client: batching commands

[emersion]

Right now clients always wait for the server reply for a command before sending the next one. This causes some unnecessary latency due to roundtrips.

Possible extended API to allow clients to send command batches:

type ClientBatch struct {}

func (*Client) Batch() *ClientBatch

func (*ClientBatch) Mail(from string) *ClientBatch
func (*ClientBatch) Rcpt(to string) *ClientBatch
func (*ClientBatch) Data() io.WriteCloser
func (*ClientBatch) Wait() error

[imirkin]

It's not generally legal to do this, but the PIPELINING extension (RFC 2920) allows it for certain commands, when advertised by the server. Specifically from section 3.1:

In particular, the commands RSET, MAIL FROM,
SEND FROM, SOML FROM, SAML FROM, and RCPT TO can all appear anywhere
in a pipelined command group. The EHLO, DATA, VRFY, EXPN, TURN,
QUIT, and NOOP commands can only appear as the last command in a
group since their success or failure produces a change of state which
the client SMTP must accommodate. (NOOP is included in this group so
it can be used as a synchronization point.)

Note that you have to wait for the response from DATA, which may be an error or a success, and even if an earlier command returned failure, DATA may return success, at which point you have to provide it with valid data.

I guess Data() should return a writer + error though, in addition to the explicit Wait() (which would not be necessary to call under usual circumstances). Thoughts?