Pre-Vote and CheckQuorum — fixing the disruptive server

1# Pre-Vote — added BEFORE the real RequestVote.
2on election_timeout:
3    reset election_timeout
4    # PROBE: ask peers if they would grant at term+1,
5    # WITHOUT bumping currentTerm.
6    pre_votes = { self }
7    for peer in cluster_minus_self:
8        send PreVote {
9            term:         currentTerm + 1,  # hypothetical
10            candidateId:  self,
11            lastLogIndex: log.lastIndex,
12            lastLogTerm:  log.lastTerm,
13        } -> peer
14    # Only if a majority would grant, bump currentTerm
15    # and fire the real RequestVote.
16 
17on receive PreVote(req):
18    # No state mutation — this is read-only.
19    would_grant = req.term > currentTerm
20        AND isAtLeastAsUpToDate(req.lastLogIndex,
21                                req.lastLogTerm, log)
22        AND not heard_from_leader_recently()
23    reply { term: currentTerm, voteGranted: would_grant }