perf(client): use transmitResultPool to reduce heap allocs

Struct pkg/varlog.(transmitResult) is an element of transmitQueue. It is
allocated in heap memory to be pushed into transmitQueue. This PR introduces
transmitResultPool to reuse transmitResult as possible.

pkg/varlog/subscribe.go

@@ -118,13 +118,31 @@ func (pq *PriorityQueue) Pop() interface{} {
 	return item
 }
 
+var transmitResultPool = sync.Pool{
+	New: func() any {
+		return &transmitResult{}
+	},
+}
+
 type transmitResult struct {
 	logStreamID   types.LogStreamID
 	storageNodeID types.StorageNodeID
 	result        client.SubscribeResult
 }
 
-func (t transmitResult) Priority() uint64 {
+func newTransmitResult(snid types.StorageNodeID, lsid types.LogStreamID) *transmitResult {
+	t := transmitResultPool.Get().(*transmitResult)
+	t.storageNodeID = snid
+	t.logStreamID = lsid
+	return t
+}
+
+func (t *transmitResult) Release() {
+	*t = transmitResult{}
+	transmitResultPool.Put(t)
+}
+
+func (t *transmitResult) Priority() uint64 {
 	return uint64(t.result.GLSN)
 }
 
@@ -133,37 +151,33 @@ type transmitQueue struct {
 	mu sync.Mutex
 }
 
-func (tq *transmitQueue) Push(r transmitResult) {
+func (tq *transmitQueue) Push(r *transmitResult) {
 	tq.mu.Lock()
 	defer tq.mu.Unlock()
 
 	heap.Push(tq.pq, r)
 }
 
-func (tq *transmitQueue) Pop() (transmitResult, bool) {
+func (tq *transmitQueue) Pop() (*transmitResult, bool) {
 	tq.mu.Lock()
 	defer tq.mu.Unlock()
 
 	if tq.pq.Len() == 0 {
-		return transmitResult{
-			result: client.InvalidSubscribeResult,
-		}, false
+		return nil, false
 	}
 
-	return heap.Pop(tq.pq).(transmitResult), true
+	return heap.Pop(tq.pq).(*transmitResult), true
 }
 
-func (tq *transmitQueue) Front() (transmitResult, bool) {
+func (tq *transmitQueue) Front() (*transmitResult, bool) {
 	tq.mu.Lock()
 	defer tq.mu.Unlock()
 
 	if tq.pq.Len() == 0 {
-		return transmitResult{
-			result: client.InvalidSubscribeResult,
-		}, false
+		return nil, false
 	}
 
-	return (*tq.pq)[0].(transmitResult), true
+	return (*tq.pq)[0].(*transmitResult), true
 }
 
 type subscriber struct {
@@ -229,11 +243,7 @@ func (s *subscriber) subscribe(ctx context.Context) {
 		case <-ctx.Done():
 			return
 		case res, ok := <-s.resultC:
-			r := transmitResult{
-				storageNodeID: s.storageNodeID,
-				logStreamID:   s.logStreamID,
-			}
-
+			r := newTransmitResult(s.storageNodeID, s.logStreamID)
 			if ok {
 				r.result = res
 			} else {
@@ -387,7 +397,7 @@ func (p *transmitter) handleTimeout(ctx context.Context) {
 	p.refreshSubscriber(ctx) //nolint:errcheck,revive // TODO: Handle an error returned.
 }
 
-func (p *transmitter) handleError(r transmitResult) error {
+func (p *transmitter) handleError(r *transmitResult) error {
 	s, ok := p.subscribers[r.logStreamID]
 	if !ok {
 		return nil
@@ -400,7 +410,7 @@ func (p *transmitter) handleError(r transmitResult) error {
 	return r.result.Error
 }
 
-func (p *transmitter) handleResult(r transmitResult) error {
+func (p *transmitter) handleResult(r *transmitResult) error {
 	var err error
 
 	/* NOTE Ignore transmitResult with GLSN less than p.wanted.
@@ -432,6 +442,7 @@ func (p *transmitter) transmitLoop(ctx context.Context) bool {
 		if res.result.GLSN <= p.wanted {
 			res, _ := p.transmitQ.Pop()
 			err := p.handleResult(res)
+			res.Release()
 			if p.wanted == p.end ||
 				errors.Is(err, verrors.ErrTrimmed) {
 				return false