[#302] pkg/container: Document default values set in `NewAnnouncement`

Document field values of instance constructed via `NewAnnouncement`.
Assert the values in corresponding unit test.

Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>

pkg/container/announcement.go

@@ -10,6 +10,11 @@ import (
 type UsedSpaceAnnouncement container.UsedSpaceAnnouncement
 
 // NewAnnouncement initialize empty UsedSpaceAnnouncement message.
+//
+// Defaults:
+//  - epoch: 0;
+//  - usedSpace: 0;
+//  - cid: nil.
 func NewAnnouncement() *UsedSpaceAnnouncement {
 	return NewAnnouncementFromV2(new(container.UsedSpaceAnnouncement))
 }

pkg/container/announcement_test.go

@@ -72,6 +72,22 @@ func TestUsedSpaceAnnouncement_ToV2(t *testing.T) {
 
 		require.Nil(t, x.ToV2())
 	})
+
+	t.Run("default values", func(t *testing.T) {
+		announcement := container.NewAnnouncement()
+
+		// check initial values
+		require.Zero(t, announcement.Epoch())
+		require.Zero(t, announcement.UsedSpace())
+		require.Nil(t, announcement.ContainerID())
+
+		// convert to v2 message
+		announcementV2 := announcement.ToV2()
+
+		require.Zero(t, announcementV2.GetEpoch())
+		require.Zero(t, announcementV2.GetUsedSpace())
+		require.Nil(t, announcementV2.GetContainerID())
+	})
 }
 
 func TestNewAnnouncementFromV2(t *testing.T) {