ralloc: Implement a new ralloc_adopt() API.

ralloc_adopt() reparents all children from one context to another.
Conceptually, ralloc_adopt(new_ctx, old_ctx) behaves like this
pseudocode:

   foreach child of old_ctx:
      ralloc_steal(new_ctx, child)

However, ralloc provides no way to iterate over a memory context's
children, and ralloc_adopt does this task more efficiently anyway.

One potential use of this is to implement a memory-sweeper pass: first,
steal all of a context's memory to a temporary context.  Then, walk over
anything that should be kept, and ralloc_steal it back to the original
context.  Finally, free the temporary context.  This works when the
context is something that can't be freed (i.e. an important structure).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>

src/util/ralloc.c

@@ -271,6 +271,32 @@ ralloc_steal(const void *new_ctx, void *ptr)
    add_child(parent, info);
 }
 
+void
+ralloc_adopt(const void *new_ctx, void *old_ctx)
+{
+   ralloc_header *new_info, *old_info, *child;
+
+   if (unlikely(old_ctx == NULL))
+      return;
+
+   old_info = get_header(old_ctx);
+   new_info = get_header(new_ctx);
+
+   /* If there are no children, bail. */
+   if (unlikely(old_info->child == NULL))
+      return;
+
+   /* Set all the children's parent to new_ctx; get a pointer to the last child. */
+   for (child = old_info->child; child->next != NULL; child = child->next) {
+      child->parent = new_info;
+   }
+
+   /* Connect the two lists together; parent them to new_ctx; make old_ctx empty. */
+   child->next = new_info->child;
+   new_info->child = old_info->child;
+   old_info->child = NULL;
+}
+
 void *
 ralloc_parent(const void *ptr)
 {

src/util/ralloc.h

@@ -234,6 +234,13 @@ void ralloc_free(void *ptr);
  */
 void ralloc_steal(const void *new_ctx, void *ptr);
 
+/**
+ * Reparent all children from one context to another.
+ *
+ * This effectively calls ralloc_steal(new_ctx, child) for all children of \p old_ctx.
+ */
+void ralloc_adopt(const void *new_ctx, void *old_ctx);
+
 /**
  * Return the given pointer's ralloc context.
  */