config: new interpolation function jsonencode

config/interpolate_funcs.go

@@ -7,6 +7,7 @@ import (
 	"crypto/sha256"
 	"encoding/base64"
 	"encoding/hex"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"io/ioutil"
@@ -40,6 +41,7 @@ func Funcs() map[string]ast.Function {
 		"formatlist":   interpolationFuncFormatList(),
 		"index":        interpolationFuncIndex(),
 		"join":         interpolationFuncJoin(),
+		"jsonencode":   interpolationFuncJSONEncode(),
 		"length":       interpolationFuncLength(),
 		"lower":        interpolationFuncLower(),
 		"md5":          interpolationFuncMd5(),
@@ -364,6 +366,23 @@ func interpolationFuncJoin() ast.Function {
 	}
 }
 
+// interpolationFuncJSONEncode implements the "jsonencode" function that encodes
+// a string as its JSON representation.
+func interpolationFuncJSONEncode() ast.Function {
+	return ast.Function{
+		ArgTypes:   []ast.Type{ast.TypeString},
+		ReturnType: ast.TypeString,
+		Callback: func(args []interface{}) (interface{}, error) {
+			s := args[0].(string)
+			jEnc, err := json.Marshal(s)
+			if err != nil {
+				return "", fmt.Errorf("failed to encode JSON data '%s'", s)
+			}
+			return string(jEnc), nil
+		},
+	}
+}
+
 // interpolationFuncReplace implements the "replace" function that does
 // string replacement.
 func interpolationFuncReplace() ast.Function {

config/interpolate_funcs_test.go

@@ -437,6 +437,48 @@ func TestInterpolateFuncJoin(t *testing.T) {
 	})
 }
 
+func TestInterpolateFuncJSONEncode(t *testing.T) {
+	testFunction(t, testFunctionConfig{
+		Vars: map[string]ast.Variable{
+			"easy": ast.Variable{
+				Value: "test",
+				Type:  ast.TypeString,
+			},
+			"hard": ast.Variable{
+				Value: " foo \\ \n \t \" bar ",
+				Type:  ast.TypeString,
+			},
+		},
+		Cases: []testFunctionCase{
+			{
+				`${jsonencode("test")}`,
+				`"test"`,
+				false,
+			},
+			{
+				`${jsonencode(easy)}`,
+				`"test"`,
+				false,
+			},
+			{
+				`${jsonencode(hard)}`,
+				`" foo \\ \n \t \" bar "`,
+				false,
+			},
+			{
+				`${jsonencode("")}`,
+				`""`,
+				false,
+			},
+			{
+				`${jsonencode()}`,
+				nil,
+				true,
+			},
+		},
+	})
+}
+
 func TestInterpolateFuncReplace(t *testing.T) {
 	testFunction(t, testFunctionConfig{
 		Cases: []testFunctionCase{

website/source/docs/configuration/interpolation.html.md

@@ -150,6 +150,9 @@ The supported built-in functions are:
       only possible with splat variables from resources with a count
       greater than one. Example: `join(",", aws_instance.foo.*.id)`
 
+  * `jsonencode(string)` - Returns a JSON-encoded representation of the given
+    string (including double quotes).
+
   * `length(list)` - Returns a number of members in a given list
       or a number of characters in a given string.
       * `${length(split(",", "a,b,c"))}` = 3