[list] Add list_first_entry()

There are several points in the iPXE codebase where
list_for_each_entry() is (ab)used to extract only the first entry from
a list.  Add a macro list_first_entry() to make this code easier to
read.

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

src/net/80211/net80211.c

@@ -668,11 +668,11 @@ struct io_buffer * net80211_mgmt_dequeue ( struct net80211_device *dev,
 			*signal = rxi->signal;
 		free ( rxi );
 
-		list_for_each_entry ( iobuf, &dev->mgmt_queue, list ) {
-			list_del ( &iobuf->list );
-			return iobuf;
-		}
-		assert ( 0 );
+		assert ( ! list_empty ( &dev->mgmt_queue ) );
+		iobuf = list_first_entry ( &dev->mgmt_queue, struct io_buffer,
+					   list );
+		list_del ( &iobuf->list );
+		return iobuf;
 	}
 
 	return NULL;

src/net/infiniband.c

@@ -982,12 +982,12 @@ struct ib_device * find_ibdev ( union ib_gid *gid ) {
 struct ib_device * last_opened_ibdev ( void ) {
 	struct ib_device *ibdev;
 
-	list_for_each_entry ( ibdev, &open_ib_devices, open_list ) {
-		assert ( ibdev->open_count != 0 );
-		return ibdev;
-	}
+	ibdev = list_first_entry ( &open_ib_devices, struct ib_device, list );
+	if ( ! ibdev )
+		return NULL;
 
-	return NULL;
+	assert ( ibdev->open_count != 0 );
+	return ibdev;
 }
 
 /* Drag in IPoIB */

src/net/netdevice.c

@@ -328,11 +328,12 @@ void netdev_poll ( struct net_device *netdev ) {
 struct io_buffer * netdev_rx_dequeue ( struct net_device *netdev ) {
 	struct io_buffer *iobuf;
 
-	list_for_each_entry ( iobuf, &netdev->rx_queue, list ) {
-		list_del ( &iobuf->list );
-		return iobuf;
-	}
-	return NULL;
+	iobuf = list_first_entry ( &netdev->rx_queue, struct io_buffer, list );
+	if ( ! iobuf )
+		return NULL;
+
+	list_del ( &iobuf->list );
+	return iobuf;
 }
 
 /**
@@ -592,12 +593,13 @@ struct net_device * find_netdev_by_location ( unsigned int bus_type,
 struct net_device * last_opened_netdev ( void ) {
 	struct net_device *netdev;
 
-	list_for_each_entry ( netdev, &open_net_devices, open_list ) {
-		assert ( netdev_is_open ( netdev ) );
-		return netdev;
-	}
+	netdev = list_first_entry ( &open_net_devices, struct net_device,
+				    list );
+	if ( ! netdev )
+		return NULL;
 
-	return NULL;
+	assert ( netdev_is_open ( netdev ) );
+	return netdev;
 }
 
 /**

src/net/tcp.c

@@ -1014,9 +1014,8 @@ static void tcp_process_rx_queue ( struct tcp_connection *tcp ) {
 	 * queue, since tcp_discard() may remove packets from the RX
 	 * queue while we are processing.
 	 */
-	while ( ! list_empty ( &tcp->rx_queue ) ) {
-		list_for_each_entry ( iobuf, &tcp->rx_queue, list )
-			break;
+	while ( ( iobuf = list_first_entry ( &tcp->rx_queue, struct io_buffer,
+					     list ) ) ) {
 
 		/* Stop processing when we hit the first gap */
 		tcpqhdr = iobuf->data;