[Raft] Add prevote to make it better (#3322)

* prevote

* remove term in AskForVoteResp

* address @kikimo's comments

* rename proposedTerm_ to votedTerm_

* fix leader can't work when term changed during sending logs

Co-authored-by: yaphet <4414314+darionyaphet@users.noreply.github.com>

src/interface/raftex.thrift

@@ -50,6 +50,7 @@ struct AskForVoteRequest {
     5: TermID       term;               // Proposed term
     6: LogID        last_log_id;        // The last received log id
     7: TermID       last_log_term;      // The term receiving the last log
+    8: bool         is_pre_vote;        // Is pre vote or not
 }
 
 

src/kvstore/Listener.cpp

@@ -49,7 +49,7 @@ void Listener::start(std::vector<HostAddr>&& peers, bool) {
 
   lastLogId_ = wal_->lastLogId();
   lastLogTerm_ = wal_->lastLogTerm();
-  term_ = proposedTerm_ = lastLogTerm_;
+  term_ = lastLogTerm_;
 
   // Set the quorum number
   quorum_ = (peers.size() + 1) / 2;
@@ -273,10 +273,9 @@ void Listener::resetListener() {
   reset();
   VLOG(1) << folly::sformat(
       "The listener has been reset : leaderCommitId={},"
-      "proposedTerm={}, lastLogTerm={}, term={},"
+      "lastLogTerm={}, term={},"
       "lastApplyLogId={}",
       leaderCommitId_,
-      proposedTerm_,
       lastLogTerm_,
       term_,
       lastApplyLogId_);