slirp: handle deferred ECONNREFUSED on non-blocking TCP sockets

slirp currently only handles ECONNREFUSED in the case where connect()
returns immediately with that error; since we use non-blocking sockets,
most of the time we won't receive the error until we later try to read
from the socket.  Ensure that we deliver the appropriate RST to the
guest in this case.

Signed-off-by: Steven Luo <steven+qemu@steven676.net>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>

slirp/socket.c

@@ -188,7 +188,7 @@ soread(struct socket *so)
 			DEBUG_MISC((dfd, " --- soread() disconnected, nn = %d, errno = %d-%s\n", nn, errno,strerror(errno)));
 			sofcantrcvmore(so);
 
-			if (err == ECONNRESET
+			if (err == ECONNRESET || err == ECONNREFUSED
 			    || err == ENOTCONN || err == EPIPE) {
 				tcp_drop(sototcpcb(so), err);
 			} else {

slirp/tcp_input.c

@@ -725,6 +725,12 @@ tcp_input(struct mbuf *m, int iphlen, struct socket *inso, unsigned short af)
 	    so->so_ti = ti;
 	    tp->t_timer[TCPT_KEEP] = TCPTV_KEEP_INIT;
 	    tp->t_state = TCPS_SYN_RECEIVED;
+	    /*
+	     * Initialize receive sequence numbers now so that we can send a
+	     * valid RST if the remote end rejects our connection.
+	     */
+	    tp->irs = ti->ti_seq;
+	    tcp_rcvseqinit(tp);
 	    tcp_template(tp);
 	  }
 	  return;