Resolve some comments

Signed-off-by: caojiamingalan <[email protected]>

raft.go

@@ -347,7 +347,9 @@ type raft struct {
 	// 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
+	// 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
@@ -428,8 +430,7 @@ func newRaft(c *Config) *raft {
 		disableProposalForwarding: c.DisableProposalForwarding,
 		msgsAfterAppendPool: &sync.Pool{
 			New: func() interface{} {
-				sl := make([]pb.Message, 0, 8)
-				return sl
+				return make([]pb.Message, 0, 8)
 			},
 		},
 	}

rawnode.go

@@ -253,11 +253,18 @@ func newStorageAppendMsg(r *raft, rd Ready) pb.Message {
 	// handling to use a fast-path in r.raftLog.term() before the newly appended
 	// entries are removed from the unstable log.
 
-	//msgsAfterAppend is a pooled slice, so we need to make a deep copy
-	m.Responses = make([]pb.Message, len(r.msgsAfterAppend))
-	copy(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
 }
@@ -431,7 +438,8 @@ func (rn *RawNode) acceptReady(rd Ready) {
 		}
 	}
 	rn.raft.msgs = nil
-	//release the msgsAfterAppend slice and put it into the msgsAfterAppendPool
+	// 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])
 	}