optimize AtomicError (#82)

This commit optimize AtomicError using atomic.Value. Benchmarks:

name               old time/op  new time/op  delta
AtomicError/Load-6   305ns ±11%    12ns ± 6%  -96.18%  (p=0.000 n=10+10)
AtomicError/Set-6   314ns ±16%    14ns ± 2%  -95.61%  (p=0.000 n=10+9)

core/errorx/atomicerror.go

@@ -1,21 +1,18 @@
 package errorx
 
-import "sync"
+import "sync/atomic"
 
 type AtomicError struct {
-	err  error
-	lock sync.Mutex
+	err atomic.Value // error
 }
 
 func (ae *AtomicError) Set(err error) {
-	ae.lock.Lock()
-	ae.err = err
-	ae.lock.Unlock()
+	ae.err.Store(err)
 }
 
 func (ae *AtomicError) Load() error {
-	ae.lock.Lock()
-	err := ae.err
-	ae.lock.Unlock()
-	return err
+	if v := ae.err.Load(); v != nil {
+		return v.(error)
+	}
+	return nil
 }

core/errorx/atomicerror_test.go

@@ -2,6 +2,8 @@ package errorx
 
 import (
 	"errors"
+	"sync"
+	"sync/atomic"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -19,3 +21,53 @@ func TestAtomicErrorNil(t *testing.T) {
 	var err AtomicError
 	assert.Nil(t, err.Load())
 }
+
+func BenchmarkAtomicError(b *testing.B) {
+	var aerr AtomicError
+	wg := sync.WaitGroup{}
+
+	b.Run("Load", func(b *testing.B) {
+		var done uint32
+		go func() {
+			for {
+				if atomic.LoadUint32(&done) != 0 {
+					break
+				}
+				wg.Add(1)
+				go func() {
+					aerr.Set(errDummy)
+					wg.Done()
+				}()
+			}
+		}()
+		b.ResetTimer()
+		for i := 0; i < b.N; i++ {
+			_ = aerr.Load()
+		}
+		b.StopTimer()
+		atomic.StoreUint32(&done, 1)
+		wg.Wait()
+	})
+	b.Run("Set", func(b *testing.B) {
+		var done uint32
+		go func() {
+			for {
+				if atomic.LoadUint32(&done) != 0 {
+					break
+				}
+				wg.Add(1)
+				go func() {
+					_ = aerr.Load()
+					wg.Done()
+				}()
+			}
+		}()
+		b.ResetTimer()
+		for i := 0; i < b.N; i++ {
+			aerr.Set(errDummy)
+		}
+		b.StopTimer()
+		atomic.StoreUint32(&done, 1)
+		wg.Wait()
+	})
+}