separate maybeCompactRaftLog function to compact raft log independently from snapshots

Signed-off-by: Clement <gh.2lgqz@aleeas.com>

server/etcdserver/server.go

@@ -980,6 +980,7 @@ func (s *EtcdServer) applyAll(ep *etcdProgress, apply *toApply) {
 	<-apply.notifyc
 
 	s.triggerSnapshot(ep)
+	s.maybeCompactRaftLog(ep)
 	select {
 	// snapshot requested via send()
 	case m := <-s.r.msgSnapC:
@@ -2170,6 +2171,21 @@ func (s *EtcdServer) snapshot(snapi uint64, confState raftpb.ConfState) {
 		"saved snapshot",
 		zap.Uint64("snapshot-index", snap.Metadata.Index),
 	)
+}
+
+func (s *EtcdServer) maybeCompactRaftLog(ep *etcdProgress) {
+	// Retain all log entries up to the latest snapshot index to ensure any member can recover from that snapshot.
+	// Beyond the snapshot index, preserve the most recent s.Cfg.SnapshotCatchUpEntries entries in memory.
+	// This allows slow followers to catch up by synchronizing entries instead of requiring a full snapshot transfer.
+	// Only compact raft log once every N applies
+	if ep.snapi <= s.Cfg.SnapshotCatchUpEntries  {
+		return
+	}
+
+	// make sure compacti >= 0, because s.r.raftStorage.Compact(compacti) always returns ErrCompacted if compacti <= 0
+	compacti := ep.snapi - s.Cfg.SnapshotCatchUpEntries
+
+	lg := s.Logger()
 
 	// When sending a snapshot, etcd will pause compaction.
 	// After receives a snapshot, the slow follower needs to get all the entries right after
@@ -2181,13 +2197,7 @@ func (s *EtcdServer) snapshot(snapi uint64, confState raftpb.ConfState) {
 		return
 	}
 
-	// keep some in memory log entries for slow followers.
+	err := s.r.raftStorage.Compact(compacti)
-	compacti := uint64(1)
-	if snapi > s.Cfg.SnapshotCatchUpEntries {
-		compacti = snapi - s.Cfg.SnapshotCatchUpEntries
-	}
-
-	err = s.r.raftStorage.Compact(compacti)
 	if err != nil {
 		// the compaction was done asynchronously with the progress of raft.
 		// raft log might already been compact.