library/GSM_SystemInformation: add dec_SystemInformationSafe()

Some types of System Information (mostly the Rest Octets) are not
fully implemented, so calling the generic dec_SystemInformation()
may result in a DTE.  Let's add dec_SystemInformationSafeBT() with
"prototype(backtrack)", so it would return a non-zero integer if
decoding fails.  Let's add a wrapper dec_SystemInformationSafe()
that would additionally check the RR Protocol Discriminator.

Change-Id: Id4d73e0f3347e1d4c4c77aec75b767311d662292
Related: OS#4662

bts/BTS_Tests.ttcn

@@ -3534,7 +3534,6 @@ private function f_TC_imm_ass(integer num_total, float sleep_s, float exp_pass)
 		repeat;
 		}
 	[] L1CTL.receive(tr_L1CTL_DATA_IND(t_RslChanNr_PCH_AGCH(0), ?)) -> value l1_dl {
-		/* somehow dec_SystemInformation will try to decode even non-RR as SI */
 		var GsmRrMessage rr := dec_GsmRrMessage(l1_dl.payload.data_ind.payload);
 		if (not match(rr, tr_IMM_ASS(42, ?, 5, ?, ?))) {
 			/* FIXME: Why are we seeing paging requests on PCH/AGCH? */
@@ -3936,15 +3935,12 @@ return SystemInformationVectorPerTc {
 	T.start;
 	alt {
 	[] pt.receive(tr_L1CTL_DATA_IND(t_RslChanNr_BCCH(0), ?)) -> value l1_dl {
-		/* somehow dec_SystemInformation will try to decode even non-RR as SI */
-		if (not (l1_dl.payload.data_ind.payload[1] ==  '06'O)) {
-			log("Ignoring non-RR SI ", l1_dl);
+		var SystemInformationFn sig := { frame_number := l1_dl.dl_info.frame_nr };
+		if (dec_SystemInformationSafe(l1_dl.payload.data_ind.payload, sig.si) != 0) {
+			log("Ignoring non-RR or invalid SI ", l1_dl);
 			repeat;
 		}
-		var SystemInformationFn sig := {
-			frame_number := l1_dl.dl_info.frame_nr,
-			si := dec_SystemInformation(l1_dl.payload.data_ind.payload)
-		}
+
 		var integer tc := f_gsm_compute_tc(sig.frame_number);
 		log("SI received at TC=", tc, ": ", sig.si);
 		/* append to the per-TC bucket */
@@ -5227,17 +5223,13 @@ private function f_get_si(L1CTL_PT pt, RrMessageType si_type)
 runs on test_CT return SystemInformation {
 	var L1ctlDlMessage l1_dl;
 	var SystemInformation si;
+	var integer rc;
 	timer T := 5.0;
 	T.start;
 	alt {
 	[] pt.receive(tr_L1CTL_DATA_IND(t_RslChanNr_BCCH(0), ?)) -> value l1_dl {
-		/* somehow dec_SystemInformation will try to decode even non-RR as SI */
-		if (not (l1_dl.payload.data_ind.payload[1] ==  '06'O)) {
-			log("Ignoring non-RR SI ", l1_dl);
-			repeat;
-		}
-		si := dec_SystemInformation(l1_dl.payload.data_ind.payload)
-		if (si.header.message_type != si_type) {
+		rc := dec_SystemInformationSafe(l1_dl.payload.data_ind.payload, si);
+		if (rc != 0 or si.header.message_type != si_type) {
 			repeat;
 		}
 		}

library/GSM_SystemInformation.ttcn

@@ -264,4 +264,29 @@ module GSM_SystemInformation {
 		}
 	}
 
+	external function dec_SystemInformationSafeBT(in octetstring stream, out SystemInformation si)
+		return integer /* Decoding result: successful (0) or unsuccessful (1) */
+		with { extension "prototype(backtrack) decode(RAW)" };
+
+	/* Some types of System Information (mostly the Rest Octets) are not fully implemented,
+	 * so calling the generic dec_SystemInformation() may result in a DTE.  This function
+	 * additionally checks RR Protocol Discriminator, and should be used in the most cases. */
+	function dec_SystemInformationSafe(in octetstring stream, out SystemInformation si)
+	return integer {
+		/* Try to decode a given octetstring as System Information */
+		if (dec_SystemInformationSafeBT(stream, si) != 0) {
+			log("Failed to decode (RR) System Information: ", stream);
+			return 1;
+		}
+
+		/* Check the protocol discriminator (we expect RR messages) */
+		if (si.header.rr_protocol_discriminator != bit2int('0110'B)) {
+			log("Protocol discriminator is not RR (!= '0110'B): ",
+			    si.header.rr_protocol_discriminator);
+			return 1;
+		}
+
+		return 0;
+	}
+
 } with { encode "RAW"; variant "FIELDORDER(msb)" }