[#644] Support keepalive during listing

Send whitespaces every time as new object in list is ready
to prevent client from context cancelling.

Signed-off-by: Denis Kirillov <d.kirillov@yadro.com>

api/handler/object_list_test.go

@@ -1,7 +1,9 @@
 package handler
 
 import (
+	"bytes"
 	"context"
+	"encoding/xml"
 	"fmt"
 	"net/http"
 	"net/http/httptest"
@@ -841,6 +843,101 @@ func TestListingsWithInvalidEncodingType(t *testing.T) {
 	listObjectsV1Err(hc, bktName, "invalid", apierr.GetAPIError(apierr.ErrInvalidEncodingMethod))
 }
 
+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) {
+		ch := make(chan struct{})
+		go func() {
+			defer close(ch)
+
+			for range whitespaces {
+				ch <- struct{}{}
+				time.Sleep(dur)
+			}
+		}()
+
+		buf := bytes.NewBuffer(nil)
+		stop := periodicXMLWriter(buf, time.Nanosecond, ch)
+
+		// 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, string(expected), buf.String())
+
+		t.Run("no additional data after stop", func(t *testing.T) {
+			time.Sleep(2 * dur)
+			require.Equal(t, string(expected), buf.String())
+		})
+	})
+
+	t.Run("does not write data", func(t *testing.T) {
+		buf := bytes.NewBuffer(nil)
+
+		ch := make(chan struct{})
+		go func() {
+			defer close(ch)
+
+			for range whitespaces {
+				time.Sleep(2 * dur)
+				ch <- struct{}{}
+			}
+		}()
+
+		stop := periodicXMLWriter(buf, time.Nanosecond, ch)
+		require.False(t, stop())
+		require.Empty(t, buf.Bytes())
+	})
+
+	t.Run("throttling works", func(t *testing.T) {
+		buf := bytes.NewBuffer(nil)
+
+		ch := make(chan struct{})
+		go func() {
+			defer close(ch)
+
+			for range whitespaces {
+				ch <- struct{}{}
+				time.Sleep(dur / 2)
+			}
+		}()
+
+		stop := periodicXMLWriter(buf, dur, ch)
+		// 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, string(expected[:len(expected)-4]), buf.String())
+	})
+
+	t.Run("disabled", func(t *testing.T) {
+		buf := bytes.NewBuffer(nil)
+
+		ch := make(chan struct{})
+		go func() {
+			defer close(ch)
+
+			for range whitespaces {
+				select {
+				case ch <- struct{}{}:
+				default:
+				}
+				time.Sleep(dur / 2)
+			}
+		}()
+
+		stop := periodicXMLWriter(buf, 0, ch)
+		// N number of whitespaces + half durations to guarantee at least N writes in buffer
+		time.Sleep(whitespaces*dur + dur/2)
+		require.False(t, stop())
+		require.Empty(t, buf.String())
+	})
+}
+
 func checkVersionsNames(t *testing.T, versions *ListObjectsVersionsResponse, names []string) {
 	for i, v := range versions.Version {
 		require.Equal(t, names[i], v.Key)