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

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)
 }