fix: log.With() race condition (#64)

* fix log.With() race condition

- added small unittest to highlight the problem
- only visible when running with -race option

* rollback fix

* fix race condition

logger.go

@@ -285,7 +285,9 @@ func (l *Logger) SetCallerOffset(offset int) {
 
 // With returns a new logger with the given keyvals added.
 func (l *Logger) With(keyvals ...interface{}) *Logger {
+	l.mu.Lock()
 	sl := *l
+	l.mu.Unlock()
 	sl.b = bytes.Buffer{}
 	sl.mu = &sync.RWMutex{}
 	sl.helpers = &sync.Map{}

logger_test.go

@@ -2,6 +2,7 @@ package log
 
 import (
 	"bytes"
+	"sync"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -151,3 +152,34 @@ func TestLogWithPrefix(t *testing.T) {
 		})
 	}
 }
+
+func TestLogWithRaceCondition(t *testing.T) {
+	var buf bytes.Buffer
+	cases := []struct {
+		name string
+	}{
+		{
+			name: "must be run with -race",
+		},
+	}
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			buf.Reset()
+			l := New(&buf)
+
+			var done sync.WaitGroup
+			done.Add(2)
+
+			go func() {
+				l.With("arg1", "val1", "arg2", "val2")
+				done.Done()
+			}()
+
+			go func() {
+				l.Info("kinda long log message")
+				done.Done()
+			}()
+			done.Wait()
+		})
+	}
+}