Rollup merge of rust-lang#38986 - APTy:fix-nonblocking-test, r=aturon

std/net/udp: Improve set_nonblocking test

While writing a separate change, I noticed the current test for `UdpSocket::set_nonblocking()` is fairly ineffective.

This fixes the test so that it validates that a correct error is returned on calls to `recv()` when no data is available.

Author: GuillaumeGomez

src/libstd/net/udp.rs

@@ -883,11 +883,23 @@ mod tests {
 
     #[test]
     fn set_nonblocking() {
-        let addr = next_test_ip4();
+        each_ip(&mut |addr, _| {
+            let socket = t!(UdpSocket::bind(&addr));
 
-        let stream = t!(UdpSocket::bind(&addr));
+            t!(socket.set_nonblocking(true));
+            t!(socket.set_nonblocking(false));
+
+            t!(socket.connect(addr));
 
-        t!(stream.set_nonblocking(true));
-        t!(stream.set_nonblocking(false));
+            t!(socket.set_nonblocking(false));
+            t!(socket.set_nonblocking(true));
+
+            let mut buf = [0];
+            match socket.recv(&mut buf) {
+                Ok(_) => panic!("expected error"),
+                Err(ref e) if e.kind() == ErrorKind::WouldBlock => {}
+                Err(e) => panic!("unexpected error {}", e),
+            }
+        })
     }
 }