net/smtp: added Noop to Client

This adds a Noop() function to the net/stmp client.

It allows for testing if a connaction is still healthy.

Fixes #22321

Change-Id: I023b613b1536ea21274cc36d41f5720c9bbdecbc
Reviewed-on: https://go-review.googlesource.com/71650
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/smtp/smtp.go

@@ -398,6 +398,16 @@ func (c *Client) Reset() error {
 	return err
 }
 
+// Noop sends the NOOP command to the server. It does nothing but check
+// that the connaction to the server is okay.
+func (c *Client) Noop() error {
+	if err := c.hello(); err != nil {
+		return err
+	}
+	_, _, err := c.cmd(250, "NOOP")
+	return err
+}
+
 // Quit sends the QUIT command and closes the connection to the server.
 func (c *Client) Quit() error {
 	if err := c.hello(); err != nil {

src/net/smtp/smtp_test.go

@@ -478,6 +478,8 @@ func TestHello(t *testing.T) {
 					t.Errorf("Want error, got none")
 				}
 			}
+		case 9:
+			err = c.Noop()
 		default:
 			t.Fatalf("Unhandled command")
 		}
@@ -510,6 +512,7 @@ var helloServer = []string{
 	"250 Reset ok\n",
 	"221 Goodbye\n",
 	"250 Sender ok\n",
+	"250 ok\n",
 }
 
 var baseHelloClient = `EHLO customhost
@@ -526,6 +529,7 @@ var helloClient = []string{
 	"RSET\n",
 	"QUIT\n",
 	"VRFY test@example.com\n",
+	"NOOP\n",
 }
 
 func TestSendMail(t *testing.T) {