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Paxos says that on receiving the result of a prepare from a quorum of acceptors, the proposer should propose the value of the higher-number proposal accepted amongst the ones returned by the acceptors, and only propose his own value if no acceptor has send us back a previously accepted value.
But in PrepareCallback we only keep the more recent inProgress commit regardless of whether is has an update. Which means we could ignore a value already accepted by some acceptors if any of the acceptor send us a more recent ballot than the other acceptor but with no values. The net effect is that we can mistakenly accept two different values for the same round.
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Jonathan Ellis
added a comment - +1
Committed, thanks
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Sylvain Lebresne
added a comment - Committed, thanks
Attaching fix: as far as checking if we should finish an inProgress round, we only need to keep the most recent inProgress commit that has a value. But so as to not break the optimization of
CASSANDRA-5667, the patch also keep the most recent inProgress, regardless of whether it has a value or not.