Free wireguard_device after netif is removed

The wireguard_device stored as the netif state pointer must remain valid until
after the netif is removed.  During removal, packets may be sent to disconnect
TCP sockets and such requiring a valid device for the wireguardif_output
function.

Move the device free from the wireguardif_shutdown function into a new
wireguardif_fini function which is called after the netif_remove.  Add a device
check in wireguardif_output as a precaution.

Make an explicit call to clear the IPv4 address of the Wireguard interface when
disconnecting.  The call mimics the behavior in netif_remove however it is
performed before the Wireguard peers are shutdown.  As a result, the TCP reset
packets can be sent.

src/esp_wireguard.c

@@ -299,6 +299,10 @@ esp_err_t esp_wireguard_disconnect(wireguard_ctx_t *ctx)
         goto fail;
     }
 
+    // Clear the IP address to gracefully disconnect any clients while the
+    // peers are still valid
+    netif_set_ipaddr(ctx->netif, IP4_ADDR_ANY4);
+
     lwip_err = wireguardif_disconnect(ctx->netif, wireguard_peer_index);
     if (lwip_err != ERR_OK) {
         ESP_LOGW(TAG, "wireguardif_disconnect: peer_index: %" PRIu8 " err: %i", wireguard_peer_index, lwip_err);
@@ -312,6 +316,7 @@ esp_err_t esp_wireguard_disconnect(wireguard_ctx_t *ctx)
     wireguard_peer_index = WIREGUARDIF_INVALID_INDEX;
     wireguardif_shutdown(ctx->netif);
     netif_remove(ctx->netif);
+    wireguardif_fini(ctx->netif);
     netif_set_default(ctx->netif_default);
     ctx->netif = NULL;
 

src/wireguardif.c

@@ -198,8 +198,14 @@ static err_t wireguardif_output_to_peer(struct netif *netif, struct pbuf *q, con
 // The ipaddr here is the one inside the VPN which we use to lookup the correct peer/endpoint
 static err_t wireguardif_output(struct netif *netif, struct pbuf *q, const ip4_addr_t *ip4addr) {
 	struct wireguard_device *device = (struct wireguard_device *)netif->state;
-	// Send to peer that matches dest IP
 	ip_addr_t ipaddr;
+
+	if (!device) {
+		ESP_LOGE(TAG, "wireguardif_output NULL device");
+		return ERR_RTE;
+	}
+
+	// Send to peer that matches dest IP
 	ip_addr_copy_from_ip4(ipaddr, *ip4addr);
 	struct wireguard_peer *peer = peer_lookup_by_allowed_ip(device, &ipaddr);
 	if (peer) {
@@ -1037,6 +1043,14 @@ void wireguardif_shutdown(struct netif *netif) {
 		udp_remove(device->udp_pcb);
 		device->udp_pcb = NULL;
 	}
+}
+
+void wireguardif_fini(struct netif *netif) {
+	LWIP_ASSERT("netif != NULL", (netif != NULL));
+	LWIP_ASSERT("state != NULL", (netif->state != NULL));
+
+	struct wireguard_device *device = (struct wireguard_device *)netif->state;
+
 	// remove device context.
 	free(device);
 	netif->state = NULL;

src/wireguardif.h

@@ -131,6 +131,9 @@ err_t wireguardif_disconnect(struct netif *netif, u8_t peer_index);
 // Shutdown the WireGuard interface
 void wireguardif_shutdown(struct netif *netif);
 
+// Finalize the WireGuard interface after the netif is removed
+void wireguardif_fini(struct netif *netif);
+
 // Is the given peer "up"? A peer is up if it has a valid session key it can communicate with
 err_t wireguardif_peer_is_up(struct netif *netif, u8_t peer_index, ip_addr_t *current_ip, u16_t *current_port);