[Libguestfs] hivex lib: Add function hivex_node_num_children

[Richard W.M. Jones]

On Sat, Feb 13, 2016 at 10:18:22PM -0800, Kevin Haroldsen wrote:
> 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.

You've come to the right place!  The patch looks fine at first glance
but I'll have to have a proper look at it tomorrow.

Thanks, Rich.

> 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)
> 
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