bgpd: Reset BGP session only if it was a real BFD DOWN event

Without this patch we always see a double-reset, e.g.:

```
2024/11/04 12:42:43.010 BGP: [VQY9X-CQZKG] bgp_peer_bfd_update_source: address [0.0.0.0->172.18.0.3] to [172.18.0.2->172.18.0.3]
2024/11/04 12:42:43.010 BGP: [X8BD9-8RKN4] bgp_peer_bfd_update_source: interface none to eth0
2024/11/04 12:42:43.010 BFD: [MSVDW-Y8Z5Q] ptm-del-dest: deregister peer [mhop:no peer:172.18.0.3 local:0.0.0.0 vrf:default cbit:0x00 minimum-ttl:255]
2024/11/04 12:42:43.010 BFD: [NYF5K-SE3NS] ptm-del-session: [mhop:no peer:172.18.0.3 local:0.0.0.0 vrf:default] refcount=0
2024/11/04 12:42:43.010 BFD: [NW21R-MRYNT] session-delete: mhop:no peer:172.18.0.3 local:0.0.0.0 vrf:default
2024/11/04 12:42:43.010 BGP: [P3D3N-3277A] 172.18.0.3 [FSM] Timer (routeadv timer expire)
2024/11/04 12:42:43.010 BFD: [YA0Q5-C0BPV] control-packet: no session found [mhop:no peer:172.18.0.3 local:172.18.0.2 port:11]
2024/11/04 12:42:43.010 BFD: [MSVDW-Y8Z5Q] ptm-add-dest: register peer [mhop:no peer:172.18.0.3 local:172.18.0.2 vrf:default cbit:0x00 minimum-ttl:255]
2024/11/04 12:42:43.011 BFD: [PSB4R-8T1TJ] session-new: mhop:no peer:172.18.0.3 local:172.18.0.2 vrf:default ifname:eth0
2024/11/04 12:42:43.011 BGP: [Q4BCV-6FHZ5] zclient_bfd_session_update: 172.18.0.2/32 -> 172.18.0.3/32 (interface eth0) VRF default(0) (CPI bit no): Down
2024/11/04 12:42:43.011 BGP: [MKVHZ-7MS3V] bfd_session_status_update: neighbor 172.18.0.3 vrf default(0) bfd state Up -> Down
2024/11/04 12:42:43.011 BGP: [HZN6M-XRM1G] %NOTIFICATION: sent to neighbor 172.18.0.3 6/10 (Cease/BFD Down) 0 bytes
2024/11/04 12:42:43.011 BGP: [QFMSE-NPSNN] zclient_bfd_session_update:   sessions updated: 1
2024/11/04 12:42:43.011 BGP: [ZWCSR-M7FG9] 172.18.0.3 [FSM] BGP_Stop (Established->Clearing), fd 22
```

Reset is due to the source address change.

With this patch, we reset the session only if it's a _REAL_ BFD down event, which
means we trigger session reset if BFD session is established earlier than BGP.

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>

bgpd/bgp_bfd.c

@@ -53,14 +53,23 @@ static void bfd_session_status_update(struct bfd_session_params *bsp,
 					peer->host);
 			return;
 		}
-		peer->last_reset = PEER_DOWN_BFD_DOWN;
 
-		/* rfc9384 */
-		if (BGP_IS_VALID_STATE_FOR_NOTIF(peer->connection->status))
-			bgp_notify_send(peer->connection, BGP_NOTIFY_CEASE,
-					BGP_NOTIFY_CEASE_BFD_DOWN);
-
-		BGP_EVENT_ADD(peer->connection, BGP_Stop);
+		/* Once the BFD session is UP, and later BGP session is UP,
+		 * BFD notices that peer->su_local changed, and BFD session goes down.
+		 * We should trigger BGP session reset if BFD session is UP
+		 * only when BGP session is UP already.
+		 * Otherwise, we end up resetting BGP session when BFD session is UP,
+		 * when the source address is changed, e.g. 0.0.0.0 -> 10.0.0.1.
+		 */
+		if (bss->last_event > peer->uptime) {
+			peer->last_reset = PEER_DOWN_BFD_DOWN;
+			/* rfc9384 */
+			if (BGP_IS_VALID_STATE_FOR_NOTIF(peer->connection->status))
+				bgp_notify_send(peer->connection, BGP_NOTIFY_CEASE,
+						BGP_NOTIFY_CEASE_BFD_DOWN);
+
+			BGP_EVENT_ADD(peer->connection, BGP_Stop);
+		}
 	}
 
 	if (bss->state == BSS_UP && bss->previous_state != BSS_UP &&