qlog: defer creation of the qlog file in QLOGDIR until the first write

If the syscall to create the qlog file takes a non-negligible amount of
time, this can block the connection's run loop, or, even worse, the
server's run loop.

By creating the qlog file on the first write, it will be created by a
the qlogger's Go routine, and not be able to block the run loop. The
qlogger uses a buffered channel (capacity: 50). The qlogger will
therefore only block if more than 50 qlog events are queued before the
syscall returns.

qlog/qlog_dir.go

@@ -4,14 +4,55 @@ import (
 	"bufio"
 	"context"
 	"fmt"
-	"log"
+	"io"
 	"os"
 	"strings"
 
 	"github.com/quic-go/quic-go/internal/utils"
 	"github.com/quic-go/quic-go/logging"
 )
 
+// lazyWriteCloser creates the qlog file when the first log line is written.
+// This means that creating the qlog (there's no guarantee how long the syscalls will take)
+// won't block the connection, since qlogs are serialized from a separate Go routine,
+// that takes events from a buffered channel (size: eventChanSize).
+type lazyWriteCloser struct {
+	createFile    func() (*os.File, error)
+	createFileErr error
+	io.WriteCloser
+}
+
+func (w *lazyWriteCloser) init() error {
+	if w.createFileErr != nil {
+		return w.createFileErr
+	}
+	if w.createFile == nil {
+		return nil
+	}
+	f, err := w.createFile()
+	if err != nil {
+		w.createFileErr = err
+		return err
+	}
+	w.createFile = nil
+	w.WriteCloser = utils.NewBufferedWriteCloser(bufio.NewWriter(f), f)
+	return nil
+}
+
+func (w *lazyWriteCloser) Write(b []byte) (int, error) {
+	if err := w.init(); err != nil {
+		return 0, err
+	}
+	return w.WriteCloser.Write(b)
+}
+
+func (w *lazyWriteCloser) Close() error {
+	if err := w.init(); err != nil {
+		return err
+	}
+	return w.WriteCloser.Close()
+}
+
 // DefaultTracer creates a qlog file in the qlog directory specified by the QLOGDIR environment variable.
 // File names are <odcid>_<perspective>.qlog.
 // Returns nil if QLOGDIR is not set.
@@ -34,16 +75,17 @@ func qlogDirTracer(p logging.Perspective, connID logging.ConnectionID, label str
 	if qlogDir == "" {
 		return nil
 	}
-	if _, err := os.Stat(qlogDir); os.IsNotExist(err) {
-		if err := os.MkdirAll(qlogDir, 0o755); err != nil {
-			log.Fatalf("failed to create qlog dir %s: %v", qlogDir, err)
+	return NewConnectionTracer(&lazyWriteCloser{createFile: func() (*os.File, error) {
+		if _, err := os.Stat(qlogDir); os.IsNotExist(err) {
+			if err := os.MkdirAll(qlogDir, 0o755); err != nil {
+				return nil, fmt.Errorf("failed to create qlog dir %s: %v", qlogDir, err)
+			}
 		}
-	}
-	path := fmt.Sprintf("%s/%s_%s.qlog", strings.TrimRight(qlogDir, "/"), connID, label)
-	f, err := os.Create(path)
-	if err != nil {
-		log.Printf("Failed to create qlog file %s: %s", path, err.Error())
-		return nil
-	}
-	return NewConnectionTracer(utils.NewBufferedWriteCloser(bufio.NewWriter(f), f), p, connID)
+		path := fmt.Sprintf("%s/%s_%s.qlog", strings.TrimRight(qlogDir, "/"), connID, label)
+		f, err := os.Create(path)
+		if err != nil {
+			return nil, fmt.Errorf("failed to create qlog file %s: %s", path, err.Error())
+		}
+		return f, nil
+	}}, p, connID)
 }