Add a new API to reset the whole state machine.

examples/calculator/calc_state_machine.hxx

@@ -179,6 +179,8 @@ public:
 
         ptr<snapshot_ctx> ctx = entry->second;
         cur_value_ = ctx->value_;
+        // Now the last committed_idx is the snapshot's last log index.
+        last_committed_idx_ = s.get_last_log_idx();
         return true;
     }
 
@@ -213,6 +215,10 @@ public:
         }
     }
 
+    void reset() override {
+        cur_value_ = 0;
+    }
+
     int64_t get_current_value() const { return cur_value_; }
 
 private:

examples/echo/echo_state_machine.hxx

@@ -110,6 +110,8 @@ public:
             ptr<buffer> snp_buf = s.serialize();
             last_snapshot_ = snapshot::deserialize(*snp_buf);
         }
+        // Now the last committed_idx is the snapshot's last log index.
+        last_committed_idx_ = s.get_last_log_idx();
         return true;
     }
 
@@ -140,6 +142,10 @@ public:
         when_done(ret, except);
     }
 
+    void reset() override {
+        // echo state machine has no state to reset.
+    }
+
 private:
     // Last committed Raft log number.
     std::atomic<uint64_t> last_committed_idx_;

include/libnuraft/state_machine.hxx

@@ -327,6 +327,12 @@ public:
      */
     virtual bool allow_leadership_transfer() { return true; }
 
+    /**
+     * Reset the state machine clearing all data and related state,
+     * should be invoked before follower apply snapshot from leader.
+     */
+    virtual void reset(){};
+
     /**
      * Parameters for `adjust_commit_index` API.
      */

src/handle_snapshot_sync.cxx

@@ -559,6 +559,8 @@ bool raft_server::handle_snapshot_sync_req(snapshot_sync_req& req, std::unique_l
             stop_election_timer();
             p_in("successfully compact the log store, will now ask the "
                  "statemachine to apply the snapshot");
+            p_in("firstly reset the statemachine");
+            state_machine_->reset();
             if (!state_machine_->apply_snapshot(req.get_snapshot())) {
                 // LCOV_EXCL_START
                 p_er("failed to apply the snapshot after log compacted, "