName := range cft.Tags {
 				if n.Content[0].Value == funcName {
@@ -85,6 +86,7 @@ func formatNode(n *yaml.Node) *yaml.Node {
 					n.Style = newNode.Content[0].Style
 				}
 			}
+
 		}
 	}
 

test/templates/getazs-null.yaml

@@ -0,0 +1,13 @@
+Parameters:
+  VpcId:
+    Type: String
+
+Resources:
+
+  PrivateSubnet1Subnet:
+    Type: AWS::EC2::Subnet
+    Properties:
+      AvailabilityZone: !Select
+        - 0
+        - !GetAZs ""
+      VpcId: !Ref VpcId