Library: Add compatibility for legacy Tokio 0.2 (#40)

Adds support to the library for tokio 0.2 backward-compatibility. This should hopefully benefit, and prevent lavalink-rs from being blocked on this feature.

These can be reached using, e.g., `gateway-tokio-02`, `driver-tokio-02`, `serenity-rustls-tokio-02`, and `serenity-native-tokio-02` features.

Naturally, this requires some jiggering about with features and the underlying CI, which has been taken care of. Twilight can't be handled in this way, as their last tokio 0.2 version uses the deprecated Discord Gateway v6.

src/driver/tasks/udp_rx.rs

@@ -21,7 +21,10 @@ use discortp::{
 };
 use flume::Receiver;
 use std::{collections::HashMap, sync::Arc};
-use tokio::net::UdpSocket;
+#[cfg(not(feature = "tokio-02-marker"))]
+use tokio::{net::UdpSocket, select};
+#[cfg(feature = "tokio-02-marker")]
+use tokio_compat::{net::udp::RecvHalf, select};
 use tracing::{error, info, instrument, warn};
 use xsalsa20poly1305::XSalsa20Poly1305 as Cipher;
 
@@ -236,14 +239,18 @@ struct UdpRx {
     config: Config,
     packet_buffer: [u8; VOICE_PACKET_MAX],
     rx: Receiver<UdpRxMessage>,
+
+    #[cfg(not(feature = "tokio-02-marker"))]
     udp_socket: Arc<UdpSocket>,
+    #[cfg(feature = "tokio-02-marker")]
+    udp_socket: RecvHalf,
 }
 
 impl UdpRx {
     #[instrument(skip(self))]
     async fn run(&mut self, interconnect: &mut Interconnect) {
         loop {
-            tokio::select! {
+            select! {
                 Ok((len, _addr)) = self.udp_socket.recv_from(&mut self.packet_buffer[..]) => {
                     self.process_udp_message(interconnect, len);
                 }
@@ -385,6 +392,7 @@ impl UdpRx {
     }
 }
 
+#[cfg(not(feature = "tokio-02-marker"))]
 #[instrument(skip(interconnect, rx, cipher))]
 pub(crate) async fn runner(
     mut interconnect: Interconnect,
@@ -409,6 +417,31 @@ pub(crate) async fn runner(
     info!("UDP receive handle stopped.");
 }
 
+#[cfg(feature = "tokio-02-marker")]
+#[instrument(skip(interconnect, rx, cipher))]
+pub(crate) async fn runner(
+    mut interconnect: Interconnect,
+    rx: Receiver<UdpRxMessage>,
+    cipher: Cipher,
+    config: Config,
+    udp_socket: RecvHalf,
+) {
+    info!("UDP receive handle started.");
+
+    let mut state = UdpRx {
+        cipher,
+        decoder_map: Default::default(),
+        config,
+        packet_buffer: [0u8; VOICE_PACKET_MAX],
+        rx,
+        udp_socket,
+    };
+
+    state.run(&mut interconnect).await;
+
+    info!("UDP receive handle stopped.");
+}
+
 #[inline]
 fn rtp_valid(packet: RtpPacket<'_>) -> bool {
     packet.get_version() == RTP_VERSION && packet.get_payload_type() == RTP_PROFILE_TYPE