text/template: use reflect.Value.FieldByIndexErr

to avoid a crash on a nil pointer as an embedded field.

Fixes #48215

Change-Id: I214faa6e3cf08cdec1c01035e4bbca0900c6a408
Reviewed-on: https://go-review.googlesource.com/c/go/+/357963
Trust: Rob Pike <[email protected]>
Run-TryBot: Rob Pike <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

src/text/template/exec.go

@@ -635,10 +635,13 @@ func (s *state) evalField(dot reflect.Value, fieldName string, node parse.Node,
  case reflect.Struct:
  tField, ok := receiver.Type().FieldByName(fieldName)
  if ok {
- field := receiver.FieldByIndex(tField.Index)
+ field, err := receiver.FieldByIndexErr(tField.Index)
  if !tField.IsExported() {
  s.errorf("%s is an unexported field of struct type %s", fieldName, typ)
  }
+ if err != nil {
+ s.errorf("%v", err)
+ }
  // If it's a function, we must call it.
  if hasArgs {
  s.errorf("%s has arguments but cannot be invoked as function", fieldName)

src/text/template/exec_test.go

@@ -1787,3 +1787,26 @@ func TestIssue39807(t *testing.T) {
 
  wg.Wait()
 }
+
+// Issue 48215: embedded nil pointer causes panic.
+// Fixed by adding FieldByIndexErr to the reflect package.
+func TestIssue48215(t *testing.T) {
+ type A struct {
+ S string
+ }
+ type B struct {
+ *A
+ }
+ tmpl, err := New("").Parse(`{{ .S }}`)
+ if err != nil {
+ t.Fatal(err)
+ }
+ err = tmpl.Execute(io.Discard, B{})
+ // We expect an error, not a panic.
+ if err == nil {
+ t.Fatal("did not get error for nil embedded struct")
+ }
+ if !strings.Contains(err.Error(), "reflect: indirection through nil pointer to embedded struct field A") {
+ t.Fatal(err)
+ }
+}