feat(client): add receive datagram abstraction

crates/irtt-client/src/receive.rs

@@ -0,0 +1,76 @@
+use std::{io, net::UdpSocket};
+
+use crate::{metadata::ReceiveMeta, timing::ClientTimestamp};
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub(crate) struct ReceivedDatagram {
+    pub(crate) len: usize,
+    pub(crate) received_at: ClientTimestamp,
+    pub(crate) meta: ReceiveMeta,
+}
+
+pub(crate) fn recv_datagram(
+    socket: &UdpSocket,
+    buf: &mut [u8],
+) -> Result<ReceivedDatagram, io::Error> {
+    let len = socket.recv(buf)?;
+    let received_at = ClientTimestamp::now();
+
+    Ok(ReceivedDatagram {
+        len,
+        received_at,
+        meta: ReceiveMeta::default(),
+    })
+}
+
+#[cfg(test)]
+mod tests {
+    use std::net::UdpSocket;
+
+    use crate::{metadata::ReceiveMeta, receive::recv_datagram, timing::ClientTimestamp};
+
+    fn connected_loopback_pair() -> (UdpSocket, UdpSocket) {
+        let a = UdpSocket::bind("127.0.0.1:0").unwrap();
+        let b = UdpSocket::bind("127.0.0.1:0").unwrap();
+        a.connect(b.local_addr().unwrap()).unwrap();
+        b.connect(a.local_addr().unwrap()).unwrap();
+        (a, b)
+    }
+
+    #[test]
+    fn fallback_receive_returns_length() {
+        let (sender, receiver) = connected_loopback_pair();
+        sender.send(b"hello").unwrap();
+
+        let mut buf = [0_u8; 16];
+        let datagram = recv_datagram(&receiver, &mut buf).unwrap();
+
+        assert_eq!(datagram.len, 5);
+        assert_eq!(&buf[..datagram.len], b"hello");
+    }
+
+    #[test]
+    fn fallback_receive_returns_default_metadata() {
+        let (sender, receiver) = connected_loopback_pair();
+        sender.send(b"meta").unwrap();
+
+        let mut buf = [0_u8; 16];
+        let datagram = recv_datagram(&receiver, &mut buf).unwrap();
+
+        assert_eq!(datagram.meta, ReceiveMeta::default());
+    }
+
+    #[test]
+    fn fallback_receive_captures_timestamp_after_successful_receive() {
+        let (sender, receiver) = connected_loopback_pair();
+        sender.send(b"time").unwrap();
+
+        let before = ClientTimestamp::now();
+        let mut buf = [0_u8; 16];
+        let datagram = recv_datagram(&receiver, &mut buf).unwrap();
+        let after = ClientTimestamp::now();
+
+        assert!(datagram.received_at.mono >= before.mono);
+        assert!(datagram.received_at.mono <= after.mono);
+    }
+}