tstest/natlab/vnet: add pcap support

Updates #13038

Change-Id: I89ce2129fee856f97986d6313d2b661c76476c0c
Signed-off-by: Maisem Ali <maisem@tailscale.com>

tstest/natlab/vnet/pcap.go

@@ -0,0 +1,39 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package vnet
+
+import (
+	"io"
+	"os"
+	"sync"
+
+	"github.com/google/gopacket"
+	"github.com/google/gopacket/pcapgo"
+)
+
+type pcapWriter struct {
+	f *os.File
+
+	mu sync.Mutex
+	w  *pcapgo.NgWriter
+}
+
+func (p *pcapWriter) WritePacket(ci gopacket.CaptureInfo, data []byte) error {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+	if p.w == nil {
+		return io.ErrClosedPipe
+	}
+	return p.w.WritePacket(ci, data)
+}
+
+func (p *pcapWriter) Close() error {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+	if p.w != nil {
+		p.w.Flush()
+		p.w = nil
+	}
+	return p.f.Close()
+}