text_formatter: fix race

examples/basic/basic.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"github.com/Sirupsen/logrus"
+	"os"
 )
 
 var log = logrus.New()

formatter_bench_test.go

@@ -80,11 +80,14 @@ func BenchmarkLargeJSONFormatter(b *testing.B) {
 }
 
 func doBenchmark(b *testing.B, formatter Formatter, fields Fields) {
+	logger := New()
+
 	entry := &Entry{
 		Time:    time.Time{},
 		Level:   InfoLevel,
 		Message: "message",
 		Data:    fields,
+		Logger:  logger,
 	}
 	var d []byte
 	var err error

text_formatter.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"sort"
 	"strings"
+	"sync"
 	"time"
 )
 
@@ -49,8 +50,8 @@ type TextFormatter struct {
 	DisableSorting bool
 
 	// Whether the logger's out is to a terminal
-	isTerminal         bool
-	terminalDetermined bool
+	isTerminal   bool
+	terminalOnce sync.Once
 }
 
 func (f *TextFormatter) Format(entry *Entry) ([]byte, error) {
@@ -71,10 +72,11 @@ func (f *TextFormatter) Format(entry *Entry) ([]byte, error) {
 
 	prefixFieldClashes(entry.Data)
 
-	if !f.terminalDetermined {
-		f.isTerminal = IsTerminal(entry.Logger.Out)
-		f.terminalDetermined = true
-	}
+	f.terminalOnce.Do(func() {
+		if entry.Logger != nil {
+			f.isTerminal = IsTerminal(entry.Logger.Out)
+		}
+	})
 
 	isColored := (f.ForceColors || f.isTerminal) && !f.DisableColors