fix file watcher event bug on Linux

the watcher can not normally get events of file changes on linux. it just can get the first two changes.

config/default_test.go

@@ -1,6 +1,7 @@
 package config
 
 import (
+	"encoding/json"
 	"fmt"
 	"os"
 	"path/filepath"
@@ -8,10 +9,15 @@ import (
 	"testing"
 	"time"
 
+	"github.com/google/uuid"
 	"github.com/micro/go-micro/config/source/env"
 	"github.com/micro/go-micro/config/source/file"
 )
 
+var (
+	sep = string(os.PathSeparator)
+)
+
 func createFileForIssue18(t *testing.T, content string) *os.File {
 	data := []byte(content)
 	path := filepath.Join(os.TempDir(), fmt.Sprintf("file.%d", time.Now().UnixNano()))
@@ -116,3 +122,57 @@ func TestConfigMerge(t *testing.T) {
 			actualHost)
 	}
 }
+
+func TestFileChange(t *testing.T) {
+	// create a temp file
+	fileName := uuid.New().String() + "testWatcher.json"
+	f, err := os.OpenFile("."+sep+fileName, os.O_WRONLY|os.O_CREATE, 0666)
+	if err != nil {
+		t.Error(err)
+	}
+	defer f.Close()
+	defer os.Remove("." + sep + fileName)
+
+	// load the file
+	if err := Load(file.NewSource(
+		file.WithPath("." + sep + fileName),
+	)); err != nil {
+		t.Error(err)
+	}
+
+	// watch changes
+	watcher, err := Watch()
+	if err != nil {
+		t.Error(err)
+	}
+	changeTimes := 0
+	go func() {
+		for {
+			v, err := watcher.Next()
+			if err != nil {
+				t.Error(err)
+				return
+			}
+			changeTimes++
+			t.Logf("file change，%s", string(v.Bytes()))
+		}
+	}()
+
+	content := map[int]string{}
+	// change the file
+	for i := 0; i < 5; i++ {
+		content[i] = time.Now().String()
+		bytes, _ := json.Marshal(content)
+		f.Truncate(0)
+		f.Seek(0, 0)
+		if _, err := f.Write(bytes); err != nil {
+			t.Error(err)
+		}
+
+		time.Sleep(time.Second)
+	}
+
+	if changeTimes != 4 {
+		t.Error(fmt.Errorf("watcher error: change times %d is not enough", changeTimes))
+	}
+}