Fix a bug where the leader is never elected even if the majority of the members are alive.

src/ra_server.erl

@@ -919,6 +919,20 @@ handle_candidate(#pre_vote_rpc{term = Term} = Msg,
 handle_candidate(#request_vote_rpc{}, State = #{current_term := Term}) ->
     Reply = #request_vote_result{term = Term, vote_granted = false},
     {candidate, State, [{reply, Reply}]};
+handle_candidate(#pre_vote_rpc{term = Term, last_log_index = PeerLastIdx} = PreVote,
+                 #{current_term := CurTerm,
+                   cfg := #cfg{log_id = LogId}} = State)
+  when Term =:= CurTerm ->
+    %% This clause is necessary to prevent the liveness issue reported in https://github.com/rabbitmq/ra/issues/439
+    {LastIdx, _} = last_idx_term(State),
+    case PeerLastIdx > LastIdx of
+        true ->
+            ?INFO("~ts: candidate pre_vote_rpc with higher last index received ~b -> ~b",
+                  [LogId, LastIdx, PeerLastIdx]),
+            process_pre_vote(candidate, PreVote, State);
+        false ->
+            {candidate, State, []}
+    end;
 handle_candidate(#pre_vote_rpc{}, State) ->
     %% just ignore pre_votes that aren't of a higher term
     {candidate, State, []};

test/ra_server_SUITE.erl

@@ -33,6 +33,7 @@ all() ->
      higher_term_detected,
      pre_vote_election,
      pre_vote_election_reverts,
+     candidate_receives_pre_vote,
      leader_receives_pre_vote,
      candidate_election,
      is_new,
@@ -1927,6 +1928,28 @@ pre_vote_election_reverts(_Config) ->
         = ra_server:handle_pre_vote(ISR, State),
     ok.
 
+candidate_receives_pre_vote(_Config) ->
+    % candidate ignores an pre_vote_rpc with lower term and lower index
+    Token = make_ref(),
+    State = (base_state(5, ?FUNCTION_NAME))#{votes => 1},
+    PreVoteRpc = #pre_vote_rpc{term = 5, candidate_id = ?N1,
+                               token = Token,
+                               machine_version = 0,
+                               last_log_index = 3, last_log_term = 5},
+    {candidate, #{}, []}
+        = ra_server:handle_candidate(PreVoteRpc, State),
+
+    % candidate abdicates for higher term
+    {follower, #{current_term := 6}, _}
+        = ra_server:handle_candidate(PreVoteRpc#pre_vote_rpc{term = 6}, State),
+
+    % candidate replies `#pre_vote_result{vote_granted=true}` for higher index
+    {candidate, #{},
+     [{reply, #pre_vote_result{token = Token, vote_granted = true}}]}
+        = ra_server:handle_candidate(PreVoteRpc#pre_vote_rpc{last_log_index = 4}, State),
+
+    ok.
+
 leader_receives_pre_vote(_Config) ->
     % leader should emit rpcs to all nodes immediately upon receiving
     % an pre_vote_rpc to put upstart followers back in their place