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Gracefully handle resending a node to prune_tree()

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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().
This commit is contained in:
Michał Kępień
2024-02-29 17:38:52 +01:00
parent a2b9adb1c7
commit f6289ad931
1 changed files with 3 additions and 1 deletions
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lib/dns/rbtdb.c
+3 -1
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@@ -2004,7 +2004,9 @@ send_to_prune_tree(dns_rbtdb_t *rbtdb, dns_rbtnode_t *node,
isc_rwlocktype_t nlocktype) {
bool pruning_queued = !ISC_LIST_EMPTY(rbtdb->prunenodes[node->locknum]);
INSIST(!ISC_LINK_LINKED(node, prunelink));
if (ISC_LINK_LINKED(node, prunelink)) {
return;
}
new_reference(rbtdb, node, nlocktype);
ISC_LIST_APPEND(rbtdb->prunenodes[node->locknum], node, prunelink);