Add a callback to jsonpb.Unmarshaler to report unknown fields.

jsonpb/jsonpb.go

@@ -590,6 +590,10 @@ type Unmarshaler struct {
 	// Whether to allow messages to contain unknown fields, as opposed to
 	// failing to unmarshal.
 	AllowUnknownFields bool
+
+	// If non-nil, AllowedUnknownField will be called for each unknown field
+	// encountered and allowed during the unmarshalling.
+	AllowedUnknownField func(fieldName string)
 }
 
 // UnmarshalNext unmarshals the next protocol buffer from a JSON object stream.
@@ -901,14 +905,21 @@ func (u *Unmarshaler) unmarshalValue(target reflect.Value, inputValue json.RawMe
 				}
 			}
 		}
-		if !u.AllowUnknownFields && len(jsonFields) > 0 {
-			// Pick any field to be the scapegoat.
-			var f string
-			for fname := range jsonFields {
-				f = fname
-				break
+		if len(jsonFields) > 0 {
+			if !u.AllowUnknownFields {
+				// Pick any field to be the scapegoat.
+				var f string
+				for fname := range jsonFields {
+					f = fname
+					break
+				}
+				return fmt.Errorf("unknown field %q in %v", f, targetType)
+			}
+			if u.AllowedUnknownField != nil {
+				for fname := range jsonFields {
+					u.AllowedUnknownField(fname)
+				}
 			}
-			return fmt.Errorf("unknown field %q in %v", f, targetType)
 		}
 		return nil
 	}

jsonpb/jsonpb_test.go

@@ -37,6 +37,7 @@ import (
 	"io"
 	"math"
 	"reflect"
+	"sort"
 	"strings"
 	"testing"
 
@@ -665,6 +666,25 @@ func TestUnmarshalNullObject(t *testing.T) {
 	}
 }
 
+func TestUnmarshalAllowedUnknownFields(t *testing.T) {
+	var msg pb.Simple
+	var unknownFields []string
+	unmarshaler := &Unmarshaler{
+		AllowUnknownFields: true,
+		AllowedUnknownField: func(fname string) {
+			unknownFields = append(unknownFields, fname)
+		},
+	}
+	json := `{"oBool": true, "unknown-a": null, "unknown-b": null}`
+	if err := unmarshaler.Unmarshal(strings.NewReader(json), &msg); err != nil {
+		t.Fatal(err)
+	}
+	sort.Strings(unknownFields) // order of fields is unspecified
+	if want := []string{"unknown-a", "unknown-b"}; !reflect.DeepEqual(unknownFields, want) {
+		t.Errorf("got %v want %v", unknownFields, want)
+	}
+}
+
 func TestUnmarshalNext(t *testing.T) {
 	// We only need to check against a few, not all of them.
 	tests := unmarshalingTests[:5]