add cnf/parser/nginx/encoder writer error

internal/cnf/parser/nginx/encoder_test.go

@@ -2,6 +2,7 @@ package nginx
 
 import (
 	"bytes"
+	"errors"
 	"strings"
 	"testing"
 )
@@ -166,6 +167,36 @@ func TestDefaultIndent(t *testing.T) {
 	}
 }
 
+var writerError = errors.New("error")
+
+type defectiveWriter struct{}
+
+func (d defectiveWriter) Write(buf []byte) (int, error) {
+	return 0, writerError
+}
+
+func TestWriteError(t *testing.T) {
+
+	input := "section {\nkey value;\n}\n"
+
+	// create reader/writer
+	r, w := strings.NewReader(input), &defectiveWriter{}
+
+	// parse input
+	receiver := new(Line)
+	decoder := NewDecoder(r)
+	if err := decoder.Decode(receiver); err != nil {
+		t.Fatalf("unexpected error <%s>", err)
+	}
+
+	// encode back to writer
+	encoder := NewEncoder(w)
+	if err := encoder.Encode(receiver); err != writerError {
+		t.Fatalf("expected error <%s>, got <%s>", writerError, err)
+	}
+
+}
+
 func escape(raw string) string {
 
 	escaped := make([]rune, 0)