add on_install_snapshot callback for statemachine

src/braft/fsm_caller.cpp

@@ -496,6 +496,10 @@ void FSMCaller::do_stop_following(const LeaderChangeContext& stop_following_cont
     _fsm->on_stop_following(stop_following_context);
 }
 
+void FSMCaller::on_install_snapshot_start(const PeerId& peer_id) {
+    _fsm->on_install_snapshot_start(peer_id);
+}
+
 void FSMCaller::describe(std::ostream &os, bool use_html) {
     const char* newline = (use_html) ? "<br>" : "\n";
     TaskType cur_task = _cur_task;

src/braft/fsm_caller.h

@@ -116,6 +116,7 @@ class BAIDU_CACHELINE_ALIGNMENT FSMCaller {
     int on_leader_start(int64_t term, int64_t lease_epoch);
     int on_start_following(const LeaderChangeContext& start_following_context);
     int on_stop_following(const LeaderChangeContext& stop_following_context);
+    void on_install_snapshot_start(const PeerId& peer_id);
     BRAFT_MOCK int on_error(const Error& e);
     int64_t last_applied_index() const {
         return _last_applied_index.load(butil::memory_order_relaxed);

src/braft/node.cpp

@@ -1365,6 +1365,11 @@ void NodeImpl::on_error(const Error& e) {
     lck.unlock();
 }
 
+void NodeImpl::install_snapshot_start(const PeerId& peer_id) {
+    _fsm_caller->on_install_snapshot_start(peer_id);
+}
+
+
 void NodeImpl::handle_vote_timeout() {
     std::unique_lock<raft_mutex_t> lck(_mutex);
 

src/braft/node.h

@@ -241,6 +241,9 @@ friend class VoteBallotCtx;
 
     bool disable_cli() const { return _options.disable_cli; }
     bool is_witness() const { return _options.witness; }
+
+    // Called when leader start to install snapshot to remote peer
+    void install_snapshot_start(const PeerId& peer_id);
 private:
 friend class butil::RefCountedThreadSafe<NodeImpl>;
 

src/braft/raft.cpp

@@ -313,6 +313,7 @@ void StateMachine::on_configuration_committed(const Configuration& conf, int64_t
 
 void StateMachine::on_stop_following(const LeaderChangeContext&) {}
 void StateMachine::on_start_following(const LeaderChangeContext&) {}
+void StateMachine::on_install_snapshot_start(const PeerId& peer_id) {}
 
 BootstrapOptions::BootstrapOptions()
     : last_log_index(0)

src/braft/raft.h

@@ -265,6 +265,11 @@ class StateMachine {
     // the very leader whom the follower starts to follow.
     // User can reset the node's information as it starts to follow some leader.
     virtual void on_start_following(const ::braft::LeaderChangeContext& ctx);
+
+    // Invoked when the leader start to send snapshot to |peer_id|
+    // Default: Do nothing
+    virtual void on_install_snapshot_start(const PeerId& peer_id);
+
 };
 
 enum State {

src/braft/replicator.cpp

@@ -789,6 +789,8 @@ void Replicator::_install_snapshot() {
                                             add_one_more_task(true)) {
         return _block(butil::gettimeofday_us(), EBUSY);
     }
+
+    node_impl->install_snapshot_start(_options.peer_id);
 
     // pre-set replicator state to INSTALLING_SNAPSHOT, so replicator could be
     // blocked if something is wrong, such as throttled for a period of time