reset max_apply_unpersisted_log_limit in become_follower

Signed-off-by: glorv <[email protected]>

harness/tests/integration_cases/test_raft.rs

@@ -1874,12 +1874,15 @@ fn test_leader_stepdown_when_quorum_lost() {
 
     sm.become_candidate();
     sm.become_leader();
+    sm.raft_log.max_apply_unpersisted_log_limit = 100;
 
     for _ in 0..=sm.election_timeout() {
         sm.tick();
     }
 
     assert_eq!(sm.state, StateRole::Follower);
+    // check after become follower, the limit is reset.
+    assert_eq!(sm.raft_log.max_apply_unpersisted_log_limit, 0);
 }
 
 #[test]

src/raft.rs

@@ -1152,6 +1152,12 @@ impl<T: Storage> Raft<T> {
         let from_role = self.state;
         self.state = StateRole::Follower;
         self.pending_request_snapshot = pending_request_snapshot;
+        // TODO: In theory, we should better control this in the caller,
+        // but bacuase the caller only know the leadership changes after
+        // call `ready()` and the committed entries is fetched within `ready()`,
+        // so we hard code this logic here. We may need to remove this hard code
+        // when we want to also support apply unpersisted log on follower.
+        self.raft_log.max_apply_unpersisted_log_limit = 0;
         info!(
             self.logger,
             "became follower at term {term}",