The original implementation allows a list to be corrupted by list
operations on the removed node. Existing code attempts to avoid this by
using external state to determine whether a node is in a list, but this
is fragile and fails when the state that holds the flag value is changed
after the node is removed, e.g. in preparation for re-using the node.
Follow Linux in invalidating the link pointers in a removed node. Add
API so that detection of particpation in a list is available at the node
abstraction.
This solution relies on the following steady-state invariants:
* A node (as opposed to a list) will never be adjacent to itself in a
list;
* The next and prev pointers of a node are always either both null or
both non-null.
Signed-off-by: Peter A. Bigot <pab@pabigot.com>
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