QUIC sniffer: handle multiple initial packets (#3802)

* QUIC sniffer: handle multiple initial packets
Basically copied from Vigilans/v2ray-core@8f33db0

Co-Authored-By: Vigilans <[email protected]>

* Remove unnecessary file

---------

Co-authored-by: Vigilans <[email protected]>

app/dispatcher/default.go

@@ -41,8 +41,14 @@ func (r *cachedReader) Cache(b *buf.Buffer) {
 	if !mb.IsEmpty() {
 		r.cache, _ = buf.MergeMulti(r.cache, mb)
 	}
-	b.Clear()
-	rawBytes := b.Extend(buf.Size)
+	cacheLen := r.cache.Len()
+	if cacheLen <= b.Cap() {
+		b.Clear()
+	} else {
+		b.Release()
+		*b = *buf.NewWithSize(cacheLen)
+	}
+	rawBytes := b.Extend(cacheLen)
 	n := r.cache.Copy(rawBytes)
 	b.Resize(0, int32(n))
 	r.Unlock()

common/buf/buffer.go

@@ -207,6 +207,21 @@ func (b *Buffer) Len() int32 {
 	return b.end - b.start
 }
 
+// Cap returns the capacity of the buffer content.
+func (b *Buffer) Cap() int32 {
+	if b == nil {
+		return 0
+	}
+	return int32(len(b.v))
+}
+
+// NewWithSize creates a Buffer with 0 length and capacity with at least the given size.
+func NewWithSize(size int32) *Buffer {
+	return &Buffer{
+		v:         bytespool.Alloc(size),
+	}
+}
+
 // IsEmpty returns true if the buffer is empty.
 func (b *Buffer) IsEmpty() bool {
 	return b.Len() == 0