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>

qlog/qlog_dir_test.go

@@ -0,0 +1,56 @@
+package qlog
+
+import (
+	"context"
+	"os"
+	"path"
+
+	. "github.com/onsi/ginkgo/v2"
+	. "github.com/onsi/gomega"
+	"github.com/quic-go/quic-go/internal/protocol"
+	"github.com/quic-go/quic-go/logging"
+)
+
+var _ = Describe("qlog dir tests", Serial, func() {
+	var originalQlogDirValue string
+	var tempTestDirPath string
+	ctx := context.Background()
+	perspective := logging.PerspectiveClient
+	connID, _ := protocol.GenerateConnectionIDForInitial()
+
+	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())
+	})
+
+	It("environment variable is set", func() {
+		qlogDir := path.Join(tempTestDirPath, "qlogs")
+		err := os.Setenv("QLOGDIR", qlogDir)
+		Expect(err).ToNot(HaveOccurred())
+		tracer := DefaultTracer(ctx, perspective, connID)
+		Expect(tracer).ToNot(BeNil())
+		tracer.Close()
+		_, err = os.Stat(qlogDir)
+		qlogDirCreated := !os.IsNotExist(err)
+		Expect(qlogDirCreated).To(BeTrue())
+		childs, err := os.ReadDir(qlogDir)
+		Expect(err).ToNot(HaveOccurred())
+		Expect(len(childs)).To(Equal(1))
+	})
+
+	It("environment variable is not set", func() {
+		err := os.Setenv("QLOGDIR", "")
+		Expect(err).ToNot(HaveOccurred())
+		tracer := DefaultTracer(ctx, perspective, connID)
+		Expect(tracer).To(BeNil())
+	})
+})