net: allow to set linger on TcpSocket (#4324)

For now, this is only allowed on TcpStream. This is a problem when one
want to disable lingering (i.e. set it to Duration(0, 0)). Without being
able to set it prior to the connect call, if the connect future is
dropped it would leave sockets in a TIME_WAIT state.

Co-authored-by: Fabien Gaud <[email protected]>

tokio/tests/tcp_socket.rs

@@ -1,6 +1,7 @@
 #![warn(rust_2018_idioms)]
 #![cfg(feature = "full")]
 
+use std::time::Duration;
 use tokio::net::TcpSocket;
 use tokio_test::assert_ok;
 
@@ -58,3 +59,16 @@ async fn bind_before_connect() {
     // Accept
     let _ = assert_ok!(srv.accept().await);
 }
+
+#[tokio::test]
+async fn basic_linger() {
+    // Create server
+    let addr = assert_ok!("127.0.0.1:0".parse());
+    let srv = assert_ok!(TcpSocket::new_v4());
+    assert_ok!(srv.bind(addr));
+
+    assert!(srv.linger().unwrap().is_none());
+
+    srv.set_linger(Some(Duration::new(0, 0))).unwrap();
+    assert_eq!(srv.linger().unwrap(), Some(Duration::new(0, 0)));
+}