From bfd8333c3e047571006bfa00180ccd6365279ae9 Mon Sep 17 00:00:00 2001
From: changyic <changyic@synology.com>
Date: Mon, 2 Dec 2019 01:28:08 +0000
Subject: [PATCH] Fix writeBody may get panic because of nil w.partWriter

- If (*messageWriter).createPart fails, w.err will be set and
  w.partWriter will be nil.
- (*messageWriter).writeBody do not check the w.err and the
  w.partWriter. It may try to use the nil w.partWriter, causing
  "panic: runtime error: invalid memory address or nil pointer
  dereference".
- Adding the check of w.err, just like (*messageWriter).Write, do
  nothing in (*messageWriter).writeBody if w.err is not nil.
---
 writeto.go | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/writeto.go b/writeto.go
index faf6124..2577fe4 100644
--- a/writeto.go
+++ b/writeto.go
@@ -258,6 +258,10 @@ func (w *messageWriter) writeHeaders(h map[string][]string) {
 }
 
 func (w *messageWriter) writeBody(f func(io.Writer) error, enc Encoding) {
+	if w.err != nil {
+		return
+	}
+
 	var subWriter io.Writer
 	if w.depth == 0 {
 		w.writeString("\r\n")