net: fetch valid conn. to determine MSS in data_is_sent_and_acked()

Packets sent out through net_tx_fiber go through psock_send() where
they wait for data_is_sent_and_acked() to process them.
data_is_sent_and_acked() looks at the underlying connection's
MSS (maximum segment size) before putting them on the wire through
uip_send(). The trouble is that that linkage between the outgoing
buffer and the connection hasn't been established at the point
data_is_sent_and_acked() is called--this normally happens through
a call to uip_set_conn().
So data_is_sent_and_acked() fetches an invalid connection handle
and makes its choice using an arbitrary MSS. In my particular case,
this arbitrary value was 0, and so packets weren't being sent out.

Change-Id: I42e8ae104ac20f8df8780c8aee6964ed37113ba0
Signed-off-by: Rohit Grover <rohit.grover@arm.com>

include/net/net_context.h

@@ -20,3 +20,9 @@
  * limitations under the License.
  */
 
+#ifndef __NET_CONTEXT_H
+#define __NET_CONTEXT_H
+
+void *net_context_get_internal_connection(struct net_context *context);
+
+#endif /* #ifndef __NET_CONTEXT_H */

net/ip/contiki/ip/psock.c

@@ -35,6 +35,7 @@
 #include <string.h>
 
 #include <net/ip_buf.h>
+#include <net/net_context.h>
 
 #ifdef CONFIG_NETWORK_IP_STACK_DEBUG_TCP_PSOCK
 #define DEBUG 1
@@ -152,18 +153,24 @@ data_is_sent_and_acked(CC_REGISTER_ARG struct psock *s)
 	 s->sendptr, s->sendlen,
 	 uip_mss(s->net_buf));
 
+  struct uip_conn *conn = net_context_get_internal_connection(ip_buf_context(s->net_buf));
+  if (!conn) {
+        s->state = STATE_BLOCKED_SEND;
+        return 0;
+  }
+
   if(s->state != STATE_DATA_SENT || uip_rexmit(s->net_buf)) {
-    if(s->sendlen > uip_mss(s->net_buf)) {
-      uip_send(s->net_buf, s->sendptr, uip_mss(s->net_buf));
+    if(s->sendlen > conn->mss) {
+      uip_send(s->net_buf, s->sendptr, conn->mss);
     } else {
       uip_send(s->net_buf, s->sendptr, s->sendlen);
     }
     s->state = STATE_DATA_SENT;
     return 0;
   } else if(s->state == STATE_DATA_SENT && uip_acked(s->net_buf)) {
-    if(s->sendlen > uip_mss(s->net_buf)) {
-      s->sendlen -= uip_mss(s->net_buf);
-      s->sendptr += uip_mss(s->net_buf);
+    if(s->sendlen > conn->mss) {
+      s->sendlen -= conn->mss;
+      s->sendptr += conn->mss;
     } else {
       s->sendptr += s->sendlen;
       s->sendlen = 0;

net/ip/net_context.c

@@ -32,6 +32,7 @@
 
 #include <net/net_ip.h>
 #include <net/net_socket.h>
+#include <net/net_context.h>
 
 #include "contiki/ip/simple-udp.h"
 #include "contiki/ipv6/uip-ds6.h"