From 092997ab67eb2529c5b40556e1d301c8220c315d Mon Sep 17 00:00:00 2001
From: Jung-Sang Ahn <jungsang.ahn@gmail.com>
Date: Tue, 24 Sep 2024 12:37:09 -0700
Subject: [PATCH] Add a callback to handle misbehaving messages from peer
 (#537)

* Callback can decide whether to ignore or to die.
---
 include/libnuraft/callback.hxx | 21 +++++++++++++++++++++
 src/handle_append_entries.cxx  | 16 ++++++++++++++--
 2 files changed, 35 insertions(+), 2 deletions(-)

diff --git a/include/libnuraft/callback.hxx b/include/libnuraft/callback.hxx
index de40f327..5c1ea581 100644
--- a/include/libnuraft/callback.hxx
+++ b/include/libnuraft/callback.hxx
@@ -18,6 +18,9 @@ limitations under the License.
 #ifndef _CALLBACK_H_
 #define _CALLBACK_H_
 
+#include "req_msg.hxx"
+#include "resp_msg.hxx"
+
 #include <cstdint>
 #include <functional>
 #include <string>
@@ -214,6 +217,18 @@ public:
          * ctx: null.
          */
         FollowerLost = 28,
+
+        /**
+         * When the server receives a misbehaving message from a peer,
+         * the callback has the ability to either ignore the message
+         * or respond normally by adjusting ReqResp.resp as indicated by ctx.
+         *
+         * Furthermore, the callback can opt to terminate
+         * if the situation is deemed critical.
+         *
+         * ctx: pointer to `ReqResp` instance.
+         */
+        ReceivedMisbehavingMessage = 29,
     };
 
     struct Param {
@@ -232,6 +247,12 @@ public:
         void* ctx;
     };
 
+    struct ReqResp {
+        ReqResp() : req(nullptr), resp(nullptr) {}
+        req_msg* req;
+        ptr<resp_msg> resp;
+    };
+
     enum ReturnCode {
         Ok = 0,
         ReturnNull = -1,
diff --git a/src/handle_append_entries.cxx b/src/handle_append_entries.cxx
index d23d97a8..a07d5b88 100644
--- a/src/handle_append_entries.cxx
+++ b/src/handle_append_entries.cxx
@@ -593,9 +593,21 @@ ptr<resp_msg> raft_server::handle_append_entries(req_msg& req)
             become_follower();
         } else if (role_ == srv_role::leader) {
             p_wn( "Receive AppendEntriesRequest from another leader (%d) "
-                  "with same term, there must be a bug. Ignore it instead of exit.",
+                  "with same term, there must be a bug. Invoking the callback.",
                   req.get_src() );
-            return nullptr;
+
+            cb_func::Param param(id_, leader_, -1, &req);
+            cb_func::ReqResp req_resp;
+            req_resp.req = &req;
+
+            ctx_->cb_func_.call(cb_func::ReceivedMisbehavingMessage, &param);
+            if (!req_resp.resp.get()) {
+                p_wn("callback function didn't return response, ignore this request");
+            } else {
+                p_wn("callback function returned response, send it back");
+            }
+            return req_resp.resp;
+
         } else {
             update_target_priority();
             // Modified by JungSang Ahn, Mar 28 2018: