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CheckInFlight counted every view data that carried the in-flight proposal toward condition A2, even when its sender had not prepared it. A node that merely holds a proposal in flight has not committed to it, so a view change could decide to re-commit a proposal that no quorum ever prepared. Count a view data toward A2 only when its sender reports the proposal as prepared. Condition A1 still counts unprepared holders as nodes that do not object, and condition B still counts them as supporting that nothing is prepared. Signed-off-by: Fedor Partanskiy <[email protected]>
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I think this can stop a view change from ever completing. With n=4, if node 2 crashes while node 1 is prepared on p, node 3 has only pre-prepared it and node 4 has nothing, then the only view data any new leader can collect are those three. A2 now needs 2 prepared entries but only node 1 has one, and B fails because node 1 is prepared, so CheckInFlight returns no decision in every view until node 2 comes back. On main, A holds and the view change completes. That's why PBFT's A2 counts pre-prepared rather than prepared: its liveness argument relies on "a request that prepares at a nonfaulty replica pre-prepares at at least f+1 nonfaulty replicas" (Castro & Liskov, TOCS 2002, §4.5.1). (Drafted with Claude's help.) |
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CheckInFlight counted every view data that carried the in-flight proposal toward condition A2, even when its sender had not prepared it. A node that merely holds a proposal in flight has not committed to it, so a view change could decide to re-commit a proposal that no quorum ever prepared.
Count a view data toward A2 only when its sender reports the proposal as prepared. Condition A1 still counts unprepared holders as nodes that do not object, and condition B still counts them as supporting that nothing is prepared.