fix: handle-vote should compare last_log_id in dictionary order, not …

…in vector order

A log `{term:2, index:1}` is definitely greater than log `{term:1, index:2}` in raft spec.
Comparing log id in the way of `term1 >= term2 && index1 >= index2` blocks election:
no one can become a leader.

async-raft/src/core/vote.rs

@@ -29,6 +29,7 @@ impl<D: AppData, R: AppDataResponse, N: RaftNetwork<D>, S: RaftStorage<D, R>> Ra
             return Ok(VoteResponse {
                 term: self.current_term,
                 vote_granted: false,
+                last_log_id: self.last_log_id,
             });
         }
 
@@ -44,6 +45,7 @@ impl<D: AppData, R: AppDataResponse, N: RaftNetwork<D>, S: RaftStorage<D, R>> Ra
                 return Ok(VoteResponse {
                     term: self.current_term,
                     vote_granted: false,
+                    last_log_id: self.last_log_id,
                 });
             }
         }
@@ -58,19 +60,17 @@ impl<D: AppData, R: AppDataResponse, N: RaftNetwork<D>, S: RaftStorage<D, R>> Ra
             self.save_hard_state().await?;
         }
 
-        // TODO: bug: (2,1), (1,2)
         // Check if candidate's log is at least as up-to-date as this node's.
         // If candidate's log is not at least as up-to-date as this node, then reject.
-        let client_is_uptodate =
-            (msg.last_log_id.term >= self.last_log_id.term) && (msg.last_log_id.index >= self.last_log_id.index);
-        if !client_is_uptodate {
+        if msg.last_log_id < self.last_log_id {
             tracing::debug!(
                 { candidate = msg.candidate_id },
                 "rejecting vote request as candidate's log is not up-to-date"
             );
             return Ok(VoteResponse {
                 term: self.current_term,
                 vote_granted: false,
+                last_log_id: self.last_log_id,
             });
         }
 
@@ -83,11 +83,13 @@ impl<D: AppData, R: AppDataResponse, N: RaftNetwork<D>, S: RaftStorage<D, R>> Ra
             Some(candidate_id) if candidate_id == &msg.candidate_id => Ok(VoteResponse {
                 term: self.current_term,
                 vote_granted: true,
+                last_log_id: self.last_log_id,
             }),
             // This node has already voted for a different candidate.
             Some(_) => Ok(VoteResponse {
                 term: self.current_term,
                 vote_granted: false,
+                last_log_id: self.last_log_id,
             }),
             // This node has not yet voted for the current term, so vote for the candidate.
             None => {
@@ -99,6 +101,7 @@ impl<D: AppData, R: AppDataResponse, N: RaftNetwork<D>, S: RaftStorage<D, R>> Ra
                 Ok(VoteResponse {
                     term: self.current_term,
                     vote_granted: true,
+                    last_log_id: self.last_log_id,
                 })
             }
         }
@@ -107,15 +110,33 @@ impl<D: AppData, R: AppDataResponse, N: RaftNetwork<D>, S: RaftStorage<D, R>> Ra
 
 impl<'a, D: AppData, R: AppDataResponse, N: RaftNetwork<D>, S: RaftStorage<D, R>> CandidateState<'a, D, R, N, S> {
     /// Handle response from a vote request sent to a peer.
-    #[tracing::instrument(level = "trace", skip(self, res, target))]
+    #[tracing::instrument(level = "debug", skip(self))]
     pub(super) async fn handle_vote_response(&mut self, res: VoteResponse, target: NodeId) -> RaftResult<()> {
         // If peer's term is greater than current term, revert to follower state.
+
         if res.term > self.core.current_term {
             self.core.update_current_term(res.term, None);
-            self.core.update_current_leader(UpdateCurrentLeader::Unknown);
-            self.core.set_target_state(State::Follower);
             self.core.save_hard_state().await?;
-            tracing::debug!("reverting to follower state due to greater term observed in RequestVote RPC response");
+
+            self.core.update_current_leader(UpdateCurrentLeader::Unknown);
+
+            // If a quorum of nodes have higher `last_log_id`, I have no chance to become a leader.
+            // TODO(xp): This is a simplified impl: revert to follower as soon as seeing a higher `last_log_id`.
+            //           When reverted to follower, it waits for heartbeat for 2 second before starting a new round of
+            //           election.
+            if self.core.last_log_id < res.last_log_id {
+                self.core.set_target_state(State::Follower);
+                tracing::debug!("reverting to follower state due to greater term observed in RequestVote RPC response");
+            } else {
+                tracing::debug!(
+                    id = %self.core.id,
+                    self_term=%self.core.current_term,
+                    res_term=%res.term,
+                    self_last_log_id=%self.core.last_log_id,
+                    res_last_log_id=%res.last_log_id,
+                    "I have lower term but higher or euqal last_log_id, keep trying to elect"
+                );
+            }
             return Ok(());
         }
 

async-raft/src/raft.rs

@@ -677,8 +677,12 @@ impl VoteRequest {
 pub struct VoteResponse {
     /// The current term of the responding node, for the candidate to update itself.
     pub term: u64,
+
     /// Will be true if the candidate received a vote from the responder.
     pub vote_granted: bool,
+
+    /// The last log id stored on the remote voter.
+    pub last_log_id: LogId,
 }
 
