Add multi buf funcs

Author: nekohasekai

common/buf/multi.go

@@ -1,6 +1,9 @@
 package buf
 
-import "github.com/sagernet/sing/common"
+import (
+	"github.com/sagernet/sing/common"
+	"io"
+)
 
 func LenMulti(buffers []*Buffer) int {
 	var n int
@@ -10,6 +13,34 @@ func LenMulti(buffers []*Buffer) int {
 	return n
 }
 
+func WriteMulti(writer io.Writer, buffers []*Buffer) error {
+	for _, buffer := range buffers {
+		_, err := writer.Write(buffer.Bytes())
+		if err != nil {
+			ReleaseMulti(buffers)
+			return err
+		}
+		buffer.Release()
+	}
+	return nil
+}
+
+func MergeMulti(buffers []*Buffer) *Buffer {
+	switch len(buffers) {
+	case 0:
+		panic("empty buffers")
+	case 1:
+		return buffers[0]
+	default:
+		newBuffer := NewSize(LenMulti(buffers))
+		for _, buffer := range buffers {
+			common.Must1(newBuffer.Write(buffer.Bytes()))
+			buffer.Release()
+		}
+		return newBuffer
+	}
+}
+
 func ToSliceMulti(buffers []*Buffer) [][]byte {
 	return common.Map(buffers, func(it *Buffer) []byte {
 		return it.Bytes()