net/smtp: preserve Auth errors

If authentication failed, the initial error was being thrown away.

Fixes #5700.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/10744043

src/pkg/net/smtp/smtp.go

@@ -196,7 +196,9 @@ func (c *Client) Auth(a Auth) error {
 		default:
 			err = &textproto.Error{Code: code, Msg: msg64}
 		}
-		resp, err = a.Next(msg, code == 334)
+		if err == nil {
+			resp, err = a.Next(msg, code == 334)
+		}
 		if err != nil {
 			// abort the AUTH
 			c.cmd(501, "*")

src/pkg/net/smtp/smtp_test.go

@@ -504,3 +504,47 @@ SendMail is working for me.
 .
 QUIT
 `
+
+func TestAuthFailed(t *testing.T) {
+	server := strings.Join(strings.Split(authFailedServer, "\n"), "\r\n")
+	client := strings.Join(strings.Split(authFailedClient, "\n"), "\r\n")
+	var cmdbuf bytes.Buffer
+	bcmdbuf := bufio.NewWriter(&cmdbuf)
+	var fake faker
+	fake.ReadWriter = bufio.NewReadWriter(bufio.NewReader(strings.NewReader(server)), bcmdbuf)
+	c, err := NewClient(fake, "fake.host")
+	if err != nil {
+		t.Fatalf("NewClient: %v", err)
+	}
+	defer c.Close()
+
+	c.tls = true
+	c.serverName = "smtp.google.com"
+	err = c.Auth(PlainAuth("", "user", "pass", "smtp.google.com"))
+
+	if err == nil {
+		t.Error("Auth: expected error; got none")
+	} else if err.Error() != "535 Invalid credentials\nplease see www.example.com" {
+		t.Errorf("Auth: got error: %v, want: %s", err, "535 Invalid credentials\nplease see www.example.com")
+	}
+
+	bcmdbuf.Flush()
+	actualcmds := cmdbuf.String()
+	if client != actualcmds {
+		t.Errorf("Got:\n%s\nExpected:\n%s", actualcmds, client)
+	}
+}
+
+var authFailedServer = `220 hello world
+250-mx.google.com at your service
+250 AUTH LOGIN PLAIN
+535-Invalid credentials
+535 please see www.example.com
+221 Goodbye
+`
+
+var authFailedClient = `EHLO localhost
+AUTH PLAIN AHVzZXIAcGFzcw==
+*
+QUIT
+`