[#302] pkg/checksum: Convert nil `Checksum` to nil message

Document that `Checksum.ToV2` method return `nil`
when called on `nil`. Document that `NewChecksumFromV2`
function return `nil` when called on `nil`. Write
corresponding unit tests.

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

pkg/checksum.go

@@ -28,6 +28,8 @@ const (
 )
 
 // NewChecksumFromV2 wraps v2 Checksum message to Checksum.
+//
+// Nil refs.Checksum converts to nil.
 func NewChecksumFromV2(cV2 *refs.Checksum) *Checksum {
 	return (*Checksum)(cV2)
 }
@@ -72,6 +74,9 @@ func (c *Checksum) SetTillichZemor(v [64]byte) {
 	checksum.SetSum(v[:])
 }
 
+// ToV2 converts Checksum to v2 Checksum message.
+//
+// Nil Checksum converts to nil.
 func (c *Checksum) ToV2() *refs.Checksum {
 	return (*refs.Checksum)(c)
 }

pkg/checksum_test.go

@@ -99,3 +99,19 @@ func TestChecksumEncoding(t *testing.T) {
 		require.Equal(t, cs, cs2)
 	})
 }
+
+func TestNewChecksumFromV2(t *testing.T) {
+	t.Run("from nil", func(t *testing.T) {
+		var x *refs.Checksum
+
+		require.Nil(t, NewChecksumFromV2(x))
+	})
+}
+
+func TestChecksum_ToV2(t *testing.T) {
+	t.Run("nil", func(t *testing.T) {
+		var x *Checksum
+
+		require.Nil(t, x.ToV2())
+	})
+}