Close conn when receiving SCTP_ASSOC_CHANGE notification

It was seen on a real pcap trace (sctp & gsmtap_log) that the kernel
stack may decide to kill the connection (sending an ABORT) if it fails
to transmit some data after a while:
ABORT Cause code: "Protocol violation (0x000d)",
      Cause Information: "Association exceeded its max_retrans count".
When this occurs, the kernel sends the
MSG_NOTIFICATION,SCTP_ASSOC_CHANGE,SCTP_COMM_LOST notification when
reading from the socket with sctp_recvmsg(). This basically signals that
the socket conn is dead, and subsequent writes to it will result in
send() failures (and receive SCTP_SEND_FAILED notification upon follow
up reads).
It's important to notice that after those events, there's no other sort
of different event like SHUTDOWN coming in, so that's the time at which
we must tell the user to close the socket.
Hence, let's signal the caller that the socket is dead by returning 0,
to comply with usual recv() API.

Related: SYS#6113
Change-Id: If35efd404405f926a4a6cc45862eeadd1b04e08c

src/osmo-hnbgw/hnbgw.c

@@ -258,6 +258,13 @@ static int hnb_read_cb(struct osmo_stream_srv *conn)
 		switch (notif->sn_header.sn_type) {
 		case SCTP_ASSOC_CHANGE:
 			switch (notif->sn_assoc_change.sac_state) {
+			case SCTP_COMM_LOST:
+				LOGHNB(hnb, DMAIN, LOGL_NOTICE,
+				       "sctp_recvmsg(%s) = SCTP_COMM_LOST, closing conn\n",
+				       osmo_sock_get_name2(osmo_stream_srv_get_ofd(conn)->fd));
+				osmo_stream_srv_destroy(conn);
+				rc = -1;
+				break;
 			case SCTP_RESTART:
 				LOGHNB(hnb, DMAIN, LOGL_NOTICE, "HNB SCTP conn RESTARTed, marking as HNBAP-unregistered\n");
 				hnb->hnb_registered = false;