Fix TcpStream::connect_timeout on linux

Linux appears to set POLLOUT when a conection's refused, which is pretty
weird. Invert the check to look for an error explicitly. Also add an
explict test for this case.

Closes #45265.

src/libstd/net/tcp.rs

@@ -1579,6 +1579,21 @@ mod tests {
                 "bad error: {} {:?}", e, e.kind());
     }
 
+    #[test]
+    fn connect_timeout_unbound() {
+        // bind and drop a socket to track down a "probably unassigned" port
+        let socket = TcpListener::bind("127.0.0.1:0").unwrap();
+        let addr = socket.local_addr().unwrap();
+        drop(socket);
+
+        let timeout = Duration::from_secs(1);
+        let e = TcpStream::connect_timeout(&addr, timeout).unwrap_err();
+        assert!(e.kind() == io::ErrorKind::ConnectionRefused ||
+                e.kind() == io::ErrorKind::TimedOut ||
+                e.kind() == io::ErrorKind::Other,
+                "bad error: {} {:?}", e, e.kind());
+    }
+
     #[test]
     fn connect_timeout_valid() {
         let listener = TcpListener::bind("127.0.0.1:0").unwrap();

src/libstd/sys/unix/net.rs

@@ -176,11 +176,16 @@ impl Socket {
                 }
                 0 => {}
                 _ => {
-                    if pollfd.revents & libc::POLLOUT == 0 {
+                    // linux returns POLLOUT|POLLERR|POLLHUP for refused connections (!), so look
-                        if let Some(e) = self.take_error()? {
+                    // for POLLHUP rather than read readiness
+                    if pollfd.revents & libc::POLLHUP != 0 {
+                        let e = self.take_error()?
+                            .unwrap_or_else(|| {
+                                io::Error::new(io::ErrorKind::Other, "no error set after POLLHUP")
+                            });
                         return Err(e);
                     }
-                    }
+
                     return Ok(());
                 }
             }