Keep dispatching packets from a socket as long as there are any.

Typically, the poll function is used as a part of a larger RTOS.
If we only dispatch one packet per socket per poll() call,
then we have to wait for a complete scheduler roundtrip,
which is potentially a lot of time, and as a result we are
not filling the peer's window efficiently.

src/iface/ethernet.rs

@@ -165,36 +165,38 @@ impl<'a, 'b, 'c, DeviceT: Device + 'a> Interface<'a, 'b, 'c, DeviceT> {
         let mut limits = self.device.limits();
         limits.max_transmission_unit -= EthernetFrame::<&[u8]>::header_len();
 
-        for socket in sockets.iter_mut() {
-            let mut device_result = Ok(());
-            let socket_result =
-                match socket {
-                    &mut Socket::Raw(ref mut socket) =>
-                        socket.dispatch(|response| {
-                            device_result = self.dispatch(timestamp, Packet::Raw(response));
-                            device_result
-                        }),
-                    &mut Socket::Udp(ref mut socket) =>
-                        socket.dispatch(|response| {
-                            device_result = self.dispatch(timestamp, Packet::Udp(response));
-                            device_result
-                        }),
-                    &mut Socket::Tcp(ref mut socket) =>
-                        socket.dispatch(timestamp, &limits, |response| {
-                            device_result = self.dispatch(timestamp, Packet::Tcp(response));
-                            device_result
-                        }),
-                    &mut Socket::__Nonexhaustive => unreachable!()
-                };
-            match (device_result, socket_result) {
-                (Err(Error::Unaddressable), _) => break, // no one to transmit to
-                (Err(Error::Exhausted), _) => break,     // nowhere to transmit
-                (Ok(()), Err(Error::Exhausted)) => (),   // nothing to transmit
-                (Err(err), _) | (_, Err(err)) => {
-                    net_debug!("cannot dispatch egress packet: {}", err);
-                    return Err(err)
+        'iface: for socket in sockets.iter_mut() {
+            'socket: loop {
+                let mut device_result = Ok(());
+                let socket_result =
+                    match socket {
+                        &mut Socket::Raw(ref mut socket) =>
+                            socket.dispatch(|response| {
+                                device_result = self.dispatch(timestamp, Packet::Raw(response));
+                                device_result
+                            }),
+                        &mut Socket::Udp(ref mut socket) =>
+                            socket.dispatch(|response| {
+                                device_result = self.dispatch(timestamp, Packet::Udp(response));
+                                device_result
+                            }),
+                        &mut Socket::Tcp(ref mut socket) =>
+                            socket.dispatch(timestamp, &limits, |response| {
+                                device_result = self.dispatch(timestamp, Packet::Tcp(response));
+                                device_result
+                            }),
+                        &mut Socket::__Nonexhaustive => unreachable!()
+                    };
+                match (device_result, socket_result) {
+                    (Err(Error::Exhausted), _)      => break 'iface,  // nowhere to transmit
+                    (Err(Error::Unaddressable), _)  => break 'socket, // no one to transmit to
+                    (Ok(()), Err(Error::Exhausted)) => break 'socket, // nothing to transmit
+                    (_, Err(err)) => {
+                        net_debug!("cannot dispatch egress packet: {}", err);
+                        return Err(err)
+                    }
+                    (_, Ok(())) => ()
                 }
-                (Ok(()), Ok(())) => ()
             }
         }