UX: better error message for json errors while parsing options (#4602)

This predominantly tries to actually list the problematic part of the
options.

Author: mstoykov

internal/js/bundle.go

@@ -217,6 +217,7 @@ func (b *Bundle) populateExports(updateOptions bool, bi *BundleInstance) error {
 				dec.DisallowUnknownFields()
 				if err = dec.Decode(&b.Options); err != nil {
 					if uerr := json.Unmarshal(data, &b.Options); uerr != nil {
+						uerr = beautifyJSONUnmarshalError(data, uerr)
 						err = errext.WithAbortReasonIfNone(
 							errext.WithExitCodeIfNone(uerr, exitcodes.InvalidConfig),
 							errext.AbortedByScriptError,
@@ -247,6 +248,19 @@ func (b *Bundle) populateExports(updateOptions bool, bi *BundleInstance) error {
 	return nil
 }
 
+func beautifyJSONUnmarshalError(data []byte, err error) error {
+	unmarshalTypError := new(json.UnmarshalTypeError)
+	if errors.As(err, &unmarshalTypError) {
+		e := unmarshalTypError
+		previousNewLineIndex := max(bytes.LastIndexByte(data[:e.Offset], '\n'), 0)
+		nextNewLineIndex := max(bytes.IndexByte(data[e.Offset:], '\n'), len(data)-1)
+
+		info := strings.TrimSpace(string(data[previousNewLineIndex:nextNewLineIndex]))
+		err = fmt.Errorf("parsing options from script got error while parsing %q: %w", info, e)
+	}
+	return err
+}
+
 // Instantiate creates a new runtime from this bundle.
 func (b *Bundle) Instantiate(ctx context.Context, vuID uint64) (*BundleInstance, error) {
 	// Instantiate the bundle into a new VM using a bound init context. This uses a context with a

internal/js/bundle_test.go

@@ -230,11 +230,24 @@ func TestNewBundle(t *testing.T) {
 			invalidOptions := map[string]struct {
 				Expr, Error string
 			}{
-				"Array":    {`[]`, "json: cannot unmarshal array into Go value of type lib.Options"},
-				"Function": {`function(){}`, "error parsing script options: json: unsupported type: func(sobek.FunctionCall) sobek.Value"},
+				"Array": {
+					`[]`,
+					`parsing options from script got error while parsing "[": ` +
+						`json: cannot unmarshal array into Go value of type lib.Options`,
+				},
+				"Bad value": {
+					`{"tags":["something"]}`,
+					`parsing options from script got error while parsing "{\"tags\":[\"something\"]": ` +
+						`json: cannot unmarshal array into Go struct field Options.tags of type map[string]string`,
+				},
+				"Function": {
+					`function(){}`,
+					"error parsing script options: json: unsupported type: func(sobek.FunctionCall) sobek.Value",
+				},
 			}
 			for name, data := range invalidOptions {
 				t.Run(name, func(t *testing.T) {
+					t.Parallel()
 					_, err := getSimpleBundle(t, "/script.js", fmt.Sprintf(`
 						export let options = %s;
 						export default function() {};