Incorrect handling of the interrupted AUTH command

[kayrus]

Example server logs:

...
AUTH PLAIN *masked*
454 4.7.0 auth failed
* -> produces 'Command too short: "*"'
501 5.5.2 Bad command
QUIT
221 2.0.0 Bye

Expected behavior:

...
AUTH PLAIN *masked*
454 4.7.0 auth failed
*
501 5.0.0 Negotiation cancelled
QUIT
221 2.0.0 Bye

See related code:

client:

go-smtp/client.go

Line 353 in 608f3c2

c.cmd(501, "*")

server code which is triggered:

go-smtp/parse.go

Line 18 in 608f3c2

return "", "", fmt.Errorf("Command too short: %q", line)

via

go-smtp/server.go

Lines 175 to 181 in 608f3c2

	line, err := c.ReadLine()
	if err == nil {
	cmd, arg, err := parseCmd(line)
	if err != nil {
	c.protocolError(501, EnhancedCode{5, 5, 2}, "Bad command")
	continue
	}

server code, which expected be triggered:

go-smtp/conn.go

Lines 568 to 572 in 608f3c2

	if encoded == "*" {
	// https://tools.ietf.org/html/rfc4954#page-4
	c.WriteResponse(501, EnhancedCode{5, 0, 0}, "Negotiation cancelled")
	return
	}

[emersion]

That's not my reading of the RFC here. To my understanding * is only valid as a response to a 334, ie. it's not valid as a response to a 454.