Add a callback for leader resignation (#484)

* Callback can decline the resignation request.

include/libnuraft/callback.hxx

@@ -199,7 +199,14 @@ public:
          * Snapshot creation begins.
          * ctx: pointer to `uint64_t` (committed_idx).
          */
-        SnapshotCreationBegin = 26
+        SnapshotCreationBegin = 26,
+
+        /**
+         * Got a resgination request either automatically or manually.
+         * ctx: null.
+         */
+        ResignationFromLeader = 27,
+
     };
 
     struct Param {

src/raft_server.cxx

@@ -1202,6 +1202,17 @@ void raft_server::yield_leadership(bool immediate_yield,
         return;
     }
 
+    // Callback if necessary.
+    cb_func::Param param(id_, leader_, successor_id, nullptr);
+    cb_func::ReturnCode cb_ret =
+        ctx_->cb_func_.call(cb_func::ResignationFromLeader, &param);
+
+    // If callback function decided to refuse this request, return here.
+    if (cb_ret != cb_func::Ok) {
+        p_in("[RESIGNATION REQUEST] refused by callback function");
+        return;
+    }
+
     recur_lock(lock_);
 
     if (immediate_yield) {

tests/unit/raft_package_asio.hxx

@@ -235,6 +235,10 @@ public:
         }
     }
 
+    void dbgLog(const std::string& msg) {
+        _s_info(myLogWrapper->getLogger()) << msg;
+    }
+
     TestMgr* getTestMgr() const {
         return static_cast<TestMgr*>(sMgr.get());
     }

tests/unit/raft_server_test.cxx

@@ -1436,7 +1436,34 @@ int leadership_takeover_by_request_test() {
     // Request leadership by the current leader, should fail.
     CHK_FALSE( s1.raftServer->request_leadership() );
 
-    // S3 requests the leadership from S1.
+    // Set callback function to refuse resignation.
+    bool refuse_request = true;
+    s1.ctx->set_cb_func([&](cb_func::Type t, cb_func::Param* p) -> cb_func::ReturnCode {
+        if (t != cb_func::Type::ResignationFromLeader) {
+            return cb_default(t, p);
+        }
+        if (refuse_request) {
+            return cb_func::ReturnCode::ReturnNull;
+        }
+        return cb_func::ReturnCode::Ok;
+    });
+
+    // S3 requests the leadership from S1, and it is supposed to be declined.
+    s1.dbgLog(" --- request leadership ---");
+    CHK_TRUE( s3.raftServer->request_leadership() );
+    // Send request.
+    s3.fNet->execReqResp();
+
+    // Send heartbeat.
+    s1.fTimer->invoke( timer_task_type::heartbeat_timer );
+    s1.fNet->execReqResp();
+    s1.fNet->execReqResp();
+
+    // S1 should still be the leader.
+    CHK_TRUE( s1.raftServer->is_leader() );
+
+    // S3 requests the leadership from S1. Now it should succeed.
+    refuse_request = false;
     s1.dbgLog(" --- request leadership ---");
     CHK_TRUE( s3.raftServer->request_leadership() );
     // Send request.