Add rollback_config API to state machine (#468)

* State machine API should be invoked for not only app logs, but also
config logs, not to miss any index number.

include/libnuraft/state_machine.hxx

@@ -122,6 +122,18 @@ public:
     virtual void rollback(const ulong log_idx,
                           buffer& data) {}
 
+    /**
+     * (Optional)
+     * Handler on the rollback of a configuration change.
+     * The configuration can be either committed or uncommitted one,
+     * and that can be checked by the given `log_idx`, comparing it with
+     * the current `cluster_config`'s log index.
+     *
+     * @param log_idx Raft log number of the configuration change.
+     * @param new_conf The cluster configuration to be rolled back.
+     */
+    virtual void rollback_config(const ulong log_idx, ptr<cluster_config>& new_conf) { }
+
     /**
      * (Optional)
      * Extended version of `rollback`, for users want to keep

src/handle_append_entries.cxx

@@ -765,15 +765,18 @@ ptr<resp_msg> raft_server::handle_append_entries(req_msg& req)
             for ( uint64_t ii = 0; ii < my_last_log_idx - log_idx + 1; ++ii ) {
                 uint64_t idx = my_last_log_idx - ii;
                 ptr<log_entry> old_entry = log_store_->entry_at(idx);
+                ptr<buffer> buf = old_entry->get_buf_ptr();
                 if (old_entry->get_val_type() == log_val_type::app_log) {
-                    ptr<buffer> buf = old_entry->get_buf_ptr();
                     buf->pos(0);
                     state_machine_->rollback_ext
                         ( state_machine::ext_op_params( idx, buf ) );
                     p_in( "rollback log %" PRIu64 ", term %" PRIu64,
                           idx, old_entry->get_term() );
 
                 } else if (old_entry->get_val_type() == log_val_type::conf) {
+                    ptr<cluster_config> conf_to_rollback =
+                        cluster_config::deserialize(*buf);
+                    state_machine_->rollback_config(idx, conf_to_rollback);
                     p_in( "revert from a prev config change to config at %" PRIu64,
                           get_config()->get_log_idx() );
                     config_changing_ = false;