[netdevice] Provide a test mechanism for discarding packets at random

Setting NETDEV_DISCARD_RATE to a non-zero value will cause one in
every NETDEV_DISCARD_RATE packets to be discarded at random on both
the transmit and receive datapaths, allowing the robustness of
upper-layer network protocols to be tested even in simulation
environments that provide wholly reliable packet transmission.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/config/general.h

  *
  */
 
+#define	NETDEV_DISCARD_RATE 0	/* Drop every N packets (0=>no drop) */
 #undef	BUILD_SERIAL		/* Include an automatic build serial
 				 * number.  Add "bs" to the list of
 				 * make targets.  For example:

src/net/netdevice.c

 #include <byteswap.h>
 #include <string.h>
 #include <errno.h>
+#include <config/general.h>
 #include <ipxe/if_ether.h>
 #include <ipxe/iobuf.h>
 #include <ipxe/tables.h>
 	DBGC ( netdev, "NETDEV %p transmitting %p (%p+%zx)\n",
 	       netdev, iobuf, iobuf->data, iob_len ( iobuf ) );
 
+	/* Enqueue packet */
 	list_add_tail ( &iobuf->list, &netdev->tx_queue );
 
+	/* Avoid calling transmit() on unopened network devices */
 	if ( ! netdev_is_open ( netdev ) ) {
 		rc = -ENETUNREACH;
 		goto err;
 	}
-		
+
+	/* Discard packet (for test purposes) if applicable */
+	if ( ( NETDEV_DISCARD_RATE > 0 ) &&
+	     ( ( random() % NETDEV_DISCARD_RATE ) == 0 ) ) {
+		rc = -EAGAIN;
+		goto err;
+	}
+
+	/* Transmit packet */
 	if ( ( rc = netdev->op->transmit ( netdev, iobuf ) ) != 0 )
 		goto err;
 
 	DBGC ( netdev, "NETDEV %p received %p (%p+%zx)\n",
 	       netdev, iobuf, iobuf->data, iob_len ( iobuf ) );
 
+	/* Discard packet (for test purposes) if applicable */
+	if ( ( NETDEV_DISCARD_RATE > 0 ) &&
+	     ( ( random() % NETDEV_DISCARD_RATE ) == 0 ) ) {
+		netdev_rx_err ( netdev, iobuf, -EAGAIN );
+		return;
+	}
+
 	/* Enqueue packet */
 	list_add_tail ( &iobuf->list, &netdev->rx_queue );