lib: linklist: add listnode_add_before()

This utility function, to join the zoo that the Quagga linked-list
implementation has accumulated, does an insert-before while returning
the newly allocated node.

It is similar to:
- listnode_add_after(), but
  - complementary direction
  - returns allocated node
- list_add_node_prev(), but
  - supports before == NULL
  - returns allocated node

In general, the entire linked-list implementation is in bad shape, and
while it needs a cleanup / rewrite / replacement, this would both cause
significant conflicts and block other cleanups...

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>

lib/linklist.c

@@ -159,6 +159,44 @@ listnode_add_after (struct list *list, struct listnode *pp, void *val)
   list->count++;
 }
 
+struct listnode *
+listnode_add_before (struct list *list, struct listnode *pp, void *val)
+{
+  struct listnode *nn;
+
+  assert (val != NULL);
+
+  nn = listnode_new ();
+  nn->data = val;
+
+  if (pp == NULL)
+    {
+      if (list->tail)
+        list->tail->next = nn;
+      else
+        list->head = nn;
+
+      nn->prev = list->tail;
+      nn->next = pp;
+
+      list->tail = nn;
+    }
+  else
+    {
+      if (pp->prev)
+	pp->prev->next = nn;
+      else
+	list->head = nn;
+
+      nn->prev = pp->prev;
+      nn->next = pp;
+
+      pp->prev = nn;
+    }
+  list->count++;
+  return nn;
+}
+
 /* Move given listnode to tail of the list */
 void
 listnode_move_to_tail (struct list *l, struct listnode *n)

lib/linklist.h

@@ -68,6 +68,7 @@ extern void list_free (struct list *);
 extern void listnode_add (struct list *, void *);
 extern void listnode_add_sort (struct list *, void *);
 extern void listnode_add_after (struct list *, struct listnode *, void *);
+extern struct listnode *listnode_add_before (struct list *, struct listnode *, void *);
 extern void listnode_move_to_tail (struct list *, struct listnode *);
 extern void listnode_delete (struct list *, void *);
 extern struct listnode *listnode_lookup (struct list *, void *);