Allow specifying a default value to lookup()

Fixes #4474, where lookup() calls fail out the entire interpolation when
the provided key value is not found in the map. This will allow using
coalesce() along with lookup() to greatly improve module flexibility.

config/interpolate_funcs.go

@@ -550,16 +550,31 @@ func interpolationFuncLookup(vs map[string]ast.Variable) ast.Function {
 	return ast.Function{
 		ArgTypes:     []ast.Type{ast.TypeMap, ast.TypeString},
 		ReturnType:   ast.TypeString,
+		Variadic:     true,
+		VariadicType: ast.TypeString,
 		Callback: func(args []interface{}) (interface{}, error) {
+			defaultValue := ""
+			defaultValueSet := false
+			if len(args) > 2 {
+				defaultValue = args[2].(string)
+				defaultValueSet = true
+			}
+			if len(args) > 3 {
+				return "", fmt.Errorf("lookup() takes no more than three arguments")
+			}
 			index := args[1].(string)
 			mapVar := args[0].(map[string]ast.Variable)
 
 			v, ok := mapVar[index]
 			if !ok {
+				if defaultValueSet {
+					return defaultValue, nil
+				} else {
 					return "", fmt.Errorf(
 						"lookup failed to find '%s'",
 						args[1].(string))
 				}
+			}
 			if v.Type != ast.TypeString {
 				return "", fmt.Errorf(
 					"lookup for '%s' has bad type %s",

config/interpolate_funcs_test.go

@@ -713,12 +713,33 @@ func TestInterpolateFuncLookup(t *testing.T) {
 				true,
 			},
 
+			// Supplied default with valid key
+			{
+				`${lookup(var.foo, "bar", "")}`,
+				"baz",
+				false,
+			},
+
+			// Supplied default with invalid key
+			{
+				`${lookup(var.foo, "zip", "")}`,
+				"",
+				false,
+			},
+
 			// Too many args
 			{
-				`${lookup(var.foo, "bar", "baz")}`,
+				`${lookup(var.foo, "bar", "", "abc")}`,
 				nil,
 				true,
 			},
+
+			// Non-empty default
+			{
+				`${lookup(var.foo, "zap", "xyz")}`,
+				"xyz",
+				false,
+			},
 		},
 	})
 }