Include body MIME terminator in multipart messages

Before, the text/plain part of the multipart MIME message was not being
correctly terminated with its boundary. The multipart writer writes the
terminator when its Close is called, but since the call to Close() was
deferred, it was not being called until after the attachments were being
written resulting in the boundary not being included at all.

widgets/compose.go

@@ -335,14 +335,7 @@ func (c *Composer) WriteMessage(header *mail.Header, writer io.Writer) error {
 
 	if len(c.attachments) == 0 {
 		// don't create a multipart email if we only have text
-		header.SetContentType("text/plain", map[string]string{"charset": "UTF-8"})
-		w, err := mail.CreateSingleInlineWriter(writer, *header)
-		if err != nil {
-			return errors.Wrap(err, "CreateSingleInlineWriter")
-		}
-		defer w.Close()
-
-		return writeBody(body, w)
+		return writeInlineBody(header, body, writer)
 	}
 
 	// otherwise create a multipart email,
@@ -353,6 +346,34 @@ func (c *Composer) WriteMessage(header *mail.Header, writer io.Writer) error {
 	}
 	defer w.Close()
 
+	if err := writeMultipartBody(body, w); err != nil {
+		return errors.Wrap(err, "writeMultipartBody")
+	}
+
+	for _, a := range c.attachments {
+		if err := writeAttachment(a, w); err != nil {
+			return errors.Wrap(err, "writeAttachment")
+		}
+	}
+
+	return nil
+}
+
+func writeInlineBody(header *mail.Header, body io.Reader, writer io.Writer) error {
+	header.SetContentType("text/plain", map[string]string{"charset": "UTF-8"})
+	w, err := mail.CreateSingleInlineWriter(writer, *header)
+	if err != nil {
+		return errors.Wrap(err, "CreateSingleInlineWriter")
+	}
+	defer w.Close()
+	if _, err := io.Copy(w, body); err != nil {
+		return errors.Wrap(err, "io.Copy")
+	}
+	return nil
+}
+
+// write the message body to the multipart message
+func writeMultipartBody(body io.Reader, w *mail.Writer) error {
 	bh := mail.InlineHeader{}
 	bh.SetContentType("text/plain", map[string]string{"charset": "UTF-8"})
 
@@ -367,23 +388,9 @@ func (c *Composer) WriteMessage(header *mail.Header, writer io.Writer) error {
 		return errors.Wrap(err, "CreatePart")
 	}
 	defer bw.Close()
-
-	if err := writeBody(body, bw); err != nil {
-		return err
-	}
-
-	for _, a := range c.attachments {
-		writeAttachment(a, w)
-	}
-
-	return nil
-}
-
-func writeBody(body io.Reader, w io.Writer) error {
-	if _, err := io.Copy(w, body); err != nil {
+	if _, err := io.Copy(bw, body); err != nil {
 		return errors.Wrap(err, "io.Copy")
 	}
-
 	return nil
 }