fix: race condition when creating standard logger (#68)

Before, `StandardLog` used to _copy_ the internal buffer causing two or
more loggers to share the same buffer triggering a race.

Don't copy the internal buffer and reduce the mutex critical section
scope.

logger.go

@@ -58,10 +58,6 @@ func (l *Logger) log(level Level, msg interface{}, keyvals ...interface{}) {
 		return
 	}
 
-	l.mu.Lock()
-	defer l.mu.Unlock()
-	defer l.b.Reset()
-
 	var kvs []interface{}
 	if l.reportTimestamp {
 		kvs = append(kvs, TimestampKey, l.timeFunc())
@@ -98,6 +94,8 @@ func (l *Logger) log(level Level, msg interface{}, keyvals ...interface{}) {
 		kvs = append(kvs, ErrMissingValue)
 	}
 
+	l.mu.Lock()
+	defer l.mu.Unlock()
 	switch l.formatter {
 	case LogfmtFormatter:
 		l.logfmtFormatter(kvs...)
@@ -107,7 +105,8 @@ func (l *Logger) log(level Level, msg interface{}, keyvals ...interface{}) {
 		l.textFormatter(kvs...)
 	}
 
-	_, _ = l.w.Write(l.b.Bytes())
+	// WriteTo will reset the buffer
+	l.b.WriteTo(l.w) // nolint: errcheck
 }
 
 // Helper marks the calling function as a helper

logger_test.go

@@ -2,8 +2,10 @@ package log
 
 import (
 	"bytes"
+	"io"
 	"sync"
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/assert"
 )
@@ -183,3 +185,36 @@ func TestLogWithRaceCondition(t *testing.T) {
 		})
 	}
 }
+
+func TestRace(t *testing.T) {
+	t.Parallel()
+
+	w := io.Discard
+	l := New(w)
+	for i := 0; i < 100; i++ {
+		t.Run("race", func(t *testing.T) {
+			t.Parallel()
+			s := l.StandardLog()
+			l.Info("foo")
+			l.GetLevel()
+			l.Print("foo")
+
+			s.Print("foo")
+			s.Writer().Write([]byte("bar"))
+			s.Output(1, "baz")
+
+			l.SetOutput(w)
+			l.Debug("foo")
+			l.SetLevel(InfoLevel)
+			l.GetPrefix()
+
+			o := l.With("foo", "bar")
+			o.Print("foo")
+			o.SetTimeFormat(time.Kitchen)
+			o.Debug("foo")
+			o.SetOutput(w)
+			o.Error("foo")
+			o.SetFormatter(JSONFormatter)
+		})
+	}
+}

stdlog.go

@@ -3,7 +3,6 @@ package log
 import (
 	"log"
 	"strings"
-	"sync"
 )
 
 type stdLogWriter struct {
@@ -54,14 +53,12 @@ type StandardLogOptions struct {
 // can infer log levels from message prefix. Expected prefixes are DEBUG, INFO,
 // WARN, ERROR, and ERR.
 func (l *Logger) StandardLog(opts ...StandardLogOptions) *log.Logger {
-	nl := *l
-	nl.mu = &sync.RWMutex{}
-	nl.helpers = &sync.Map{}
+	nl := l.With()
 	// The caller stack is
 	// log.Printf() -> l.Output() -> l.out.Write(stdLogger.Write)
 	nl.callerOffset += 3
 	sl := &stdLogWriter{
-		l: &nl,
+		l: nl,
 	}
 	if len(opts) > 0 {
 		sl.opt = &opts[0]