add an integration test for DPLPMTUD (#4498)

* add an integration test for DPLPMTUD

* use a v4-only socket in DPLPMTUD test

* increase DPLPMTUD test timeout

integrationtests/self/dplpmtud_test.go

@@ -0,0 +1,101 @@
+package self_test
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"net"
+	"sync"
+	"time"
+
+	"github.com/quic-go/quic-go"
+	quicproxy "github.com/quic-go/quic-go/integrationtests/tools/proxy"
+
+	. "github.com/onsi/ginkgo/v2"
+	. "github.com/onsi/gomega"
+)
+
+var _ = Describe("DPLPMTUD", func() {
+	It("discovers the MTU", func() {
+		const rtt = 100 * time.Millisecond
+		const mtu = 1400
+
+		ln, err := quic.ListenAddr("localhost:0", getTLSConfig(), getQuicConfig(&quic.Config{DisablePathMTUDiscovery: true}))
+		Expect(err).ToNot(HaveOccurred())
+		defer ln.Close()
+		go func() {
+			defer GinkgoRecover()
+			conn, err := ln.Accept(context.Background())
+			Expect(err).ToNot(HaveOccurred())
+			str, err := conn.AcceptStream(context.Background())
+			Expect(err).ToNot(HaveOccurred())
+			_, err = io.Copy(str, str)
+			Expect(err).ToNot(HaveOccurred())
+			str.Close()
+		}()
+
+		var mx sync.Mutex
+		var maxPacketSizeClient, maxPacketSizeServer int
+		serverPort := ln.Addr().(*net.UDPAddr).Port
+		proxy, err := quicproxy.NewQuicProxy("localhost:0", &quicproxy.Opts{
+			RemoteAddr:  fmt.Sprintf("localhost:%d", serverPort),
+			DelayPacket: func(quicproxy.Direction, []byte) time.Duration { return rtt / 2 },
+			DropPacket: func(dir quicproxy.Direction, packet []byte) bool {
+				if len(packet) > mtu {
+					return true
+				}
+				mx.Lock()
+				defer mx.Unlock()
+				switch dir {
+				case quicproxy.DirectionIncoming:
+					if len(packet) > maxPacketSizeClient {
+						maxPacketSizeClient = len(packet)
+					}
+				case quicproxy.DirectionOutgoing:
+					if len(packet) > maxPacketSizeServer {
+						maxPacketSizeServer = len(packet)
+					}
+				}
+				return false
+			},
+		})
+		Expect(err).ToNot(HaveOccurred())
+		defer proxy.Close()
+
+		// Make sure to use v4-only socket here.
+		// We can't reliably set the DF bit on dual-stack sockets on macOS.
+		udpConn, err := net.ListenUDP("udp4", &net.UDPAddr{IP: net.IPv4(127, 0, 0, 1), Port: 0})
+		Expect(err).ToNot(HaveOccurred())
+		defer udpConn.Close()
+		tr := &quic.Transport{Conn: udpConn}
+		defer tr.Close()
+		conn, err := tr.Dial(
+			context.Background(),
+			proxy.LocalAddr(),
+			getTLSClientConfig(),
+			getQuicConfig(nil),
+		)
+		Expect(err).ToNot(HaveOccurred())
+		defer conn.CloseWithError(0, "")
+		str, err := conn.OpenStream()
+		Expect(err).ToNot(HaveOccurred())
+		done := make(chan struct{})
+		go func() {
+			defer close(done)
+			defer GinkgoRecover()
+			data, err := io.ReadAll(str)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(data).To(Equal(PRDataLong))
+		}()
+		_, err = str.Write(PRDataLong)
+		Expect(err).ToNot(HaveOccurred())
+		str.Close()
+		Eventually(done, 20*time.Second).Should(BeClosed())
+
+		mx.Lock()
+		defer mx.Unlock()
+		fmt.Fprintf(GinkgoWriter, "max client packet size: %d, MTU: %d\n", maxPacketSizeClient, mtu)
+		fmt.Fprintf(GinkgoWriter, "max server packet size: %d, MTU: %d\n", maxPacketSizeServer, mtu)
+		Expect(maxPacketSizeClient).To(BeNumerically(">=", mtu-25))
+	})
+})