[TextFormatter] Speed up. Fprintf is changed to buffer.Write*

As Fprintf is slower than buffer.WriteString, it's replaced
to faster call.

Signed-off-by: Anton Tiurin <noxiouz@yandex.ru>

formatter_bench_test.go

@@ -1,6 +1,7 @@
 package logrus
 
 import (
+	"fmt"
 	"testing"
 	"time"
 )
@@ -45,6 +46,15 @@ var largeFields = Fields{
 	"entries":   "yeah",
 }
 
+var errorFields = Fields{
+	"foo": fmt.Errorf("bar"),
+	"baz": fmt.Errorf("qux"),
+}
+
+func BenchmarkErrorTextFormatter(b *testing.B) {
+	doBenchmark(b, &TextFormatter{DisableColors: true}, errorFields)
+}
+
 func BenchmarkSmallTextFormatter(b *testing.B) {
 	doBenchmark(b, &TextFormatter{DisableColors: true}, smallFields)
 }

text_formatter.go

@@ -131,21 +131,28 @@ func needsQuoting(text string) bool {
 	return true
 }
 
-func (f *TextFormatter) appendKeyValue(b *bytes.Buffer, key, value interface{}) {
-	switch value.(type) {
+func (f *TextFormatter) appendKeyValue(b *bytes.Buffer, key string, value interface{}) {
+
+	b.WriteString(key)
+	b.WriteByte('=')
+
+	switch value := value.(type) {
 	case string:
-		if needsQuoting(value.(string)) {
-			fmt.Fprintf(b, "%v=%s ", key, value)
+		if needsQuoting(value) {
+			b.WriteString(value)
 		} else {
-			fmt.Fprintf(b, "%v=%q ", key, value)
+			fmt.Fprintf(b, "%q", value)
 		}
 	case error:
-		if needsQuoting(value.(error).Error()) {
-			fmt.Fprintf(b, "%v=%s ", key, value)
+		errmsg := value.Error()
+		if needsQuoting(errmsg) {
+			b.WriteString(errmsg)
 		} else {
-			fmt.Fprintf(b, "%v=%q ", key, value)
+			fmt.Fprintf(b, "%q", value)
 		}
 	default:
-		fmt.Fprintf(b, "%v=%v ", key, value)
+		fmt.Fprint(b, value)
 	}
+
+	b.WriteByte(' ')
 }