add a -qlog flag to the example client and server

.gitignore

@@ -3,6 +3,7 @@ debug.test
 main
 mockgen_tmp.go
 *.qtr
+*.qlog
 
 fuzzing/*/*.zip
 fuzzing/*/coverprofile

example/client/main.go

@@ -5,12 +5,14 @@ import (
 	"crypto/tls"
 	"crypto/x509"
 	"flag"
+	"fmt"
 	"io"
 	"log"
 	"net/http"
 	"os"
 	"sync"
 
+	"github.com/lucas-clemente/quic-go"
 	"github.com/lucas-clemente/quic-go/http3"
 	"github.com/lucas-clemente/quic-go/internal/testdata"
 	"github.com/lucas-clemente/quic-go/internal/utils"
@@ -21,6 +23,7 @@ func main() {
 	quiet := flag.Bool("q", false, "don't print the data")
 	keyLogFile := flag.String("keylog", "", "key log file")
 	insecure := flag.Bool("insecure", false, "skip certificate verification")
+	qlog := flag.Bool("qlog", false, "output a qlog (in the same directory)")
 	flag.Parse()
 	urls := flag.Args()
 
@@ -48,12 +51,26 @@ func main() {
 		log.Fatal(err)
 	}
 	testdata.AddRootCA(pool)
+
+	var qconf quic.Config
+	if *qlog {
+		qconf.GetLogWriter = func(connID []byte) io.WriteCloser {
+			filename := fmt.Sprintf("client_%x.qlog", connID)
+			f, err := os.Create(filename)
+			if err != nil {
+				log.Fatal(err)
+			}
+			log.Printf("Creating qlog file %s.\n", filename)
+			return f
+		}
+	}
 	roundTripper := &http3.RoundTripper{
 		TLSClientConfig: &tls.Config{
 			RootCAs:            pool,
 			InsecureSkipVerify: *insecure,
 			KeyLogWriter:       keyLog,
 		},
+		QuicConfig: &qconf,
 	}
 	defer roundTripper.Close()
 	hclient := &http.Client{

example/main.go

@@ -186,6 +186,7 @@ func main() {
 	www := flag.String("www", "", "www data")
 	tcp := flag.Bool("tcp", false, "also listen on TCP")
 	trace := flag.Bool("trace", false, "enable quic-trace")
+	qlog := flag.Bool("qlog", false, "output a qlog (in the same directory)")
 	flag.Parse()
 
 	logger := utils.DefaultLogger
@@ -202,9 +203,20 @@ func main() {
 	}
 
 	handler := setupHandler(*www, *trace)
-	var quicConf *quic.Config
+	quicConf := &quic.Config{}
 	if *trace {
-		quicConf = &quic.Config{QuicTracer: tracer}
+		quicConf.QuicTracer = tracer
+	}
+	if *qlog {
+		quicConf.GetLogWriter = func(connID []byte) io.WriteCloser {
+			filename := fmt.Sprintf("server_%x.qlog", connID)
+			f, err := os.Create(filename)
+			if err != nil {
+				log.Fatal(err)
+			}
+			log.Printf("Creating qlog file %s.\n", filename)
+			return f
+		}
 	}
 
 	var wg sync.WaitGroup