[21.11.x]: backport #4157 #4216
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In joint consensus raft needs majority of both new and previous quorums to make the decisions. Voting for new leader is one of the processes that requires majority agreement from both quorums. In previous implementation we waited for majority of the vote request responses. This approach would make leader election much slower in situations where quorums differ by one node. Example: current voters: `[1,2,4]`, previous voters: `[1,2,3]` In this scenario to elect leader it is enough to wait from responses from node 1 and 2 as they form majority in both quorums. In previous implementation we wait for at least `(n/2)+1` responses. Where `n` is a number of unique voter ids. In the example above `n = len([1,2,3,4]) = 4`. This way we had to wait for 3 replies while only 2 of the are enough to elect new leader. Changed implementation to check if we can make a definitive decision about vote round result after receiving each of the replies. This way we will always use smallest possible set of replies to make a decision. Signed-off-by: Michal Maslanka <michal@vectorized.io> (cherry picked from commit bca6f1d)
dotnwat
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ztlpn,
jcsp,
VadimPlh and
rystsov
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Fixes: #4188