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

Make `nil` as a default value for attributes.
Document field values of instance constructed
via `New`. Assert the values in corresponding
unit test.

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

pkg/container/container.go

@@ -20,6 +20,17 @@ type Container struct {
 	sig *pkg.Signature
 }
 
+// New creates, initializes and returns blank Container instance.
+//
+// Defaults:
+//  - token: nil;
+//  - sig: nil;
+//  - basicACL: acl.PrivateBasicRule;
+//  - version: nil;
+//  - nonce: random UUID;
+//  - attr: nil;
+//  - policy: nil;
+//  - ownerID: nil.
 func New(opts ...NewOption) *Container {
 	cnrOptions := defaultContainerOptions()
 

pkg/container/container_test.go

@@ -98,4 +98,37 @@ func TestContainer_ToV2(t *testing.T) {
 
 		require.Nil(t, x.ToV2())
 	})
+
+	t.Run("default values", func(t *testing.T) {
+		cnt := container.New()
+
+		// check initial values
+		require.Nil(t, cnt.SessionToken())
+		require.Nil(t, cnt.Signature())
+		require.Nil(t, cnt.Version())
+		require.Nil(t, cnt.Attributes())
+		require.Nil(t, cnt.PlacementPolicy())
+		require.Nil(t, cnt.OwnerID())
+
+		require.EqualValues(t, acl.PrivateBasicRule, cnt.BasicACL())
+
+		nonce, err := cnt.NonceUUID()
+		require.NoError(t, err)
+		require.NotNil(t, nonce)
+
+		// convert to v2 message
+		cntV2 := cnt.ToV2()
+
+		nonceV2, err := uuid.FromBytes(cntV2.GetNonce())
+		require.NoError(t, err)
+
+		require.Equal(t, nonce.String(), nonceV2.String())
+
+		require.Nil(t, cntV2.GetVersion())
+		require.Nil(t, cntV2.GetAttributes())
+		require.Nil(t, cntV2.GetPlacementPolicy())
+		require.Nil(t, cntV2.GetOwnerID())
+
+		require.Equal(t, uint32(acl.PrivateBasicRule), cntV2.GetBasicACL())
+	})
 }