text/template: perform value validity checks

Check reflect.Value.IsValid() before calling other reflect.Value methods
that panic on zero values.

Added tests for cases with untyped nils. They panicked without these fixes.

Removed a TODO.

Fixes #12356

Change-Id: I9b5cbed26db09a0a7c36d99a93f8b9729899d51e
Reviewed-on: https://go-review.googlesource.com/14340Reviewed-by: Rob Pike <r@golang.org>

src/text/template/exec.go

@@ -135,8 +135,6 @@ func errRecover(errp *error) {
 			*errp = err.Err // Strip the wrapper.
 		case ExecError:
 			*errp = err // Keep the wrapper.
-		case error: // TODO: This should never happen, but it does. Understand and/or fix.
-			*errp = err
 		default:
 			panic(e)
 		}

src/text/template/exec_test.go

@@ -337,6 +337,7 @@ var execTests = []execTest{
 	{"if not .BinaryFunc call", "{{ if not .BinaryFunc}}{{call .BinaryFunc `1` `2`}}{{else}}No{{end}}", "No", tVal, true},
 	{"Interface Call", `{{stringer .S}}`, "foozle", map[string]interface{}{"S": bytes.NewBufferString("foozle")}, true},
 	{".ErrFunc", "{{call .ErrFunc}}", "bla", tVal, true},
+	{"call nil", "{{call nil}}", "", tVal, false},
 
 	// Erroneous function calls (check args).
 	{".BinaryFuncTooFew", "{{call .BinaryFunc `1`}}", "", tVal, false},
@@ -425,12 +426,15 @@ var execTests = []execTest{
 	{"slice[1]", "{{index .SI 1}}", "4", tVal, true},
 	{"slice[HUGE]", "{{index .SI 10}}", "", tVal, false},
 	{"slice[WRONG]", "{{index .SI `hello`}}", "", tVal, false},
+	{"slice[nil]", "{{index .SI nil}}", "", tVal, false},
 	{"map[one]", "{{index .MSI `one`}}", "1", tVal, true},
 	{"map[two]", "{{index .MSI `two`}}", "2", tVal, true},
 	{"map[NO]", "{{index .MSI `XXX`}}", "0", tVal, true},
-	{"map[nil]", "{{index .MSI nil}}", "0", tVal, true},
+	{"map[nil]", "{{index .MSI nil}}", "", tVal, false},
+	{"map[``]", "{{index .MSI ``}}", "0", tVal, true},
 	{"map[WRONG]", "{{index .MSI 10}}", "", tVal, false},
 	{"double index", "{{index .SMSI 1 `eleven`}}", "11", tVal, true},
+	{"nil[1]", "{{index nil 1}}", "", tVal, false},
 
 	// Len.
 	{"slice", "{{len .SI}}", "3", tVal, true},

src/text/template/funcs.go

@@ -104,6 +104,21 @@ func findFunction(name string, tmpl *Template) (reflect.Value, bool) {
 	return reflect.Value{}, false
 }
 
+// prepareArg checks if value can be used as an argument of type argType, and
+// converts an invalid value to appropriate zero if possible.
+func prepareArg(value reflect.Value, argType reflect.Type) (reflect.Value, error) {
+	if !value.IsValid() {
+		if !canBeNil(argType) {
+			return reflect.Value{}, fmt.Errorf("value is nil; should be of type %s", argType)
+		}
+		value = reflect.Zero(argType)
+	}
+	if !value.Type().AssignableTo(argType) {
+		return reflect.Value{}, fmt.Errorf("value has type %s; should be %s", value.Type(), argType)
+	}
+	return value, nil
+}
+
 // Indexing.
 
 // index returns the result of indexing its first argument by the following
@@ -111,6 +126,9 @@ func findFunction(name string, tmpl *Template) (reflect.Value, bool) {
 // indexed item must be a map, slice, or array.
 func index(item interface{}, indices ...interface{}) (interface{}, error) {
 	v := reflect.ValueOf(item)
+	if !v.IsValid() {
+		return nil, fmt.Errorf("index of untyped nil")
+	}
 	for _, i := range indices {
 		index := reflect.ValueOf(i)
 		var isNil bool
@@ -125,6 +143,8 @@ func index(item interface{}, indices ...interface{}) (interface{}, error) {
 				x = index.Int()
 			case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
 				x = int64(index.Uint())
+			case reflect.Invalid:
+				return nil, fmt.Errorf("cannot index slice/array with nil")
 			default:
 				return nil, fmt.Errorf("cannot index slice/array with type %s", index.Type())
 			}
@@ -133,17 +153,18 @@ func index(item interface{}, indices ...interface{}) (interface{}, error) {
 			}
 			v = v.Index(int(x))
 		case reflect.Map:
-			if !index.IsValid() {
-				index = reflect.Zero(v.Type().Key())
-			}
-			if !index.Type().AssignableTo(v.Type().Key()) {
-				return nil, fmt.Errorf("%s is not index type for %s", index.Type(), v.Type())
+			index, err := prepareArg(index, v.Type().Key())
+			if err != nil {
+				return nil, err
 			}
 			if x := v.MapIndex(index); x.IsValid() {
 				v = x
 			} else {
 				v = reflect.Zero(v.Type().Elem())
 			}
+		case reflect.Invalid:
+			// the loop holds invariant: v.IsValid()
+			panic("unreachable")
 		default:
 			return nil, fmt.Errorf("can't index item of type %s", v.Type())
 		}
@@ -155,7 +176,11 @@ func index(item interface{}, indices ...interface{}) (interface{}, error) {
 
 // length returns the length of the item, with an error if it has no defined length.
 func length(item interface{}) (int, error) {
-	v, isNil := indirect(reflect.ValueOf(item))
+	v := reflect.ValueOf(item)
+	if !v.IsValid() {
+		return 0, fmt.Errorf("len of untyped nil")
+	}
+	v, isNil := indirect(v)
 	if isNil {
 		return 0, fmt.Errorf("len of nil pointer")
 	}
@@ -172,6 +197,9 @@ func length(item interface{}) (int, error) {
 // The function must return 1 result, or 2 results, the second of which is an error.
 func call(fn interface{}, args ...interface{}) (interface{}, error) {
 	v := reflect.ValueOf(fn)
+	if !v.IsValid() {
+		return nil, fmt.Errorf("call of nil")
+	}
 	typ := v.Type()
 	if typ.Kind() != reflect.Func {
 		return nil, fmt.Errorf("non-function of type %s", typ)
@@ -201,13 +229,11 @@ func call(fn interface{}, args ...interface{}) (interface{}, error) {
 		} else {
 			argType = dddType
 		}
-		if !value.IsValid() && canBeNil(argType) {
-			value = reflect.Zero(argType)
-		}
-		if !value.Type().AssignableTo(argType) {
-			return nil, fmt.Errorf("arg %d has type %s; should be %s", i, value.Type(), argType)
+
+		var err error
+		if argv[i], err = prepareArg(value, argType); err != nil {
+			return nil, fmt.Errorf("arg %d: %s", i, err)
 		}
-		argv[i] = value
 	}
 	result := v.Call(argv)
 	if len(result) == 2 && !result[1].IsNil() {