[tcp] Use a dedicated timer for the TIME_WAIT state

iPXE currently repurposes the retransmission timer to hold the TCP
connection in the TIME_WAIT state (i.e. waiting for up to 2*MSL in
case we are required to re-ACK our peer's FIN due to a lost ACK).
However, the fact that this timer is running will prevent such an ACK
from ever being sent, since the logic in tcp_xmit() assumes that a
running timer indicates that we ourselves are waiting for an ACK and
so blocks the transmission.  (We always wait for an ACK before sending
our next packet, to keep our transmit data path as simple as
possible.)

Fix by using an entirely separate timer for the TIME_WAIT state, so
that packets can still be sent.

Reported-by: Piotr Jaroszyński <p.jaroszynski@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/tcp.c

 	struct list_head queue;
 	/** Retransmission timer */
 	struct retry_timer timer;
+	/** Shutdown (TIME_WAIT) timer */
+	struct retry_timer wait;
 };
 
 /**
 /* Forward declarations */
 static struct interface_descriptor tcp_xfer_desc;
 static void tcp_expired ( struct retry_timer *timer, int over );
+static void tcp_wait_expired ( struct retry_timer *timer, int over );
 static int tcp_rx_ack ( struct tcp_connection *tcp, uint32_t ack,
 			uint32_t win );
 
 	ref_init ( &tcp->refcnt, NULL );
 	intf_init ( &tcp->xfer, &tcp_xfer_desc, &tcp->refcnt );
 	timer_init ( &tcp->timer, tcp_expired );
+	timer_init ( &tcp->wait, tcp_wait_expired );
 	tcp->prev_tcp_state = TCP_CLOSED;
 	tcp->tcp_state = TCP_STATE_SENT ( TCP_SYN );
 	tcp_dump_state ( tcp );
 /**
  * Retransmission timer expired
  *
- * @v timer	Retry timer
- * @v over	Failure indicator
+ * @v timer		Retransmission timer
+ * @v over		Failure indicator
  */
 static void tcp_expired ( struct retry_timer *timer, int over ) {
 	struct tcp_connection *tcp =
 		container_of ( timer, struct tcp_connection, timer );
-	int graceful_close = TCP_CLOSED_GRACEFULLY ( tcp->tcp_state );
 
 	DBGC ( tcp, "TCP %p timer %s in %s for %08x..%08x %08x\n", tcp,
 	       ( over ? "expired" : "fired" ), tcp_state ( tcp->tcp_state ),
 		 ( tcp->tcp_state == TCP_SYN_RCVD ) ||
 		 ( tcp->tcp_state == TCP_ESTABLISHED ) ||
 		 ( tcp->tcp_state == TCP_FIN_WAIT_1 ) ||
-		 ( tcp->tcp_state == TCP_TIME_WAIT ) ||
 		 ( tcp->tcp_state == TCP_CLOSE_WAIT ) ||
 		 ( tcp->tcp_state == TCP_CLOSING_OR_LAST_ACK ) );
 
-	if ( over || graceful_close ) {
-		/* If we have finally timed out and given up, or if
-		 * this is the result of a graceful close, terminate
-		 * the connection
+	if ( over ) {
+		/* If we have finally timed out and given up,
+		 * terminate the connection
 		 */
 		tcp->tcp_state = TCP_CLOSED;
 		tcp_dump_state ( tcp );
 	}
 }
 
+/**
+ * Shutdown timer expired
+ *
+ * @v timer		Shutdown timer
+ * @v over		Failure indicator
+ */
+static void tcp_wait_expired ( struct retry_timer *timer, int over __unused ) {
+	struct tcp_connection *tcp =
+		container_of ( timer, struct tcp_connection, wait );
+
+	assert ( tcp->tcp_state == TCP_TIME_WAIT );
+
+	DBGC ( tcp, "TCP %p wait complete in %s for %08x..%08x %08x\n", tcp,
+	       tcp_state ( tcp->tcp_state ), tcp->snd_seq,
+	       ( tcp->snd_seq + tcp->snd_sent ), tcp->rcv_ack );
+
+	tcp->tcp_state = TCP_CLOSED;
+	tcp_dump_state ( tcp );
+	tcp_close ( tcp, 0 );
+}
+
 /**
  * Send RST response to incoming packet
  *
 	 * timer to expire and cause the connection to be freed.
 	 */
 	if ( TCP_CLOSED_GRACEFULLY ( tcp->tcp_state ) ) {
-		start_timer_fixed ( &tcp->timer, ( 2 * TCP_MSL ) );
+		stop_timer ( &tcp->wait );
+		start_timer_fixed ( &tcp->wait, ( 2 * TCP_MSL ) );
 	}
 
 	return 0;