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An interrupted node-left operation can leave a node (even if it rejoins later) on a list of faultyNodes that do not receive cluster state updates until a LagDetector timeout
Related to https://github.com/elastic/elasticsearch/issues/91447 test failure. We believe the failure circumstances are rare: the circumstances were created by a NullPointerException that has been fixed, and what remains is hypothetical.
It's possible for a node-left task to get interrupted prior to removing the node from the master's list of faultyNodes. Nodes on the faultyNodes list do not receive cluster state updates, and are eventually removed. Subsequently, when the node attempts to rejoin, after test network disruptions have ceased, the node-join request can succeed, but the node will never receive the cluster state update, consider the node-join a failure, and will resend node-join requests until the LagDetector removes the node from the faultyNodes list.
A solution would be for a node-join request to first run a new node-left request, if the node is seen to still be present in the cluster state. Complete the node-left operation before the node-join proceeds. This will ensure that all of the node-left logic runs successfully, including removing the node from the list of faultyNodes, and there's clean state on which to apply a node-join request. A comment on the test failure has further details on this suggestion.
if the node is seen to still be present in the cluster state
... and is considered faulty. Non-faulty nodes will rejoin the cluster on (e.g.) every master election and we don't want to drop them (and all their shards) from the cluster first.
Hmm. You're right, there could be multiple node-join requests sent and received out-of-order, don't want to run node-left every time.
I was hoping to avoid directly checking for the node in the faultyNodes list: if that's the only thing that can go wrong on a node-left operation, then it seems like it would be better to directly remove the node from that list and skip the rest of the node-left logic.