make candidateFromLeadershipTransfer atomic to avoid a race (#570)

api.go

@@ -134,7 +134,7 @@ type Raft struct {
 	// candidate because the leader tries to transfer leadership. This flag is
 	// used in RequestVoteRequest to express that a leadership transfer is going
 	// on.
-	candidateFromLeadershipTransfer bool
+	candidateFromLeadershipTransfer atomic.Bool
 
 	// Stores our local server ID, used to avoid sending RPCs to ourself
 	localID ServerID

raft.go

@@ -293,7 +293,7 @@ func (r *Raft) runCandidate() {
 	// which will make other servers vote even though they have a leader already.
 	// It is important to reset that flag, because this priviledge could be abused
 	// otherwise.
-	defer func() { r.candidateFromLeadershipTransfer = false }()
+	defer func() { r.candidateFromLeadershipTransfer.Store(false) }()
 
 	electionTimeout := r.config().ElectionTimeout
 	electionTimer := randomTimeout(electionTimeout)
@@ -1390,7 +1390,7 @@ func (r *Raft) appendEntries(rpc RPC, a *AppendEntriesRequest) {
 
 	// Increase the term if we see a newer one, also transition to follower
 	// if we ever get an appendEntries call
-	if a.Term > r.getCurrentTerm() || (r.getState() != Follower && !r.candidateFromLeadershipTransfer) {
+	if a.Term > r.getCurrentTerm() || (r.getState() != Follower && !r.candidateFromLeadershipTransfer.Load()) {
 		// Ensure transition to follower
 		r.setState(Follower)
 		r.setCurrentTerm(a.Term)
@@ -1839,7 +1839,7 @@ func (r *Raft) electSelf() <-chan *voteResult {
 		Candidate:          r.trans.EncodePeer(r.localID, r.localAddr),
 		LastLogIndex:       lastIdx,
 		LastLogTerm:        lastTerm,
-		LeadershipTransfer: r.candidateFromLeadershipTransfer,
+		LeadershipTransfer: r.candidateFromLeadershipTransfer.Load(),
 	}
 
 	// Construct a function to ask for a vote
@@ -1972,7 +1972,7 @@ func (r *Raft) initiateLeadershipTransfer(id *ServerID, address *ServerAddress)
 func (r *Raft) timeoutNow(rpc RPC, req *TimeoutNowRequest) {
 	r.setLeader("", "")
 	r.setState(Candidate)
-	r.candidateFromLeadershipTransfer = true
+	r.candidateFromLeadershipTransfer.Store(true)
 	rpc.Respond(&TimeoutNowResponse{}, nil)
 }