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

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

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

pkg/netmap/policy.go

@@ -8,6 +8,12 @@ import (
 type PlacementPolicy netmap.PlacementPolicy
 
 // NewPlacementPolicy creates and returns new PlacementPolicy instance.
+//
+// Defaults:
+//  - backupFactor: 0;
+//  - replicas nil;
+//  - selectors nil;
+//  - filters nil.
 func NewPlacementPolicy() *PlacementPolicy {
 	return NewPlacementPolicyFromV2(new(netmap.PlacementPolicy))
 }
@@ -31,6 +37,10 @@ func (p *PlacementPolicy) Replicas() []*Replica {
 	rs := (*netmap.PlacementPolicy)(p).
 		GetReplicas()
 
+	if rs == nil {
+		return nil
+	}
+
 	res := make([]*Replica, 0, len(rs))
 
 	for i := range rs {
@@ -73,6 +83,10 @@ func (p *PlacementPolicy) Selectors() []*Selector {
 	rs := (*netmap.PlacementPolicy)(p).
 		GetSelectors()
 
+	if rs == nil {
+		return nil
+	}
+
 	res := make([]*Selector, 0, len(rs))
 
 	for i := range rs {

pkg/netmap/policy_test.go

@@ -151,6 +151,24 @@ func TestNewPlacementPolicy(t *testing.T) {
 
 		require.Nil(t, x.ToV2())
 	})
+
+	t.Run("default values", func(t *testing.T) {
+		pp := NewPlacementPolicy()
+
+		// check initial values
+		require.Nil(t, pp.Replicas())
+		require.Nil(t, pp.Filters())
+		require.Nil(t, pp.Selectors())
+		require.Zero(t, pp.ContainerBackupFactor())
+
+		// convert to v2 message
+		ppV2 := pp.ToV2()
+
+		require.Nil(t, ppV2.GetReplicas())
+		require.Nil(t, ppV2.GetFilters())
+		require.Nil(t, ppV2.GetSelectors())
+		require.Zero(t, ppV2.GetContainerBackupFactor())
+	})
 }
 
 func TestNewPlacementPolicyFromV2(t *testing.T) {