[#302] pkg/storagegroup: Document default values set in `New`

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

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

pkg/storagegroup/storagegroup.go

@@ -18,6 +18,12 @@ func NewFromV2(aV2 *storagegroup.StorageGroup) *StorageGroup {
 }
 
 // New creates and initializes blank StorageGroup.
+//
+// Defaults:
+//  - size: 0;
+//  - exp: 0;
+//  - members: nil;
+//  - hash: nil.
 func New() *StorageGroup {
 	return NewFromV2(new(storagegroup.StorageGroup))
 }

pkg/storagegroup/storagegroup_test.go

@@ -71,3 +71,23 @@ func TestStorageGroup_ToV2(t *testing.T) {
 		require.Nil(t, x.ToV2())
 	})
 }
+
+func TestNew(t *testing.T) {
+	t.Run("default values", func(t *testing.T) {
+		sg := storagegroup.New()
+
+		// check initial values
+		require.Nil(t, sg.Members())
+		require.Nil(t, sg.ValidationDataHash())
+		require.Zero(t, sg.ExpirationEpoch())
+		require.Zero(t, sg.ValidationDataSize())
+
+		// convert to v2 message
+		sgV2 := sg.ToV2()
+
+		require.Nil(t, sgV2.GetMembers())
+		require.Nil(t, sgV2.GetValidationHash())
+		require.Zero(t, sgV2.GetExpirationEpoch())
+		require.Zero(t, sgV2.GetValidationDataSize())
+	})
+}