tstest/natlab/vnet: add start of virtual network-based NAT Lab

Updates #13038

Change-Id: I3c74120d73149c1329288621f6474bbbcaa7e1a6
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tta/tta.go

@@ -0,0 +1,159 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// The tta server is the Tailscale Test Agent.
+//
+// It runs on each Tailscale node being integration tested and permits the test
+// harness to control the node. It connects out to the test drver (rather than
+// accepting any TCP connections inbound, which might be blocked depending on
+// the scenario being tested) and then the test driver turns the TCP connection
+// around and sends request back.
+package main
+
+import (
+	"bytes"
+	"errors"
+	"flag"
+	"fmt"
+	"io"
+	"log"
+	"net"
+	"net/http"
+	"os"
+	"os/exec"
+	"strings"
+	"sync"
+	"time"
+
+	"tailscale.com/util/set"
+	"tailscale.com/version/distro"
+)
+
+var (
+	driverAddr = flag.String("driver", "test-driver.tailscale:8008", "address of the test driver; by default we use the DNS name test-driver.tailscale which is special cased in the emulated network's DNS server")
+)
+
+type chanListener <-chan net.Conn
+
+func serveCmd(w http.ResponseWriter, cmd string, args ...string) {
+	if distro.Get() == distro.Gokrazy && !strings.Contains(cmd, "/") {
+		cmd = "/user/" + cmd
+	}
+	out, err := exec.Command(cmd, args...).CombinedOutput()
+	w.Header().Set("Content-Type", "text/plain; charset=utf-8")
+	if err != nil {
+		w.Header().Set("Exec-Err", err.Error())
+		w.WriteHeader(500)
+	}
+	w.Write(out)
+}
+
+func main() {
+	if distro.Get() == distro.Gokrazy {
+		cmdLine, _ := os.ReadFile("/proc/cmdline")
+		if !bytes.Contains(cmdLine, []byte("tailscale-tta=1")) {
+			// "Exiting immediately with status code 0 when the
+			// GOKRAZY_FIRST_START=1 environment variable is set means “don’t
+			// start the program on boot”"
+			return
+		}
+	}
+	flag.Parse()
+	log.Printf("Tailscale Test Agent running.")
+
+	var mux http.ServeMux
+	var hs http.Server
+	hs.Handler = &mux
+	var (
+		stMu   sync.Mutex
+		newSet = set.Set[net.Conn]{} // conns in StateNew
+	)
+	needConnCh := make(chan bool, 1)
+	hs.ConnState = func(c net.Conn, s http.ConnState) {
+		stMu.Lock()
+		defer stMu.Unlock()
+		switch s {
+		case http.StateNew:
+			newSet.Add(c)
+		case http.StateClosed:
+			newSet.Delete(c)
+		}
+		if len(newSet) == 0 {
+			select {
+			case needConnCh <- true:
+			default:
+			}
+		}
+	}
+	conns := make(chan net.Conn, 1)
+
+	mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
+		io.WriteString(w, "TTA\n")
+		return
+	})
+	mux.HandleFunc("/up", func(w http.ResponseWriter, r *http.Request) {
+		serveCmd(w, "tailscale", "up", "--auth-key=test")
+	})
+	mux.HandleFunc("/status", func(w http.ResponseWriter, r *http.Request) {
+		serveCmd(w, "tailscale", "status", "--json")
+	})
+	mux.HandleFunc("/ping", func(w http.ResponseWriter, r *http.Request) {
+		target := r.FormValue("target")
+		cmd := exec.Command("tailscale", "ping", target)
+		w.Header().Set("Content-Type", "text/plain; charset=utf-8")
+		w.(http.Flusher).Flush()
+		cmd.Stdout = w
+		cmd.Stderr = w
+		if err := cmd.Run(); err != nil {
+			fmt.Fprintf(w, "error: %v\n", err)
+		}
+	})
+	go hs.Serve(chanListener(conns))
+
+	var lastErr string
+	needConnCh <- true
+	for {
+		<-needConnCh
+		c, err := connect()
+		log.Printf("Connect: %v", err)
+		if err != nil {
+			s := err.Error()
+			if s != lastErr {
+				log.Printf("Connect failure: %v", s)
+			}
+			lastErr = s
+			time.Sleep(time.Second)
+			continue
+		}
+		conns <- c
+
+		time.Sleep(time.Second)
+	}
+}
+
+func connect() (net.Conn, error) {
+	c, err := net.Dial("tcp", *driverAddr)
+	if err != nil {
+		return nil, err
+	}
+	return c, nil
+}
+
+func (cl chanListener) Accept() (net.Conn, error) {
+	c, ok := <-cl
+	if !ok {
+		return nil, errors.New("closed")
+	}
+	return c, nil
+}
+
+func (cl chanListener) Close() error {
+	return nil
+}
+
+func (cl chanListener) Addr() net.Addr {
+	return &net.TCPAddr{
+		IP:   net.ParseIP("52.0.0.34"), // TS..DR(iver)
+		Port: 123,
+	}
+}