stp: pass a topology change on only from a forwarding port

stp_in() propagated the TC flag of any BPDU that did not arrive on an
alternate port, and acknowledged and propagated a TCN from any port, so a
port still in its listen period or held discarding in a dispute could flush
the addresses of every other port and move the topology change counter.
802.1D-2004 17.31 keeps the Topology Change machine in LEARNING until the
port forwards and drops rcvdTc and rcvdTcn there. Both paths now ask for
the forwarding state first; an alternate port is discarding, so the old
check is covered by the new one.

Scenario 53 plays a TC flag and a TCN into a port that is still discarding,
and then into the same port once it forwards. Scenario 51 used a TCN on a
port one tick short of its edge transition as the trigger for the clock it
tests; it now waits for the transition.

doc/stp.md

@@ -154,9 +154,10 @@ BPDUs toward the root, so the change reaches the rest of the network. With
 `stp version stp` the root port sends TCN instead, until a Config BPDU with
 TCA comes back.
 
-A TC flag received on a root or designated port is passed on the same way to
-the other ports, not back to the port it came in on. A legacy TCN is
-acknowledged with TCA and then treated like a received TC. A port that turns
+A TC flag received on a forwarding root or designated port is passed on the
+same way to the other ports, not back to the port it came in on; a port that
+is still discarding or learning drops it. A legacy TCN is acknowledged with
+TCA and then treated like a received TC. A port that turns
 alternate, loses its link, or is blocked for a loop flushes its own addresses
 without raising a topology change, and an edge port never raises one.
 

rtl837x_stp.c

@@ -1018,6 +1018,8 @@ void stp_in(void) __banked
 		return;
 
 	if (STP_I->bpdu_type == BPDU_TYPE_TCN) {
+		if (stp_state_get(port) != 0b11)
+			return;
 		stp_tx_flags_extra = BPDU_FLAG_TCACK;
 		stp_cnf_send(port);
 		uip_len = 0;
@@ -1063,7 +1065,7 @@ void stp_in(void) __banked
 
 	if ((STP_I->flags & BPDU_FLAG_TCACK) && port == stp_root_port)
 		stp_tcwhile[port] = 0;
-	if ((STP_I->flags & BPDU_FLAG_TC) && !((stp_alt >> port) & 1) && stp_tc_prop(port))
+	if ((STP_I->flags & BPDU_FLAG_TC) && stp_state_get(port) == 0b11 && stp_tc_prop(port))
 		stp_tc_count++;
 
 	if ((stp_pflags[port] & STP_PF_ROOTGUARD)

test/stp/stp_sim.c

@@ -1988,7 +1988,7 @@ static void scen_last_tc(void)
 	links_set(1 << 8);
 	tcn = root_on(8);
 	tcn.legacy = 2;
-	secs(3);
+	secs(4);
 	before = stp_tc_count;
 	bpdu_in(&tcn);
 	check(stp_tc_count != before, "a TCN moves the topology change counter");
@@ -2062,6 +2062,57 @@ static void scen_lag(void)
 	check(port_state(1) == 1 && port_state(2) == 1, "the ports start over discarding");
 }
 
+static void scen_tc_needs_forwarding(void)
+{
+	struct sim_bpdu r;
+	struct sim_bpdu worse = {
+		.port = 2, .root_prio = 0x40, .root_cost = 200000, .br_prio = 0x80, .port_id = 4, .flags = 0x3d
+	};
+	struct sim_bpdu tcn;
+	int tc;
+
+	printf("53. a topology change heard on a port that does not forward yet is not passed on\n");
+	reset_all();
+	stp_pflags[2] &= ~STP_PF_AUTOEDGE;
+	links_set((1 << 0) | (1 << 2));
+	secs(1);
+	r = root_on(0);
+	for (int i = 0; i < 3; i++) {
+		bpdu_in(&r);
+		secs(2);
+	}
+	check_state(0, 3, "root port 1 forwards");
+	check_state(2, 1, "port 3 is still discarding");
+	memcpy(worse.root_mac, ROOT_MAC, 6);
+	memcpy(worse.br_mac, PEER_MAC, 6);
+	memset(flush_count, 0, sizeof(flush_count));
+	tc = stp_tc_count;
+	bpdu_in(&worse);
+	tick(2);
+	check(stp_tc_count == tc, "a TC flag on the discarding port raises no topology change");
+	check(flush_count[0] == 0, "and the root port keeps its addresses");
+	tcn = worse;
+	tcn.legacy = 2;
+	memset(last_tx, 0, sizeof(last_tx));
+	bpdu_in(&tcn);
+	tick(2);
+	check(stp_tc_count == tc, "a TCN on the discarding port raises none either");
+	check((last_tx[2].flags & 0x80) == 0, "and is not acknowledged");
+	for (int i = 0; i < 16; i++) {
+		bpdu_in(&r);
+		secs(2);
+	}
+	check_state(2, 3, "port 3 forwards after both forward delays");
+	for (int i = 0; i < 2; i++) {
+		bpdu_in(&r);
+		secs(2);
+	}
+	tc = stp_tc_count;
+	bpdu_in(&worse);
+	tick(2);
+	check(stp_tc_count == tc + 1 && flush_count[0] > 0, "now the same TC is passed on");
+}
+
 int main(int argc, char **argv)
 {
 	verbose = argc > 1 && argv[1][0] == '-' ? (argv[1][1] == 'd' ? 2 : 1) : 0;
@@ -2117,6 +2168,7 @@ int main(int argc, char **argv)
 	scen_bpdu_handling();
 	scen_last_tc();
 	scen_lag();
+	scen_tc_needs_forwarding();
 	if (failures) {
 		printf("\n%d check(s) failed\n", failures);
 		return 1;