[Libguestfs] hivex lib: Add function hivex_node_num_children

[Kevin Haroldsen]

Hello,

I've been working on a graphical registry hive editing tool in Qt
using the hivex C library.
While creating it, I noticed that in order to determine if an
expansion element should be shown on a node, you have to determine if
a node has any children.
Currently, in order to determine if a node has any children, you must
find every child with hivex_node_children, which is a relatively
expensive operation.
So, I have created a function called hivex_node_num_children that
isolates the first few lines of hivex_node_children to simply return
the number of child nodes a node has by accessing
ntreg_nk_record->nr_subkeys.

I'm a bit of a newer developer, so I'm not incredibly familiar with
the patching process through mailing lists, nor do I have any history
with libguestfs.
Please let me know if I am doing anything wrong or what can be improved.

This is the git branch:
https://github.com/kupiakos/hivex/tree/node_children_count
This is a patch of the difference:

diff --git a/generator/generator.ml b/generator/generator.ml
index be783ae..4f878ff 100755
--- a/generator/generator.ml
+++ b/generator/generator.ml
@@ -210,6 +210,11 @@ Return the child of node with the name C<name>,
if it exists.

 The name is matched case insensitively.";

+  "node_num_children", (RSize, [AHive; ANode "node"]),
+    "return the number of children of a node",
+    "\
+Return the number of nodes as produced by C<hivex_node_children>.";
+
   "node_parent", (RNode, [AHive; ANode "node"]),
     "return the parent of node",
     "\
diff --git a/lib/node.c b/lib/node.c
index 1fb48cf..fc3ca71 100644
--- a/lib/node.c
+++ b/lib/node.c
@@ -513,6 +513,22 @@ hivex_node_children (hive_h *h, hive_node_h node)
   return children;
 }

+size_t
+hivex_node_num_children (hive_h *h, hive_node_h node)
+{
+  if (!IS_VALID_BLOCK (h, node) || !block_id_eq(h, node, "nk")) {
+    SET_ERRNO( EINVAL, "invalid block or not an 'nk' block");
+    return 0;
+  }
+
+  struct ntreg_nk_record *nk =
+    (struct ntreg_nk_record *) ((char *) h->addr + node);
+
+  size_t nr_subkeys_in_nk = le32toh(nk->nr_subkeys);
+
+  return nr_subkeys_in_nk;
+}
+
 /* Very inefficient, but at least having a separate API call
  * allows us to make it more efficient in future.
  */

--
 - Kevin Haroldsen (kupiakos)