Refactor: add noop_log_id to Leading

When a leader established, store the first log id in struct `Leading`,
i.e., the noop log id.

This log id can be used for linearizable read.

openraft/src/engine/engine_impl.rs

@@ -643,6 +643,18 @@ where C: RaftTypeConfig
             debug_assert!(_res.is_ok(), "commit vote can not fail but: {:?}", _res);
         }
 
+        // Update the noop log index
+        {
+            let vote = *self.state.vote_ref();
+            let index = self.state.last_log_id().next_index();
+
+            let leading = self.internal_server_state.leading_mut().unwrap();
+
+            // TODO: in future the leader will be able to start another new election without quit leader.
+            debug_assert!(leading.noop_log_id.is_none());
+            leading.noop_log_id = Some(LogId::new(vote.committed_leader_id().unwrap(), index));
+        }
+
         let mut rh = self.replication_handler();
 
         // It has to setup replication stream first because append_blank_log() may update the

openraft/src/engine/handler/vote_handler/mod.rs

@@ -153,7 +153,7 @@ where C: RaftTypeConfig
         // Re-create a new Leader instance.
 
         let em = &self.state.membership_state.effective();
-        let leader = Leading::new(
+        let leading = Leading::new(
             *self.state.vote_ref(),
             em.membership().to_quorum_set(),
             em.learner_ids(),
@@ -162,7 +162,7 @@ where C: RaftTypeConfig
 
         // Do not update clock_progress, until the first blank log is committed.
 
-        *self.internal_server_state = InternalServerState::Leading(Box::new(leader));
+        *self.internal_server_state = InternalServerState::Leading(Box::new(leading));
 
         self.server_state_handler().update_server_state_if_changed();
     }

openraft/src/engine/tests/elect_test.rs

@@ -49,6 +49,10 @@ fn test_elect() -> anyhow::Result<()> {
         eng.elect();
 
         assert_eq!(Vote::new_committed(1, 1), *eng.state.vote_ref());
+        assert_eq!(
+            Some(log_id(1, 1, 1)),
+            eng.internal_server_state.leading().unwrap().noop_log_id
+        );
         assert!(
             eng.internal_server_state.voting_mut().is_none(),
             "voting state is removed when becoming leader"
@@ -102,6 +106,11 @@ fn test_elect() -> anyhow::Result<()> {
         eng.elect();
 
         assert_eq!(Vote::new_committed(2, 1), *eng.state.vote_ref());
+        assert_eq!(
+            Some(log_id(2, 1, 1)),
+            eng.internal_server_state.leading().unwrap().noop_log_id
+        );
+
         assert!(
             eng.internal_server_state.voting_mut().is_none(),
             "voting state is removed when becoming leader"
@@ -151,6 +160,8 @@ fn test_elect() -> anyhow::Result<()> {
         eng.elect();
 
         assert_eq!(Vote::new(1, 1), *eng.state.vote_ref());
+        assert_eq!(None, eng.internal_server_state.leading().unwrap().noop_log_id);
+
         assert_eq!(
             Some(btreeset! {1},),
             eng.internal_server_state.leading().map(|x| x.voting().unwrap().granters().collect::<BTreeSet<_>>())

openraft/src/engine/tests/handle_vote_resp_test.rs

@@ -90,6 +90,7 @@ fn test_handle_vote_resp() -> anyhow::Result<()> {
         });
 
         assert_eq!(Vote::new(2, 1), *eng.state.vote_ref());
+        assert_eq!(None, eng.internal_server_state.leading().unwrap().noop_log_id);
         assert_eq!(
             Some(btreeset! {1},),
             eng.internal_server_state.leading().map(|x| x.voting().unwrap().granters().collect::<BTreeSet<_>>())
@@ -163,6 +164,7 @@ fn test_handle_vote_resp() -> anyhow::Result<()> {
         });
 
         assert_eq!(Vote::new(2, 1), *eng.state.vote_ref());
+        assert_eq!(None, eng.internal_server_state.leading().unwrap().noop_log_id);
         assert_eq!(
             Some(btreeset! {1},),
             eng.internal_server_state.leading().map(|x| x.voting().unwrap().granters().collect::<BTreeSet<_>>())
@@ -199,6 +201,7 @@ fn test_handle_vote_resp() -> anyhow::Result<()> {
         });
 
         assert_eq!(Vote::new(2, 1), *eng.state.vote_ref());
+        assert_eq!(None, eng.internal_server_state.leading().unwrap().noop_log_id);
         assert_eq!(
             Some(btreeset! {1,2},),
             eng.internal_server_state.leading().map(|x| x.voting().unwrap().granters().collect::<BTreeSet<_>>())
@@ -235,6 +238,10 @@ fn test_handle_vote_resp() -> anyhow::Result<()> {
         });
 
         assert_eq!(Vote::new_committed(2, 1), *eng.state.vote_ref());
+        assert_eq!(
+            Some(log_id(2, 1, 1)),
+            eng.internal_server_state.leading().unwrap().noop_log_id
+        );
         assert!(
             eng.internal_server_state.voting_mut().is_none(),
             "voting state is removed when becoming leader"

openraft/src/leader/leader.rs

@@ -31,6 +31,12 @@ pub(crate) struct Leading<NID: NodeId, QS: QuorumSet<NID>, I: Instant> {
     /// The vote this leader works in.
     pub(crate) vote: Vote<NID>,
 
+    /// The log id of the first log entry proposed by this leader,
+    /// i.e., the `noop` log(AKA blank log) after leader established.
+    ///
+    /// It is set when leader established.
+    pub(crate) noop_log_id: Option<LogId<NID>>,
+
     quorum_set: QS,
 
     /// Voting state, i.e., there is a Candidate running.
@@ -63,6 +69,7 @@ where
 
         Self {
             vote,
+            noop_log_id: None,
             quorum_set: quorum_set.clone(),
             voting: None,
             progress: VecProgress::new(