qlog: add a default tracer that writes to QLOGDIR (#4233)

* add qlog default tracer which writes to QLOGDIR

* gofumpt

* add qlog default tracer which writes to QLOGDIR

* fix flaky tests

* Update README.md

Co-authored-by: Marten Seemann <martenseemann@gmail.com>

* Update README.md

Co-authored-by: Marten Seemann <martenseemann@gmail.com>

* Update README.md

Co-authored-by: Marten Seemann <martenseemann@gmail.com>

* Update README.md

Co-authored-by: Marten Seemann <martenseemann@gmail.com>

---------

Co-authored-by: Marten Seemann <martenseemann@gmail.com>

integrationtests/self/qlog_dir_test.go

@@ -0,0 +1,90 @@
+package self_test
+
+import (
+	"context"
+	"os"
+	"path"
+	"regexp"
+
+	. "github.com/onsi/ginkgo/v2"
+	. "github.com/onsi/gomega"
+	"github.com/quic-go/quic-go"
+	"github.com/quic-go/quic-go/qlog"
+)
+
+var _ = Describe("qlog dir tests", Serial, func() {
+	var originalQlogDirValue string
+	var tempTestDirPath string
+
+	BeforeEach(func() {
+		originalQlogDirValue = os.Getenv("QLOGDIR")
+		var err error
+		tempTestDirPath, err = os.MkdirTemp("", "temp_test_dir")
+		Expect(err).ToNot(HaveOccurred())
+	})
+
+	AfterEach(func() {
+		err := os.Setenv("QLOGDIR", originalQlogDirValue)
+		Expect(err).ToNot(HaveOccurred())
+		err = os.RemoveAll(tempTestDirPath)
+		Expect(err).ToNot(HaveOccurred())
+	})
+
+	handshake := func() {
+		serverStopped := make(chan struct{})
+		server, err := quic.ListenAddr(
+			"localhost:0",
+			getTLSConfig(),
+			&quic.Config{
+				Tracer: qlog.DefaultTracer,
+			},
+		)
+		Expect(err).ToNot(HaveOccurred())
+
+		go func() {
+			defer GinkgoRecover()
+			defer close(serverStopped)
+			for {
+				if _, err := server.Accept(context.Background()); err != nil {
+					return
+				}
+			}
+		}()
+
+		conn, err := quic.DialAddr(
+			context.Background(),
+			server.Addr().String(),
+			getTLSClientConfig(),
+			&quic.Config{
+				Tracer: qlog.DefaultTracer,
+			},
+		)
+		Expect(err).ToNot(HaveOccurred())
+		conn.CloseWithError(0, "")
+		server.Close()
+		<-serverStopped
+	}
+
+	It("environment variable is set", func() {
+		qlogDir := path.Join(tempTestDirPath, "qlogs")
+		err := os.Setenv("QLOGDIR", qlogDir)
+		Expect(err).ToNot(HaveOccurred())
+		handshake()
+		_, err = os.Stat(tempTestDirPath)
+		qlogDirCreated := !os.IsNotExist(err)
+		Expect(qlogDirCreated).To(BeTrue())
+		childs, err := os.ReadDir(qlogDir)
+		Expect(err).ToNot(HaveOccurred())
+		Expect(len(childs)).To(Equal(2))
+		odcids := make([]string, 0)
+		vantagePoints := make([]string, 0)
+		qlogFileNameRegexp := regexp.MustCompile(`^([0-f]+)_(client|server).qlog$`)
+		for _, child := range childs {
+			matches := qlogFileNameRegexp.FindStringSubmatch(child.Name())
+			odcids = append(odcids, matches[1])
+			vantagePoints = append(vantagePoints, matches[2])
+		}
+		Expect(odcids[0]).To(Equal(odcids[1]))
+		Expect(vantagePoints).To(ContainElements("client", "server"))
+	})
+})