use SO_RCVBUFFORCE to force receive buffer increase on Linux (#3804)

* Add ability to force change the receive buffer size using SO_RCVBUFFORCE in Linux

* Fix imports

* Update test

* Add sys_conn_helper_not_linux

* Rename file

* ignore the error on SetReadBuffer

* also run unit tests as root

---------

Co-authored-by: Marten Seemann <martenseemann@gmail.com>

transport.go

@@ -180,10 +180,17 @@ func setReceiveBuffer(c net.PacketConn, logger utils.Logger) error {
 		logger.Debugf("Conn has receive buffer of %d kiB (wanted: at least %d kiB)", size/1024, protocol.DesiredReceiveBufferSize/1024)
 		return nil
 	}
-	if err := conn.SetReadBuffer(protocol.DesiredReceiveBufferSize); err != nil {
-		return fmt.Errorf("failed to increase receive buffer size: %w", err)
-	}
+	// Ignore the error. We check if we succeeded by querying the buffer size afterward.
+	_ = conn.SetReadBuffer(protocol.DesiredReceiveBufferSize)
 	newSize, err := inspectReadBuffer(c)
+	if newSize < protocol.DesiredReceiveBufferSize {
+		// Try again with RCVBUFFORCE on Linux
+		_ = forceSetReceiveBuffer(c, protocol.DesiredReceiveBufferSize)
+		newSize, err = inspectReadBuffer(c)
+		if err != nil {
+			return fmt.Errorf("failed to determine receive buffer size: %w", err)
+		}
+	}
 	if err != nil {
 		return fmt.Errorf("failed to determine receive buffer size: %w", err)
 	}