tcp: Implement conservative zero-window probe on ACK timeout

This probably doesn't cover all the cases where we should send a
zero-window probe, but it's rather unobtrusive and obvious, so start
from here, also because I just observed this case (without the fix
from the previous patch, to take into account window information from
keep-alive segments).

If we hit the ACK timeout, and try re-sending data from the socket,
if the window is zero, we'll just fail again, go back to the timer,
and so on, until we hit the maximum number of re-transmissions and
reset the connection.

Don't do that: forcibly try to send something by implementing the
equivalent of a zero-window probe in this case.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

tcp.c

@@ -2175,6 +2175,8 @@ void tcp_timer_handler(const struct ctx *c, union epoll_ref ref)
 			flow_dbg(conn, "ACK timeout, retry");
 			conn->retrans++;
 			conn->seq_to_tap = conn->seq_ack_from_tap;
+			if (!conn->wnd_from_tap)
+				conn->wnd_from_tap = 1; /* Zero-window probe */
 			if (tcp_set_peek_offset(conn->sock, 0)) {
 				tcp_rst(c, conn);
 			} else {