improve JSON error message (#5412) (#5433)

Co-authored-by: Zaphkiel <duambi123@gmail.com>

internal/conf/jsonwrapper/unmarshal.go

@@ -15,10 +15,13 @@ import (
 // - prevents using existing elements of slices, fixing https://github.com/golang/go/issues/21092
 // - prevents setting slices to nil
 
-func process(v reflect.Value, raw any) error {
+func process(v reflect.Value, raw any, path string) error {
 	switch v.Kind() {
 	case reflect.Slice:
 		if raw == nil {
+			if path != "" {
+				return fmt.Errorf("cannot set slice '%s' to nil", path)
+			}
 			return fmt.Errorf("cannot set slice to nil")
 		}
 
@@ -41,7 +44,11 @@ func process(v reflect.Value, raw any) error {
 				jsonKey = strings.Split(jsonKey, ",")[0]
 
 				if rawVal, ok2 := rawMap[jsonKey]; ok2 {
-					err := process(field, rawVal)
+					fieldPath := jsonKey
+					if path != "" {
+						fieldPath = path + "." + jsonKey
+					}
+					err := process(field, rawVal, fieldPath)
 					if err != nil {
 						return err
 					}
@@ -71,7 +78,7 @@ func Decode(r io.Reader, dest any) error {
 		return err
 	}
 
-	err = process(reflect.ValueOf(dest).Elem(), raw)
+	err = process(reflect.ValueOf(dest).Elem(), raw, "")
 	if err != nil {
 		return err
 	}

internal/conf/jsonwrapper/unmarshal_test.go

@@ -74,21 +74,37 @@ func TestUnmarshalPreventSliceReuse(t *testing.T) {
 }
 
 func TestUnmarshalSetSliceToNil(t *testing.T) {
-	type Data struct {
-		Items []string `json:"items"`
-	}
+	t.Run("top level", func(t *testing.T) {
+		type Data struct {
+			Items []string `json:"items"`
+		}
 
-	var data Data
+		var data Data
 
-	json := []byte(`{"items": null}`)
-	err := Unmarshal(json, &data)
-	require.EqualError(t, err, "cannot set slice to nil")
+		json := []byte(`{"items": null}`)
+		err := Unmarshal(json, &data)
+		require.EqualError(t, err, "cannot set slice 'items' to nil")
 
-	data = Data{Items: []string{"a", "b"}}
+		data = Data{Items: []string{"a", "b"}}
 
-	json = []byte(`{"items": null}`)
-	err = Unmarshal(json, &data)
-	require.EqualError(t, err, "cannot set slice to nil")
+		json = []byte(`{"items": null}`)
+		err = Unmarshal(json, &data)
+		require.EqualError(t, err, "cannot set slice 'items' to nil")
+	})
+
+	t.Run("nested", func(t *testing.T) {
+		type Inner struct {
+			Values []int `json:"values"`
+		}
+		type Outer struct {
+			Inner Inner `json:"inner"`
+		}
+
+		var data Outer
+		json := []byte(`{"inner": {"values": null}}`)
+		err := Unmarshal(json, &data)
+		require.EqualError(t, err, "cannot set slice 'inner.values' to nil")
+	})
 }
 
 func TestUnmarshalSetNullableSliceToNil(t *testing.T) {