Allow reuse of slices exposed through Ready

raft.go

@@ -346,7 +346,11 @@ type raft struct {
 	// Messages in this list have the type MsgAppResp, MsgVoteResp, or
 	// MsgPreVoteResp. See the comment in raft.send for details.
 	msgsAfterAppend []pb.Message
-
+	// msgsAfterAppendPool is a pool for msgsAfterAppend to avoid frequently
+	// reallocating the underlying array. When assigning a new slice to msgsAfterAppend,
+	// we should try to get it from the pool first; before releasing the msgsAfterAppend
+	// slice, we should put the underlying array into the pool.
+	msgsAfterAppendPool *sync.Pool
 	// the leader id
 	lead uint64
 	// leadTransferee is id of the leader transfer target when its value is not zero.
@@ -424,6 +428,11 @@ func newRaft(c *Config) *raft {
 		preVote:                   c.PreVote,
 		readOnly:                  newReadOnly(c.ReadOnlyOption),
 		disableProposalForwarding: c.DisableProposalForwarding,
+		msgsAfterAppendPool: &sync.Pool{
+			New: func() interface{} {
+				return make([]pb.Message, 0, 8)
+			},
+		},
 	}
 
 	cfg, prs, err := confchange.Restore(confchange.Changer{
@@ -545,6 +554,9 @@ func (r *raft) send(m pb.Message) {
 		// because the safety of such behavior has not been formally verified,
 		// we err on the side of safety and omit a `&& !m.Reject` condition
 		// above.
+		if cap(r.msgsAfterAppend) == 0 {
+			r.msgsAfterAppend = r.msgsAfterAppendPool.Get().([]pb.Message)
+		}
 		r.msgsAfterAppend = append(r.msgsAfterAppend, m)
 	} else {
 		if m.To == r.id {

rawnode.go

@@ -252,9 +252,19 @@ func newStorageAppendMsg(r *raft, rd Ready) pb.Message {
 	// be contained in msgsAfterAppend). This ordering allows the MsgAppResp
 	// handling to use a fast-path in r.raftLog.term() before the newly appended
 	// entries are removed from the unstable log.
-	m.Responses = r.msgsAfterAppend
+
+	// msgsAfterAppend is a pooled slice, so we need to make a deep copy
+	msgsCount, needResp := len(r.msgsAfterAppend), false
 	if needStorageAppendRespMsg(r, rd) {
-		m.Responses = append(m.Responses, newStorageAppendRespMsg(r, rd))
+		msgsCount++
+		needResp = true
+	}
+	if msgsCount != 0 {
+		// NB: allocating precisely as much as needed.
+		m.Responses = append(make([]pb.Message, 0, msgsCount), r.msgsAfterAppend...)
+		if needResp {
+			m.Responses = append(m.Responses, newStorageAppendRespMsg(r, rd))
+		}
 	}
 	return m
 }
@@ -428,6 +438,11 @@ func (rn *RawNode) acceptReady(rd Ready) {
 		}
 	}
 	rn.raft.msgs = nil
+	// Release the msgsAfterAppend slice and put it back into the msgsAfterAppendPool.
+	// This clears the slice but reserves the underlying array, which we can pool and reuse.
+	if cap(rn.raft.msgsAfterAppend) > 0 {
+		rn.raft.msgsAfterAppendPool.Put(rn.raft.msgsAfterAppend[:0])
+	}
 	rn.raft.msgsAfterAppend = nil
 	rn.raft.raftLog.acceptUnstable()
 	if len(rd.CommittedEntries) > 0 {