Fix Meta unmarshal when is Source Register (#174)

The Meta nftables expression was not filling the Register and SourceRegister
fields when unmarshalling.

Add a check for NFTA_META_SREG message when unmarshalling to fill the Meta
fields.
Add Unit Test for source and destination unmarshall.

Signed-off-by: Rafael Campos <rafael.campos.lasheras@ibm.com>

expr/expr.go

@@ -243,6 +243,9 @@ func (e *Meta) unmarshal(fam byte, data []byte) error {
 	ad.ByteOrder = binary.BigEndian
 	for ad.Next() {
 		switch ad.Type() {
+		case unix.NFTA_META_SREG:
+			e.Register = ad.Uint32()
+			e.SourceRegister = true
 		case unix.NFTA_META_DREG:
 			e.Register = ad.Uint32()
 		case unix.NFTA_META_KEY:

expr/meta_test.go

@@ -0,0 +1,62 @@
+package expr
+
+import (
+	"encoding/binary"
+	"reflect"
+	"testing"
+
+	"github.com/mdlayher/netlink"
+	"golang.org/x/sys/unix"
+)
+
+func TestMeta(t *testing.T) {
+	t.Parallel()
+	tests := []struct {
+		name string
+		meta Meta
+	}{
+		{
+			name: "Unmarshal Meta DestRegister case",
+			meta: Meta{
+				Key:            1,
+				SourceRegister: false,
+				Register:       1,
+			},
+		},
+		{
+			name: "Unmarshal Meta SourceRegister case",
+			meta: Meta{
+				Key:            1,
+				SourceRegister: true,
+				Register:       1,
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			nMeta := Meta{}
+			data, err := tt.meta.marshal(0 /* don't care in this test */)
+			if err != nil {
+				t.Fatalf("marshal error: %+v", err)
+
+			}
+			ad, err := netlink.NewAttributeDecoder(data)
+			if err != nil {
+				t.Fatalf("NewAttributeDecoder() error: %+v", err)
+			}
+			ad.ByteOrder = binary.BigEndian
+			for ad.Next() {
+				if ad.Type() == unix.NFTA_EXPR_DATA {
+					if err := nMeta.unmarshal(0, ad.Bytes()); err != nil {
+						t.Errorf("unmarshal error: %+v", err)
+						break
+					}
+				}
+			}
+			if !reflect.DeepEqual(tt.meta, nMeta) {
+				t.Fatalf("original %+v and recovered %+v Exthdr structs are different", tt.meta, nMeta)
+			}
+		})
+	}
+}