reflect: add FieldByIndexErr

This new function, although different in signature from other
reflect functions, allows the caller to avoid the panic caused by
nil embedded fields in calls to FieldByIndex.

Fixes #48218

Change-Id: I447f135bb789148c27ae3f2f23dcf43094f4c1de
Reviewed-on: https://go-review.googlesource.com/c/go/+/357962
Trust: Rob Pike <r@golang.org>
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

Author: robpike

src/reflect/value.go

@@ -5,6 +5,7 @@
 package reflect
 
 import (
+	"errors"
 	"internal/abi"
 	"internal/goarch"
 	"internal/itoa"
@@ -1232,7 +1233,8 @@ func (v Value) Field(i int) Value {
 }
 
 // FieldByIndex returns the nested field corresponding to index.
-// It panics if v's Kind is not struct.
+// It panics if evaluation requires stepping through a nil
+// pointer or a field that is not a struct.
 func (v Value) FieldByIndex(index []int) Value {
 	if len(index) == 1 {
 		return v.Field(index[0])
@@ -1252,6 +1254,29 @@ func (v Value) FieldByIndex(index []int) Value {
 	return v
 }
 
+// FieldByIndexErr returns the nested field corresponding to index.
+// It returns an error if evaluation requires stepping through a nil
+// pointer, but panics if it must step through a field that
+// is not a struct.
+func (v Value) FieldByIndexErr(index []int) (Value, error) {
+	if len(index) == 1 {
+		return v.Field(index[0]), nil
+	}
+	v.mustBe(Struct)
+	for i, x := range index {
+		if i > 0 {
+			if v.Kind() == Ptr && v.typ.Elem().Kind() == Struct {
+				if v.IsNil() {
+					return Value{}, errors.New("reflect: indirection through nil pointer to embedded struct field " + v.typ.Elem().Name())
+				}
+				v = v.Elem()
+			}
+		}
+		v = v.Field(x)
+	}
+	return v, nil
+}
+
 // FieldByName returns the struct field with the given name.
 // It returns the zero Value if no field was found.
 // It panics if v's Kind is not struct.

src/reflect/visiblefields_test.go

@@ -6,6 +6,7 @@ package reflect_test
 
 import (
 	. "reflect"
+	"strings"
 	"testing"
 )
 
@@ -328,3 +329,21 @@ func TestFields(t *testing.T) {
 		})
 	}
 }
+
+// Must not panic with nil embedded pointer.
+func TestFieldByIndexErr(t *testing.T) {
+	type A struct {
+		S string
+	}
+	type B struct {
+		*A
+	}
+	v := ValueOf(B{})
+	_, err := v.FieldByIndexErr([]int{0, 0})
+	if err == nil {
+		t.Fatal("expected error")
+	}
+	if !strings.Contains(err.Error(), "embedded struct field A") {
+		t.Fatal(err)
+	}
+}