fix a data race in KCP

transport/internet/kcp/connection.go

@@ -8,7 +8,6 @@ import (
 	"time"
 
 	"v2ray.com/core/app/log"
-	"v2ray.com/core/common"
 	"v2ray.com/core/common/buf"
 	"v2ray.com/core/common/predicate"
 )
@@ -360,16 +359,12 @@ func (v *Connection) Write(b []byte) (int, error) {
 			return totalWritten, io.ErrClosedPipe
 		}
 
-		for {
-			rb := v.sendingWorker.Push()
-			if rb == nil {
-				break
-			}
-			common.Must(rb.Reset(func(bb []byte) (int, error) {
-				return copy(bb[:v.mss], b[totalWritten:]), nil
-			}))
+		for v.sendingWorker.Push(func(bb []byte) (int, error) {
+			n := copy(bb[:v.mss], b[totalWritten:])
+			totalWritten += n
+			return n, nil
+		}) {
 			v.dataUpdater.WakeUp()
-			totalWritten += rb.Len()
 			if totalWritten == len(b) {
 				return totalWritten, nil
 			}
@@ -390,14 +385,9 @@ func (v *Connection) WriteMultiBuffer(mb buf.MultiBuffer) error {
 			return io.ErrClosedPipe
 		}
 
-		for {
-			rb := v.sendingWorker.Push()
-			if rb == nil {
-				break
-			}
-			common.Must(rb.Reset(func(bb []byte) (int, error) {
+		for v.sendingWorker.Push(func(bb []byte) (int, error) {
 			return mb.Read(bb[:v.mss])
-			}))
+		}) {
 			v.dataUpdater.WakeUp()
 			if mb.IsEmpty() {
 				return nil

transport/internet/kcp/sending.go

@@ -3,6 +3,7 @@ package kcp
 import (
 	"sync"
 
+	"v2ray.com/core/common"
 	"v2ray.com/core/common/buf"
 )
 
@@ -284,17 +285,18 @@ func (v *SendingWorker) ProcessSegment(current uint32, seg *AckSegment, rto uint
 	}
 }
 
-func (v *SendingWorker) Push() *buf.Buffer {
+func (v *SendingWorker) Push(f buf.Supplier) bool {
 	v.Lock()
 	defer v.Unlock()
 
 	if v.window.IsFull() {
-		return nil
+		return false
 	}
 
 	b := v.window.Push(v.nextNumber)
 	v.nextNumber++
-	return b
+	common.Must(b.Reset(f))
+	return true
 }
 
 func (v *SendingWorker) Write(seg Segment) error {