UDP: Store local and use local address in metadata

Closes: smoltcp-rs#903

src/socket/udp.rs

@@ -8,20 +8,27 @@ use crate::socket::PollAt;
 #[cfg(feature = "async")]
 use crate::socket::WakerRegistration;
 use crate::storage::Empty;
-use crate::wire::{IpEndpoint, IpListenEndpoint, IpProtocol, IpRepr, UdpRepr};
+use crate::wire::{IpEndpoint, IpAddress, IpListenEndpoint, IpProtocol, IpRepr, UdpRepr};
 
 /// Metadata for a sent or received UDP packet.
 #[cfg_attr(feature = "defmt", derive(defmt::Format))]
 #[derive(Debug, PartialEq, Eq, Clone, Copy)]
 pub struct UdpMetadata {
     pub endpoint: IpEndpoint,
+    /// The IP address to which an incoming datagram was sent, or to which an outgoing datagram
+    /// will be sent. Incoming datagrams always have this set. On outgoing datagrams, if it is not
+    /// set, and the socket is not bound to a single address anyway, a suitable address will be
+    /// determined using the algorithms of RFC 6724 (candidate source address selection) or some
+    /// heuristic (for IPv4).
+    pub local_address: Option<IpAddress>,
     pub meta: PacketMeta,
 }
 
 impl<T: Into<IpEndpoint>> From<T> for UdpMetadata {
     fn from(value: T) -> Self {
         Self {
             endpoint: value.into(),
+            local_address: None,
             meta: PacketMeta::default(),
         }
     }
@@ -493,6 +500,7 @@ impl<'a> Socket<'a> {
 
         let metadata = UdpMetadata {
             endpoint: remote_endpoint,
+            local_address: Some(ip_repr.dst_addr()),
             meta,
         };
 
@@ -517,19 +525,23 @@ impl<'a> Socket<'a> {
         let hop_limit = self.hop_limit.unwrap_or(64);
 
         let res = self.tx_buffer.dequeue_with(|packet_meta, payload_buf| {
-            let src_addr = match endpoint.addr {
-                Some(addr) => addr,
-                None => match cx.get_source_address(&packet_meta.endpoint.addr) {
+            let src_addr = if let Some(s) = packet_meta.local_address {
+                s
+            } else {
+                match endpoint.addr {
                     Some(addr) => addr,
-                    None => {
-                        net_trace!(
-                            "udp:{}:{}: cannot find suitable source address, dropping.",
-                            endpoint,
-                            packet_meta.endpoint
-                        );
-                        return Ok(());
-                    }
-                },
+                    None => match cx.get_source_address(&packet_meta.endpoint.addr) {
+                        Some(addr) => addr,
+                        None => {
+                            net_trace!(
+                                "udp:{}:{}: cannot find suitable source address, dropping.",
+                                endpoint,
+                                packet_meta.endpoint
+                            );
+                            return Ok(());
+                        }
+                    },
+                }
             };
 
             net_trace!(