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

@@ -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

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)
+	}
+}