qlog: disentangle the ConnectionTracer from the qlog writer (#4300)

The qlog writer simply records events, puts them into a channel, and
consumes these events in a separate Go routine (by serializing them).

The ConnectionTracer is the one generating those events.

qlog/writer_test.go

@@ -0,0 +1,49 @@
+package qlog
+
+import (
+	"bytes"
+	"errors"
+	"io"
+	"log"
+	"os"
+
+	"github.com/quic-go/quic-go/internal/protocol"
+
+	. "github.com/onsi/ginkgo/v2"
+	. "github.com/onsi/gomega"
+)
+
+type limitedWriter struct {
+	io.WriteCloser
+	N       int
+	written int
+}
+
+func (w *limitedWriter) Write(p []byte) (int, error) {
+	if w.written+len(p) > w.N {
+		return 0, errors.New("writer full")
+	}
+	n, err := w.WriteCloser.Write(p)
+	w.written += n
+	return n, err
+}
+
+var _ = Describe("Writing", func() {
+	It("stops writing when encountering an error", func() {
+		buf := &bytes.Buffer{}
+		t := NewConnectionTracer(
+			&limitedWriter{WriteCloser: nopWriteCloser(buf), N: 250},
+			protocol.PerspectiveServer,
+			protocol.ParseConnectionID([]byte{0xde, 0xad, 0xbe, 0xef}),
+		)
+		for i := uint32(0); i < 1000; i++ {
+			t.UpdatedPTOCount(i)
+		}
+
+		b := &bytes.Buffer{}
+		log.SetOutput(b)
+		defer log.SetOutput(os.Stdout)
+		t.Close()
+		Expect(b.String()).To(ContainSubstring("writer full"))
+	})
+})