tcp: Always send an ACK segment once the handshake is completed

The reporter is running a SMTP server behind pasta, and the client
waits for the server's banner before sending any data. In turn, the
server waits for our ACK after sending SYN,ACK, which never comes.

If we use the ACK_IF_NEEDED indication to tcp_send_flag(), given that
there's no pending data, we delay sending the ACK segment at the end
of the three-way handshake until we have some data to send to the
server.

This was actually intended, as I thought we would lower the latency
for new connections, but we can't assume that the client will start
sending data first (SMTP is the typical example where this doesn't
happen).

And, trying out this patch with SSH (where the client starts sending
data first), the reporter actually noticed we have a lower latency
by forcing an ACK right away. Comparing a capture before the patch:

13:07:14.007704 IP 10.1.2.1.42056 > 10.1.2.140.1234: Flags [S], seq 1797034836, win 65535, options [mss 4096,nop,wscale 7], length 0
13:07:14.007769 IP 10.1.2.140.1234 > 10.1.2.1.42056: Flags [S.], seq 2297052481, ack 1797034837, win 65480, options [mss 65480,nop,wscale 7], length 0
13:07:14.008462 IP 10.1.2.1.42056 > 10.1.2.140.1234: Flags [.], seq 1:22, ack 1, win 65535, length 21
13:07:14.008496 IP 10.1.2.140.1234 > 10.1.2.1.42056: Flags [.], ack 22, win 512, length 0
13:07:14.011799 IP 10.1.2.140.1234 > 10.1.2.1.42056: Flags [P.], seq 1:515, ack 22, win 512, length 514

and after:

13:10:26.165364 IP 10.1.2.1.59508 > 10.1.2.140.1234: Flags [S], seq 4165939595, win 65535, options [mss 4096,nop,wscale 7], length 0
13:10:26.165391 IP 10.1.2.140.1234 > 10.1.2.1.59508: Flags [S.], seq 985607380, ack 4165939596, win 65480, options [mss 65480,nop,wscale 7], length 0
13:10:26.165418 IP 10.1.2.1.59508 > 10.1.2.140.1234: Flags [.], ack 1, win 512, length 0
13:10:26.165683 IP 10.1.2.1.59508 > 10.1.2.140.1234: Flags [.], seq 1:22, ack 1, win 512, length 21
13:10:26.165698 IP 10.1.2.140.1234 > 10.1.2.1.59508: Flags [.], ack 22, win 512, length 0
13:10:26.167107 IP 10.1.2.140.1234 > 10.1.2.1.59508: Flags [P.], seq 1:515, ack 22, win 512, length 514

the latency between the initial SYN segment and the first data
transmission actually decreases from 792µs to 334µs. This is not
statistically relevant as we have a single measurement, but it can't
be that bad, either.

Reported-by: cr3bs (from IRC)
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

tcp.c

@@ -2517,7 +2517,7 @@ static void tcp_conn_from_sock_finish(struct ctx *c, struct tcp_tap_conn *conn,
 	 * dequeue waiting for SYN,ACK from tap -- check now.
 	 */
 	tcp_data_from_sock(c, conn);
-	tcp_send_flag(c, conn, ACK_IF_NEEDED);
+	tcp_send_flag(c, conn, ACK);
 }
 
 /**