feat: port-filtering experiment (#891)

Part of https://github.com/ooni/probe/issues/2005

internal/cmd/ooporthelper/main.go

@@ -0,0 +1,79 @@
+// command ooporthelper implements the Port Filtering test helper
+package main
+
+import (
+	"context"
+	"flag"
+	"net"
+	"sync"
+	"time"
+
+	"github.com/apex/log"
+	"github.com/ooni/probe-cli/v3/internal/engine/experiment/portfiltering"
+	"github.com/ooni/probe-cli/v3/internal/runtimex"
+)
+
+var (
+	srvCtx      context.Context
+	srvCancel   context.CancelFunc
+	srvWg       = new(sync.WaitGroup)
+	srvTestChan = make(chan string, len(TestPorts)) // buffered channel for testing
+	srvTest     bool
+)
+
+func init() {
+	srvCtx, srvCancel = context.WithCancel(context.Background())
+}
+
+func shutdown(ctx context.Context, l net.Listener) {
+	<-ctx.Done()
+	l.Close()
+}
+
+// TODO(DecFox): Add the ability of an echo service to generate some traffic
+func handleConnetion(ctx context.Context, conn net.Conn) {
+	defer conn.Close()
+	ctx, cancel := context.WithTimeout(ctx, 10*time.Second)
+	defer cancel()
+	<-ctx.Done()
+}
+
+func listenTCP(ctx context.Context, port string) {
+	defer srvWg.Done()
+	address := net.JoinHostPort("127.0.0.1", port)
+	listener, err := net.Listen("tcp", address)
+	runtimex.PanicOnError(err, "net.Listen failed")
+	go shutdown(ctx, listener)
+	srvTestChan <- port // send to channel to imply server will start listening on port
+	for {
+		conn, err := listener.Accept()
+		if err != nil {
+			log.Infof("listener unable to accept connections on port%s", port)
+			return
+		}
+		go handleConnetion(ctx, conn)
+	}
+}
+
+func main() {
+	logmap := map[bool]log.Level{
+		true:  log.DebugLevel,
+		false: log.InfoLevel,
+	}
+	debug := flag.Bool("debug", false, "Toggle debug mode")
+	flag.Parse()
+	log.SetLevel(logmap[*debug])
+	defer srvCancel()
+	ports := portfiltering.Ports
+	if srvTest {
+		ports = TestPorts
+	}
+	for _, port := range ports {
+		srvWg.Add(1)
+		ctx, cancel := context.WithCancel(srvCtx)
+		defer cancel()
+		go listenTCP(ctx, port)
+	}
+	<-srvCtx.Done()
+	srvWg.Wait() // wait for listeners on all ports to close
+}