Fix: save leader_id if a higher term is seen when handling append-ent…

…ries RPC

Problem:

A follower saves hard state `(term=msg.term, voted_for=None)`
when a `msg.term > local.term` when handling append-entries RPC.

This is quite enough to be correct but not perfect. Correct because:

- In one term, only an established leader will send append-entries;

- Thus, there is a quorum voted for this leader;

- Thus, no matter what `voted_for` is saved, it is still correct. E.g.
  when handling append-entries, a follower node could save hard state
  `(term=msg.term, voted_for=Some(ANY_VALUE))`.

The problem is that a follower already knows the legal leader for a term
but still does not save it. This leads to an unstable cluster state: The
test sometimes fails.

Solution:

A follower always save hard state with the id of a known legal leader.

openraft/src/core/append_entries.rs

@@ -69,7 +69,7 @@ impl<D: AppData, R: AppDataResponse, N: RaftNetwork<D>, S: RaftStorage<D, R>> Ra
             let mut report_metrics = false;
 
             if msg.term > self.current_term {
-                self.update_current_term(msg.term, None);
+                self.update_current_term(msg.term, Some(msg.leader_id));
                 self.save_hard_state().await?;
                 report_metrics = true;
             }

openraft/src/core/mod.rs

@@ -430,7 +430,7 @@ impl<D: AppData, R: AppDataResponse, N: RaftNetwork<D>, S: RaftStorage<D, R>> Ra
     }
 
     /// Encapsulate the process of updating the current term, as updating the `voted_for` state must also be updated.
-    #[tracing::instrument(level = "trace", skip(self))]
+    #[tracing::instrument(level = "debug", skip(self))]
     fn update_current_term(&mut self, new_term: u64, voted_for: Option<NodeId>) {
         if new_term > self.current_term {
             self.current_term = new_term;