libdiskfs: add diskfs_make_node_alloc to allocate fat nodes

libdiskfs has two kind of nodes, struct node and struct netnode.
struct node is used to store libdiskfs specific data, while struct
netnode contains user supplied data.  Previously, both objects were
allocated separatly, and a pointer from the node to the netnode
provided a mapping from the former to the latter.

Provide a function diskfs_make_node_alloc that allocates both nodes in
a contiguous region.

This reduces the memory allocation overhead when creating nodes.  It
also makes the relation between node and netnode a simple offset
calculation.  Provide two functions to compute the netnode address
from the node address and vice-versa.

Most notably, this makes implementing a cache on top of libdiskfs
easier.  Auxiliary data for the cache can be stored in the
user-defined netnode, and the fat node can be used as the value.

* libdiskfs/node-make.c (init_node): Move initialization here.
(diskfs_make_node): Use init_node.
(diskfs_make_node_alloc): New function to allocate fat nodes.
* libdiskfs/diskfs.h (diskfs_make_node_alloc): New declaration.
(_diskfs_sizeof_struct_node): Likewise.
(diskfs_node_disknode): Compute disknode address from node address.
(diskfs_disknode_node): And vice-versa.
* libdiskfs/init-init.c (_diskfs_sizeof_struct_node): New variable.

3 files changed, 61 insertions, 9 deletions

diff --git a/libdiskfs/diskfs.h b/libdiskfs/diskfs.h
index ae1a1502..2c68aa34 100644
--- a/libdiskfs/diskfs.h
+++ b/libdiskfs/diskfs.h
@@ -686,6 +686,33 @@ diskfs_notice_filechange (struct node *np, enum file_changed_type type,
    The new node will have one hard reference and no light references.  */
 struct node *diskfs_make_node (struct disknode *dn);
 
+/* Create a new node structure.  Also allocate SIZE bytes for the
+   disknode.  The address of the disknode can be obtained using
+   diskfs_node_disknode.  The new node will have one hard reference
+   and no light references.  */
+struct node *diskfs_make_node_alloc (size_t size);
+
+/* To avoid breaking the ABI whenever sizeof (struct node) changes, we
+   explicitly provide the size.  The following two functions will use
+   this value for offset calculations.  */
+extern const size_t _diskfs_sizeof_struct_node;
+
+/* Return the address of the disknode for NODE.  NODE must have been
+   allocated using diskfs_make_node_alloc.  */
+static inline struct disknode *
+diskfs_node_disknode (struct node *node)
+{
+  return (struct disknode *) ((char *) node + _diskfs_sizeof_struct_node);
+}
+
+/* Return the address of the node for DISKNODE.  DISKNODE must have
+   been allocated using diskfs_make_node_alloc.  */
+static inline struct node *
+diskfs_disknode_node (struct disknode *disknode)
+{
+  return (struct node *) ((char *) disknode - _diskfs_sizeof_struct_node);
+}
+
 
 /* The library also exports the following functions; they are not generally
    useful unless you are redefining other functions the library provides. */
diff --git a/libdiskfs/init-init.c b/libdiskfs/init-init.c
index 35be7edd..7a7f2485 100644
--- a/libdiskfs/init-init.c
+++ b/libdiskfs/init-init.c
@@ -25,6 +25,10 @@ the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.  */
 #include <stdio.h>
 #include <maptime.h>
 
+/* For safe inlining of diskfs_node_disknode and
+   diskfs_disknode_node.  */
+size_t const _diskfs_sizeof_struct_node = sizeof (struct node);
+
 mach_port_t diskfs_default_pager;
 mach_port_t diskfs_auth_server_port;
 volatile struct mapped_time_value *diskfs_mtime;
diff --git a/libdiskfs/node-make.c b/libdiskfs/node-make.c
index 2b6ef2af..ff0cc0d4 100644
--- a/libdiskfs/node-make.c
+++ b/libdiskfs/node-make.c
@@ -19,16 +19,9 @@
 #include <fcntl.h>
 
 
-/* Create a and return new node structure with DN as its physical disknode.
-   The node will have one hard reference and no light references.  */
-struct node *
-diskfs_make_node (struct disknode *dn)
+static struct node *
+init_node (struct node *np, struct disknode *dn)
 {
-  struct node *np = malloc (sizeof (struct node));
-
-  if (np == 0)
-    return 0;
-
   np->dn = dn;
   np->dn_set_ctime = 0;
   np->dn_set_atime = 0;
@@ -52,3 +45,31 @@ diskfs_make_node (struct disknode *dn)
 
   return np;
 }
+
+/* Create a and return new node structure with DN as its physical disknode.
+   The node will have one hard reference and no light references.  */
+struct node *
+diskfs_make_node (struct disknode *dn)
+{
+  struct node *np = malloc (sizeof (struct node));
+
+  if (np == 0)
+    return 0;
+
+  return init_node (np, dn);
+}
+
+/* Create a new node structure.  Also allocate SIZE bytes for the
+   disknode.  The address of the disknode can be obtained using
+   diskfs_node_disknode.  The new node will have one hard reference
+   and no light references.  */
+struct node *
+diskfs_make_node_alloc (size_t size)
+{
+  struct node *np = malloc (sizeof (struct node) + size);
+
+  if (np == NULL)
+    return NULL;
+
+  return init_node (np, diskfs_node_disknode (np));
+}