Driver: Split receive into its own feature (#141)

Adds the "receive" feature, which is disabled by default. When this is disabled, the UDP receive task is not compiled and not run, and as an optimisation the UDP receive buffer size is set to 0. All related events are also removed.

This also removes the UDP Tx task, and moves packet and keepalive sends back into the mixer thread. This allows us to entirely remove channels and various allocations between the mixer and an async task created only for sending data (i.e., fewer memcopies).

If "receive" is enabled, UDP sends are now non-blocking due to technical constraints -- failure to send is non-fatal, but *will* drop affected packets. Given that blocking on a UDP send indicates that the OS cannot clear send buffers fast enough, this should alleviate OS load.

Closes #131.

benches/mixing-task.rs

@@ -25,7 +25,7 @@ use songbird::{
     tracks,
     Config,
 };
-use std::io::Cursor;
+use std::{io::Cursor, net::UdpSocket};
 use tokio::runtime::{Handle, Runtime};
 use xsalsa20poly1305::{aead::NewAead, XSalsa20Poly1305 as Cipher, KEY_SIZE};
 
@@ -41,14 +41,12 @@ fn dummied_mixer(
         Receiver<CoreMessage>,
         Receiver<EventMessage>,
         Receiver<UdpRxMessage>,
-        Receiver<UdpTxMessage>,
     ),
 ) {
     let (mix_tx, mix_rx) = flume::unbounded();
     let (core_tx, core_rx) = flume::unbounded();
     let (event_tx, event_rx) = flume::unbounded();
 
-    let (udp_sender_tx, udp_sender_rx) = flume::unbounded();
     let (udp_receiver_tx, udp_receiver_rx) = flume::unbounded();
 
     let ic = Interconnect {
@@ -61,18 +59,23 @@ fn dummied_mixer(
 
     let mut out = Mixer::new(mix_rx, handle, ic, config);
 
+    let udp_tx = UdpSocket::bind("0.0.0.0:0").expect("Failed to create send port.");
+    udp_tx
+        .connect("127.0.0.1:5316")
+        .expect("Failed to connect to local dest port.");
+
     let fake_conn = MixerConnection {
         cipher: Cipher::new_from_slice(&vec![0u8; KEY_SIZE]).unwrap(),
         crypto_state: CryptoState::Normal,
         udp_rx: udp_receiver_tx,
-        udp_tx: udp_sender_tx,
+        udp_tx,
     };
 
     out.conn_active = Some(fake_conn);
 
     out.skip_sleep = true;
 
-    (out, (core_rx, event_rx, udp_receiver_rx, udp_sender_rx))
+    (out, (core_rx, event_rx, udp_receiver_rx))
 }
 
 fn mixer_float(
@@ -85,7 +88,6 @@ fn mixer_float(
         Receiver<CoreMessage>,
         Receiver<EventMessage>,
         Receiver<UdpRxMessage>,
-        Receiver<UdpTxMessage>,
     ),
 ) {
     let mut out = dummied_mixer(handle, softclip);
@@ -115,7 +117,6 @@ fn mixer_float_drop(
         Receiver<CoreMessage>,
         Receiver<EventMessage>,
         Receiver<UdpRxMessage>,
-        Receiver<UdpTxMessage>,
     ),
 ) {
     let mut out = dummied_mixer(handle, true);
@@ -143,7 +144,6 @@ fn mixer_opus(
         Receiver<CoreMessage>,
         Receiver<EventMessage>,
         Receiver<UdpRxMessage>,
-        Receiver<UdpTxMessage>,
     ),
 ) {
     // should add a single opus-based track.