lib: northbound cli support to end config nodes

Some more complex CLI usages will require northbound to support
signalizing a custom configuration node end.

For an example:

```
router bgp 100
 bgp router-id 10.254.254.1
 neighbor 10.0.0.100 remote-as 200
 !
 address-family ipv4 unicast
  network 10.0.1.0/24
  network 10.0.2.0/24
  network 10.0.3.0/24
 exit-address-family
 !
 address-family ipv6 unicast
  neighbor 10.0.0.100 activate
 exit-address-family
!
```

This commit implements a new callback called `cli_show_end` which
complements `cli_show` and is only called at the end of processing the
yang configuration node. It will be used to write the configuration
node termination like: "!" or "exit-address-family".

Signed-off-by: Rafael Zalamena <rzalamena@opensourcerouting.org>

lib/northbound.h

@@ -337,6 +337,27 @@ struct nb_callbacks {
 	 */
 	void (*cli_show)(struct vty *vty, struct lyd_node *dnode,
 			 bool show_defaults);
+
+	/*
+	 * Optional callback to show the CLI node end for lists or containers.
+	 *
+	 * vty
+	 *    The vty terminal to dump the configuration to.
+	 *
+	 * dnode
+	 *    libyang data node that should be shown in the form of a CLI
+	 *    command.
+	 *
+	 * show_defaults
+	 *    Specify whether to display default configuration values or not.
+	 *    This parameter can be ignored most of the time since the
+	 *    northbound doesn't call this callback for default leaves or
+	 *    non-presence containers that contain only default child nodes.
+	 *    The exception are commands associated to multiple configuration
+	 *    nodes, in which case it might be desirable to hide one or more
+	 *    parts of the command when this parameter is set to false.
+	 */
+	void (*cli_show_end)(struct vty *vty, struct lyd_node *dnode);
 };
 
 /*

lib/northbound_cli.c

@@ -434,10 +434,29 @@ static int nb_cli_candidate_load_transaction(struct vty *vty,
 	return CMD_SUCCESS;
 }
 
+/*
+ * ly_iter_next_is_up: detects when iterating up on the yang model.
+ *
+ * This function detects whether next node in the iteration is upwards,
+ * then return the node otherwise return NULL.
+ */
+static struct lyd_node *ly_iter_next_up(struct lyd_node *elem)
+{
+	/* Are we going downwards? Is this still not a leaf? */
+	if (!(elem->schema->nodetype & (LYS_LEAF | LYS_LEAFLIST | LYS_ANYDATA)))
+		return NULL;
+
+	/* Are there still leaves in this branch? */
+	if (elem->next != NULL)
+		return NULL;
+
+	return elem->parent;
+}
+
 void nb_cli_show_dnode_cmds(struct vty *vty, struct lyd_node *root,
 			    bool with_defaults)
 {
-	struct lyd_node *next, *child;
+	struct lyd_node *next, *child, *parent;
 
 	LY_TREE_DFS_BEGIN (root, next, child) {
 		struct nb_node *nb_node;
@@ -452,6 +471,19 @@ void nb_cli_show_dnode_cmds(struct vty *vty, struct lyd_node *root,
 
 		(*nb_node->cbs.cli_show)(vty, child, with_defaults);
 	next:
+		/*
+		 * When transiting upwards in the yang model we should
+		 * give the previous container/list node a chance to
+		 * print its close vty output (e.g. "!" or "end-family"
+		 * etc...).
+		 */
+		parent = ly_iter_next_up(child);
+		if (parent != NULL) {
+			nb_node = parent->schema->priv;
+			if (nb_node->cbs.cli_show_end)
+				(*nb_node->cbs.cli_show_end)(vty, parent);
+		}
+
 		LY_TREE_DFS_END(root, next, child);
 	}
 }