Commit 2df147cb12 made the prune_tree()
function use send_to_prune_tree() for triggering pruning of deleted leaf
nodes' parents. This enabled the following sequence of events to
happen:
1. Node A, which is a leaf node, is passed to send_to_prune_tree() and
its pruning is queued.
2. Node B is added to the RBTDB as a child of node A before the latter
gets pruned.
3. Node B, which is now a leaf node itself (and is likely to belong to
a different node bucket than node A), is passed to
send_to_prune_tree() and its pruning gets queued.
4. Node B gets pruned. Its parent, node A, now becomes a leaf again
and therefore the prune_tree() call that handled node B calls
send_to_prune_tree() for node A.
5. Since node A was already queued for pruning in step 1 (but not yet
pruned), the INSIST(!ISC_LINK_LINKED(node, prunelink)); assertion
fails for node A in send_to_prune_tree().
The above sequence of events is not a sign of pathological behavior.
Replace the assertion check with a conditional early return from
send_to_prune_tree().
f6289ad931