From 93af67783796a48b3f59bd969dc0c528c37571ec Mon Sep 17 00:00:00 2001
From: Daniel Cormier <danielc@knowbe4.com>
Date: Fri, 10 Nov 2017 11:50:13 -0500
Subject: [PATCH] net/textproto: properly write terminating sequence if
 DotWriter is closed with no writes

Fixed textproto.Writer.DotWriter() to properly write \r\n.\r\n to the buffer
when Close() is called without any bytes written. This properly writes the
terminating sequence outlined in RFC 5321 section 4.1.1.4 and RFC 3977
section 3.1.1, even when no other bytes are written.

Change-Id: I262fd2963ee76fff7ffae8e3cb0e86255694b361
Reviewed-on: https://go-review.googlesource.com/c/go/+/77350
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
---
 src/net/textproto/writer.go      |  5 +++--
 src/net/textproto/writer_test.go | 26 ++++++++++++++++++++++++++
 2 files changed, 29 insertions(+), 2 deletions(-)

diff --git a/src/net/textproto/writer.go b/src/net/textproto/writer.go
index 1bc5974c6c..33c146c022 100644
--- a/src/net/textproto/writer.go
+++ b/src/net/textproto/writer.go
@@ -58,7 +58,8 @@ type dotWriter struct {
 }
 
 const (
-	wstateBeginLine = iota // beginning of line; initial state; must be zero
+	wstateBegin     = iota // initial state; must be zero
+	wstateBeginLine        // beginning of line
 	wstateCR               // wrote \r (possibly at end of line)
 	wstateData             // writing data in middle of line
 )
@@ -68,7 +69,7 @@ func (d *dotWriter) Write(b []byte) (n int, err error) {
 	for n < len(b) {
 		c := b[n]
 		switch d.state {
-		case wstateBeginLine:
+		case wstateBegin, wstateBeginLine:
 			d.state = wstateData
 			if c == '.' {
 				// escape leading dot
diff --git a/src/net/textproto/writer_test.go b/src/net/textproto/writer_test.go
index ac03669fa2..2afef11b5e 100644
--- a/src/net/textproto/writer_test.go
+++ b/src/net/textproto/writer_test.go
@@ -33,3 +33,29 @@ func TestDotWriter(t *testing.T) {
 		t.Fatalf("wrote %q", s)
 	}
 }
+
+func TestDotWriterCloseEmptyWrite(t *testing.T) {
+	var buf bytes.Buffer
+	w := NewWriter(bufio.NewWriter(&buf))
+	d := w.DotWriter()
+	n, err := d.Write([]byte{})
+	if n != 0 || err != nil {
+		t.Fatalf("Write: %d, %s", n, err)
+	}
+	d.Close()
+	want := "\r\n.\r\n"
+	if s := buf.String(); s != want {
+		t.Fatalf("wrote %q; want %q", s, want)
+	}
+}
+
+func TestDotWriterCloseNoWrite(t *testing.T) {
+	var buf bytes.Buffer
+	w := NewWriter(bufio.NewWriter(&buf))
+	d := w.DotWriter()
+	d.Close()
+	want := "\r\n.\r\n"
+	if s := buf.String(); s != want {
+		t.Fatalf("wrote %q; want %q", s, want)
+	}
+}