TALLOC: Discussion of destructor processing

Stephen Gallagher sgallagh at redhat.com
Mon Apr 30 11:38:35 MDT 2012

I was having a discussion with Pavel Březina about talloc destructors
and he made me aware of the underlying algorithm used to process them
during a nested free operation.

So, to summarize quickly, the behavior is as follows: (pseudo-code).

if (destructor(ctx) == -1) {
     return -1;
free_children(ctx); /* This recurses */

I see several problems with this approach.

1) The results of the destructors for the children are not considered
when determining whether to report success or failure for the
talloc_free(). This means that if any part of the hierarchy except the
top-most context is freed, there will be a memory leak (with probably
unreachable memory).

2) It strikes me as strange that we're calling the destructor before
we're we're handling the destructors of the children. It's contrary to
(at least my) expectations about the order that destructors should
execute. Freeing of memory and execution of destructors should be
happening from the "bottom-up" of the hierarchy.

My vision of how this should behave is as follows: We should iterate
through the hierarchy tree, starting from the leaves. Work its way back
up, calling destructors where they exist until either we have called all
of them or any one has failed. If any failed, return -1 to the top-level
free. If all succeeded, THEN iterate the tree a second time to actually
perform the deallocation.
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