[ENet] Better handle disconnected peers in DTLS server

The ENetDTLSServer socket implementation should avoid reporting errors
during send and receive operations, unless the socket is effectively no
longer usable (and thus ENet should close it).

thirdparty/enet/godot.cpp

@@ -299,7 +299,12 @@ public:
 
 	Error sendto(const uint8_t *p_buffer, int p_len, int &r_sent, IPAddress p_ip, uint16_t p_port) {
 		String key = String(p_ip) + ":" + itos(p_port);
-		ERR_FAIL_COND_V(!peers.has(key), ERR_UNAVAILABLE);
+		if (unlikely(!peers.has(key))) {
+			// The peer might have been disconnected due to a DTLS error.
+			// We need to wait for it to time out, just mark the packet as sent.
+			r_sent = p_len;
+			return OK;
+		}
 		Ref<PacketPeerDTLS> peer = peers[key];
 		Error err = peer->put_packet(p_buffer, p_len);
 		if (err == OK) {
@@ -307,7 +312,10 @@ public:
 		} else if (err == ERR_BUSY) {
 			r_sent = 0;
 		} else {
-			r_sent = -1;
+			// The peer might have been disconnected due to a DTLS error.
+			// We need to wait for it to time out, just mark the packet as sent.
+			r_sent = p_len;
+			return OK;
 		}
 		return err;
 	}
@@ -331,7 +339,7 @@ public:
 		Error err = ERR_BUSY;
 		// TODO this needs to be fair!
 
-		for (KeyValue<String, Ref<PacketPeerDTLS>> & E : peers) {
+		for (KeyValue<String, Ref<PacketPeerDTLS>> &E : peers) {
 			Ref<PacketPeerDTLS> peer = E.value;
 			peer->poll();
 
@@ -349,7 +357,8 @@ public:
 				if (err != OK || p_len < r_read) {
 					// Something wrong with this peer, removing it.
 					remove.push_back(E.key);
-					err = FAILED;
+					err = ERR_BUSY;
+					r_read = 0;
 					continue;
 				}
 
@@ -549,7 +558,7 @@ int enet_socket_receive(ENetSocket socket, ENetAddress *address, ENetBuffer *buf
 	return read;
 }
 
-int enet_socket_get_address (ENetSocket socket, ENetAddress * address) {
+int enet_socket_get_address(ENetSocket socket, ENetAddress *address) {
 	IPAddress ip;
 	uint16_t port;
 	ENetGodotSocket *sock = (ENetGodotSocket *)socket;