common/buf/multi_buffer.go: Fix Compact() (#5015)

Fixes https://github.com/XTLS/Xray-core/issues/5012

Co-authored-by: patterniha <71074308+patterniha@users.noreply.github.com>

common/buf/buffer.go

@@ -244,6 +244,14 @@ func (b *Buffer) Cap() int32 {
 	return int32(len(b.v))
 }
 
+// Available returns the available capacity of the buffer content.
+func (b *Buffer) Available() int32 {
+	if b == nil {
+		return 0
+	}
+	return int32(len(b.v)) - b.end
+}
+
 // IsEmpty returns true if the buffer is empty.
 func (b *Buffer) IsEmpty() bool {
 	return b.Len() == 0

common/buf/multi_buffer.go

@@ -144,7 +144,7 @@ func Compact(mb MultiBuffer) MultiBuffer {
 
 	for i := 1; i < len(mb); i++ {
 		curr := mb[i]
-		if last.Len()+curr.Len() > Size {
+		if curr.Len() > last.Available() {
 			mb2 = append(mb2, last)
 			last = curr
 		} else {

common/buf/multi_buffer_test.go

@@ -175,6 +175,29 @@ func TestCompact(t *testing.T) {
 	}
 }
 
+func TestCompactWithConsumed(t *testing.T) {
+	// make a consumed buffer (a.Start != 0)
+	a := New()
+	for range 8192 {
+		common.Must2(a.WriteString("a"))
+	}
+	a.Read(make([]byte, 2))
+
+	b := New()
+	for range 2 {
+		common.Must2(b.WriteString("b"))
+	}
+
+	mb := MultiBuffer{a, b}
+	cmb := Compact(mb)
+	mbc := &MultiBufferContainer{mb}
+	mbc.Read(make([]byte, 8190))
+
+	if w := cmb.String(); w != "bb" {
+		t.Error("unexpected Compact result ", w)
+	}
+}
+
 func BenchmarkSplitBytes(b *testing.B) {
 	var mb MultiBuffer
 	raw := make([]byte, Size)