net: tcp: Correctly determine when the TCP transmit window is full

In the stack both unacked_len and send_data_total track the amount
of data for retransmission. send_data_total actually accounts the
total bytes in the buffer, where unacked_len is used to control the
retransmission progress.

Using unacked_len is sometimes reset to 0, this can lead to more data
being allowd in the send_data buffer. In worse case this can cause
depletion of the net buffers, causing a stall and crash of the connection.

The value send_data_total actually accounts the total amount of data in
the send_data buffer, so it is the proper value to used in the
tcp_window_full function.

Signed-off-by: Sjors Hettinga <s.a.hettinga@gmail.com>

subsys/net/ip/tcp.c

@@ -978,7 +978,7 @@ static int tcp_pkt_peek(struct net_pkt *to, struct net_pkt *from, size_t pos,
 
 static bool tcp_window_full(struct tcp *conn)
 {
-	bool window_full = !(conn->unacked_len < conn->send_win);
+	bool window_full = (conn->send_data_total >= conn->send_win);
 
 	NET_DBG("conn: %p window_full=%hu", conn, window_full);