Merge pull request #410 from fxamacker/fxamacker/fix-diagnose-empty-d…

…ata-error

Fix Diagnose to return io.EOF error on empty data.

This was detected while updating CBOR fuzz tests for v2.5.0-beta3.

diagnose.go

@@ -210,6 +210,9 @@ func (di *diagnose) diag(cborSequence bool) (string, error) {
 			}
 
 		case io.EOF:
+			if firstItem {
+				return di.w.String(), err
+			}
 			return di.w.String(), nil
 
 		default:

diagnose_test.go

@@ -6,6 +6,7 @@ package cbor
 import (
 	"bytes"
 	"fmt"
+	"io"
 	"reflect"
 	"strings"
 	"testing"
@@ -1079,3 +1080,41 @@ func TestDiagnoseNotwellformedData(t *testing.T) {
 		t.Errorf("Diagnose(0x%x) returned error %q", cborData, err)
 	}
 }
+
+func TestDiagnoseEmptyData(t *testing.T) {
+	var emptyData []byte
+
+	defaultMode, _ := DiagOptions{}.DiagMode()
+	sequenceMode, _ := DiagOptions{CBORSequence: true}.DiagMode()
+
+	testCases := []struct {
+		name string
+		dm   DiagMode
+	}{
+		{"default", defaultMode},
+		{"sequence", sequenceMode},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			s, err := tc.dm.Diagnose(emptyData)
+			if len(s) != 0 {
+				t.Errorf("Diagnose() didn't return empty notation for empty data")
+			}
+			if err != io.EOF {
+				t.Errorf("Diagnose() didn't return io.EOF for empty data")
+			}
+
+			s, rest, err := tc.dm.DiagnoseFirst(emptyData)
+			if len(s) != 0 {
+				t.Errorf("DiagnoseFirst() didn't return empty notation for empty data")
+			}
+			if len(rest) != 0 {
+				t.Errorf("DiagnoseFirst() didn't return empty rest for empty data")
+			}
+			if err != io.EOF {
+				t.Errorf("DiagnoseFirst() didn't return io.EOF for empty data")
+			}
+		})
+	}
+}