[#302] pkg/netmap: Document default values set in NewFilter

Make `filtersFromV2` return `nil` if passed
value is `nil`. Document field values of
instance constructed via `NewFilter`. Assert
the values in corresponding unit test.

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

pkg/netmap/filter.go

@@ -148,6 +148,13 @@ func (c *Context) matchKeyValue(f *Filter, b *Node) bool {
 }
 
 // NewFilter creates and returns new Filter instance.
+//
+// Defaults:
+//  - name: "";
+//  - key: "";
+//  - value: "";
+//  - operation: 0;
+//  - filters: nil.
 func NewFilter() *Filter {
 	return NewFilterFromV2(new(netmap.Filter))
 }
@@ -217,6 +224,10 @@ func (f *Filter) SetOperation(op Operation) {
 }
 
 func filtersFromV2(fs []*netmap.Filter) []*Filter {
+	if fs == nil {
+		return nil
+	}
+
 	res := make([]*Filter, 0, len(fs))
 
 	for i := range fs {

pkg/netmap/filter_test.go

@@ -307,3 +307,25 @@ func TestFilterEncoding(t *testing.T) {
 		require.Equal(t, f, f2)
 	})
 }
+
+func TestNewFilter(t *testing.T) {
+	t.Run("default values", func(t *testing.T) {
+		filter := NewFilter()
+
+		// check initial values
+		require.Empty(t, filter.Name())
+		require.Empty(t, filter.Key())
+		require.Empty(t, filter.Value())
+		require.Zero(t, filter.Operation())
+		require.Nil(t, filter.InnerFilters())
+
+		// convert to v2 message
+		filterV2 := filter.ToV2()
+
+		require.Empty(t, filterV2.GetName())
+		require.Empty(t, filterV2.GetKey())
+		require.Empty(t, filterV2.GetValue())
+		require.Equal(t, netmap.UnspecifiedOperation, filterV2.GetOp())
+		require.Nil(t, filterV2.GetFilters())
+	})
+}