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

@@ -548,17 +548,32 @@ func interpolationFuncSplit() ast.Function {
 // dynamic lookups of map types within a Terraform configuration.
 func interpolationFuncLookup(vs map[string]ast.Variable) ast.Function {
 	return ast.Function{
-		ArgTypes:   []ast.Type{ast.TypeMap, ast.TypeString},
-		ReturnType: ast.TypeString,
+		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 {
-				return "", fmt.Errorf(
-					"lookup failed to find '%s'",
-					args[1].(string))
+				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(

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,
+			},
 		},
 	})
 }