[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/core/process.c

 void step ( void ) {
 	struct process *process;
 
-	list_for_each_entry ( process, &run_queue, list ) {
+	if ( ( process = list_first_entry ( &run_queue, struct process,
+					    list ) ) ) {
 		list_del ( &process->list );
 		list_add_tail ( &process->list, &run_queue );
 		ref_get ( process->refcnt ); /* Inhibit destruction mid-step */
 		DBGC2 ( process, "PROCESS %p (%p) finished executing\n",
 			process, process->step );
 		ref_put ( process->refcnt ); /* Allow destruction */
-		break;
 	}
 }
 

src/hci/commands/fcmgmt_cmd.c

 		}
 	} else {
 		/* Use first port */
-		if ( list_empty ( &fc_ports ) ) {
+		port = list_first_entry ( &fc_ports, struct fc_port, list );
+		if ( ! port ) {
 			printf ( "No ports\n" );
 			return 1;
 		}
-		list_for_each_entry ( port, &fc_ports, list )
-			break;
 	}
 	assert ( port != NULL );
 

src/include/ipxe/list.h

 	list_check ( (list) );				\
 	container_of ( list, type, member ); } )
 
+/**
+ * Get the container of the first entry in a list
+ *
+ * @v list		List head
+ * @v type		Containing type
+ * @v member		Name of list field within containing type
+ * @ret first		First list entry, or NULL
+ */
+#define list_first_entry( list, type, member )		\
+	( list_empty ( (list) ) ?			\
+	  ( type * ) NULL :				\
+	  list_entry ( (list)->next, type, member ) )
+
 /**
  * Iterate over entries in a list
  *

src/net/80211/net80211.c

 			*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

 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

 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;
 }
 
 /**
 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

 	 * 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;