[#60] Implement periodic white space XML writer

Periodic white space XML writer sends XML header
and white spaces to the io.Writer.

Signed-off-by: Alex Vanin <a.vanin@yadro.com>

api/handler/multipart_upload_test.go

@@ -0,0 +1,48 @@
+package handler
+
+import (
+	"bytes"
+	"encoding/xml"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestPeriodicWriter(t *testing.T) {
+	const dur = 100 * time.Millisecond
+	const whitespaces = 8
+	expected := []byte(xml.Header)
+	for i := 0; i < whitespaces; i++ {
+		expected = append(expected, []byte(" ")...)
+	}
+
+	t.Run("writes data", func(t *testing.T) {
+		buf := bytes.NewBuffer(nil)
+		stop := periodicXMLWriter(buf, dur)
+
+		// N number of whitespaces + half durations to guarantee at least N writes in buffer
+		time.Sleep(whitespaces*dur + dur/2)
+		require.True(t, stop())
+		require.Equal(t, expected, buf.Bytes())
+
+		t.Run("no additional data after stop", func(t *testing.T) {
+			time.Sleep(2 * dur)
+			require.Equal(t, expected, buf.Bytes())
+		})
+	})
+
+	t.Run("does not write data", func(t *testing.T) {
+		buf := bytes.NewBuffer(nil)
+		stop := periodicXMLWriter(buf, dur)
+		time.Sleep(dur / 2)
+		require.False(t, stop())
+		require.Empty(t, buf.Bytes())
+
+		t.Run("disabled", func(t *testing.T) {
+			stop = periodicXMLWriter(buf, 0)
+			require.False(t, stop())
+			require.Empty(t, buf.Bytes())
+		})
+	})
+}