daemon/logger: improve jsonfilelog read benchmark

The jsonfilelog read benchmark was incorrectly reusing the same message
pointer in the producer loop. The message value would be reset after the
first call to jsonlogger.Log, resulting in all subsequent calls logging
a zero-valued message. This is not a representative workload for
benchmarking and throws off the throughput metric.

Reduce variation between benchmark runs by using a constant timestamp.

Write to the producer goroutine's error channel only on a non-nil error
to eliminate spurious synchronization between producer and consumer
goroutines external to the logger being benchmarked.

Signed-off-by: Cory Snider <csnider@mirantis.com>

daemon/logger/jsonfilelog/read_test.go

@@ -4,21 +4,20 @@ import (
 	"bytes"
 	"encoding/json"
 	"io"
+	"path/filepath"
 	"testing"
 	"time"
 
 	"github.com/docker/docker/daemon/logger"
 	"gotest.tools/v3/assert"
-	"gotest.tools/v3/fs"
 )
 
 func BenchmarkJSONFileLoggerReadLogs(b *testing.B) {
-	tmp := fs.NewDir(b, "bench-jsonfilelog")
-	defer tmp.Remove()
+	tmp := b.TempDir()
 
 	jsonlogger, err := New(logger.Info{
 		ContainerID: "a7317399f3f857173c6179d44823594f8294678dea9999662e5c625b5a1c7657",
-		LogPath:     tmp.Join("container.log"),
+		LogPath:     filepath.Join(tmp, "container.log"),
 		Config: map[string]string{
 			"labels": "first,second",
 		},
@@ -30,24 +29,32 @@ func BenchmarkJSONFileLoggerReadLogs(b *testing.B) {
 	assert.NilError(b, err)
 	defer jsonlogger.Close()
 
-	msg := &logger.Message{
-		Line:      []byte("Line that thinks that it is log line from docker\n"),
-		Source:    "stderr",
-		Timestamp: time.Now().UTC(),
+	const line = "Line that thinks that it is log line from docker\n"
+	ts := time.Date(2007, 1, 2, 3, 4, 5, 6, time.UTC)
+	msg := func() *logger.Message {
+		m := logger.NewMessage()
+		m.Line = append(m.Line, line...)
+		m.Source = "stderr"
+		m.Timestamp = ts
+		return m
 	}
 
-	buf := bytes.NewBuffer(nil)
-	assert.NilError(b, marshalMessage(msg, nil, buf))
+	var buf bytes.Buffer
+	assert.NilError(b, marshalMessage(msg(), nil, &buf))
 	b.SetBytes(int64(buf.Len()))
 
 	b.ResetTimer()
 
-	chError := make(chan error, b.N+1)
+	chError := make(chan error)
 	go func() {
 		for i := 0; i < b.N; i++ {
-			chError <- jsonlogger.Log(msg)
+			if err := jsonlogger.Log(msg()); err != nil {
+				chError <- err
+			}
+		}
+		if err := jsonlogger.Close(); err != nil {
+			chError <- err
 		}
-		chError <- jsonlogger.Close()
 	}()
 
 	lw := jsonlogger.(*JSONFileLogger).ReadLogs(logger.ReadConfig{Follow: true})
@@ -57,9 +64,7 @@ func BenchmarkJSONFileLoggerReadLogs(b *testing.B) {
 		case <-lw.WatchProducerGone():
 			return
 		case err := <-chError:
-			if err != nil {
-				b.Fatal(err)
-			}
+			b.Fatal(err)
 		}
 	}
 }