fix: handle with read the empty file (#4258)

core/filex/file_test.go

@@ -52,6 +52,7 @@ last line`
 second line
 last line
 `
+	emptyContent = ``
 )
 
 func TestFirstLine(t *testing.T) {
@@ -79,6 +80,26 @@ func TestFirstLineError(t *testing.T) {
 	assert.Error(t, err)
 }
 
+func TestFirstLineEmptyFile(t *testing.T) {
+	filename, err := fs.TempFilenameWithText(emptyContent)
+	assert.Nil(t, err)
+	defer os.Remove(filename)
+
+	val, err := FirstLine(filename)
+	assert.Nil(t, err)
+	assert.Equal(t, "", val)
+}
+
+func TestFirstLineWithoutNewline(t *testing.T) {
+	filename, err := fs.TempFilenameWithText(longLine)
+	assert.Nil(t, err)
+	defer os.Remove(filename)
+
+	val, err := FirstLine(filename)
+	assert.Nil(t, err)
+	assert.Equal(t, longLine, val)
+}
+
 func TestLastLine(t *testing.T) {
 	filename, err := fs.TempFilenameWithText(text)
 	assert.Nil(t, err)
@@ -99,6 +120,16 @@ func TestLastLineWithLastNewline(t *testing.T) {
 	assert.Equal(t, longLine, val)
 }
 
+func TestLastLineWithoutLastNewline(t *testing.T) {
+	filename, err := fs.TempFilenameWithText(longLine)
+	assert.Nil(t, err)
+	defer os.Remove(filename)
+
+	val, err := LastLine(filename)
+	assert.Nil(t, err)
+	assert.Equal(t, longLine, val)
+}
+
 func TestLastLineShort(t *testing.T) {
 	filename, err := fs.TempFilenameWithText(shortText)
 	assert.Nil(t, err)
@@ -123,3 +154,13 @@ func TestLastLineError(t *testing.T) {
 	_, err := LastLine("/tmp/does-not-exist")
 	assert.Error(t, err)
 }
+
+func TestLastLineEmptyFile(t *testing.T) {
+	filename, err := fs.TempFilenameWithText(emptyContent)
+	assert.Nil(t, err)
+	defer os.Remove(filename)
+
+	val, err := LastLine(filename)
+	assert.Nil(t, err)
+	assert.Equal(t, "", val)
+}