[#302] pkg/object: Document default values set in NewTombstone

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

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

pkg/object/tombstone.go

@@ -16,10 +16,22 @@ func NewTombstoneFromV2(tV2 *tombstone.Tombstone) *Tombstone {
 }
 
 // NewTombstone creates and initializes blank Tombstone.
+//
+// Defaults:
+//  - exp: 0;
+//  - splitID: nil;
+//  - members: nil.
 func NewTombstone() *Tombstone {
 	return NewTombstoneFromV2(new(tombstone.Tombstone))
 }
 
+// ToV2 converts Tombstone to v2 Tombstone message.
+//
+// Nil Tombstone converts to nil.
+func (ts *Tombstone) ToV2() *tombstone.Tombstone {
+	return (*tombstone.Tombstone)(ts)
+}
+
 // ExpirationEpoch return number of tombstone expiration epoch.
 func (t *Tombstone) ExpirationEpoch() uint64 {
 	return (*tombstone.Tombstone)(t).

pkg/object/tombstone_test.go

@@ -72,3 +72,21 @@ func TestNewTombstoneFromV2(t *testing.T) {
 		require.Nil(t, NewTombstoneFromV2(x))
 	})
 }
+
+func TestNewTombstone(t *testing.T) {
+	t.Run("default values", func(t *testing.T) {
+		ts := NewTombstone()
+
+		// check initial values
+		require.Nil(t, ts.SplitID())
+		require.Nil(t, ts.Members())
+		require.Zero(t, ts.ExpirationEpoch())
+
+		// convert to v2 message
+		tsV2 := ts.ToV2()
+
+		require.Nil(t, tsV2.GetSplitID())
+		require.Nil(t, tsV2.GetMembers())
+		require.Zero(t, tsV2.GetExpirationEpoch())
+	})
+}