 //////////////////////////////////////////////////////////////////////////////////////////////////

async-raft/tests/elect_compare_last_log.rs

@@ -0,0 +1,98 @@
+use std::sync::Arc;
+use std::time::Duration;
+
+use anyhow::Result;
+use async_raft::raft::Entry;
+use async_raft::raft::EntryConfigChange;
+use async_raft::raft::EntryPayload;
+use async_raft::raft::MembershipConfig;
+use async_raft::storage::HardState;
+use async_raft::Config;
+use async_raft::LogId;
+use async_raft::RaftStorage;
+use async_raft::State;
+use fixtures::RaftRouter;
+use maplit::btreeset;
+
+#[macro_use]
+mod fixtures;
+
+/// The last_log in a vote request must be greater or equal than the local one.
+///
+/// - Fake a cluster with two node: with last log {2,1} and {1,2}.
+/// - Bring up the cluster and only node 0 can become leader.
+///
+/// RUST_LOG=async_raft,memstore,elect_compare_last_log=trace cargo test -p async-raft --test elect_compare_last_log
+#[tokio::test(flavor = "multi_thread", worker_threads = 6)]
+async fn elect_compare_last_log() -> Result<()> {
+    let (_log_guard, ut_span) = init_ut!();
+    let _ent = ut_span.enter();
+
+    // Setup test dependencies.
+    let config = Arc::new(Config::build("test".into()).validate().expect("failed to build Raft config"));
+    let router = Arc::new(RaftRouter::new(config.clone()));
+
+    let sto0 = router.new_store(0);
+    let sto1 = router.new_store(1);
+
+    tracing::info!("--- fake store: sto0: last log: 2,1");
+    {
+        sto0.save_hard_state(&HardState {
+            current_term: 10,
+            voted_for: None,
+        })
+        .await?;
+
+        sto0.append_to_log(&[&Entry {
+            log_id: LogId { term: 2, index: 1 },
+            payload: EntryPayload::ConfigChange(EntryConfigChange {
+                membership: MembershipConfig {
+                    members: btreeset! {0,1},
+                    members_after_consensus: None,
+                },
+            }),
+        }])
+        .await?;
+    }
+
+    tracing::info!("--- fake store: sto1: last log: 1,2");
+    {
+        sto1.save_hard_state(&HardState {
+            current_term: 10,
+            voted_for: None,
+        })
+        .await?;
+
+        sto1.append_to_log(&[
+            &Entry {
+                log_id: LogId { term: 1, index: 1 },
+                payload: EntryPayload::ConfigChange(EntryConfigChange {
+                    membership: MembershipConfig {
+                        members: btreeset! {0,1},
+                        members_after_consensus: None,
+                    },
+                }),
+            },
+            &Entry {
+                log_id: LogId { term: 1, index: 2 },
+                payload: EntryPayload::Blank,
+            },
+        ])
+        .await?;
+    }
+
+    tracing::info!("--- bring up cluster and elect");
+
+    router.new_raft_node_with_sto(0, sto0.clone()).await;
+    router.new_raft_node_with_sto(1, sto1.clone()).await;
+
+    router
+        .wait_for_state(&btreeset! {0}, State::Leader, timeout(), "only node 0 becomes leader")
+        .await?;
+
+    Ok(())
+}
+
+fn timeout() -> Option<Duration> {
+    Some(Duration::from_millis(5000))
+}

async-raft/tests/fixtures/mod.rs

@@ -204,6 +204,10 @@ impl RaftRouter {
         self.new_raft_node_with_sto(id, memstore).await
     }
 
+    pub fn new_store(self: &Arc<Self>, id: u64) -> Arc<MemStore> {
+        Arc::new(MemStore::new(id))
+    }
+
     pub async fn new_raft_node_with_sto(self: &Arc<Self>, id: NodeId, sto: Arc<MemStore>) {
         let node = Raft::new(id, self.config.clone(), self.clone(), sto.clone());
         let mut rt = self.routing_table.write().await;