tappsbt: test decode handling of global Unknowns

In this commit, we add a new test to ensure that if a Packet is missing
the Unknown values we need to identify VPackets, the decoder fails. We
also check that decoding does not fail if Unknown values unrelated to
VPackets are included.

tappsbt/decode_test.go

@@ -10,6 +10,7 @@ import (
 	"strings"
 	"testing"
 
+	"github.com/btcsuite/btcd/btcutil/psbt"
 	"github.com/lightninglabs/taproot-assets/address"
 	"github.com/lightninglabs/taproot-assets/asset"
 	"github.com/lightninglabs/taproot-assets/commitment"
@@ -69,6 +70,73 @@ func assertEqualPackets(t *testing.T, expected, actual *VPacket) {
 	}
 }
 
+// TestGlobalUnknownFields tests that the global Unknown fields mandatory for a
+// valid VPacket are present for an encoded VPacket. We also test that when
+// decoding a VPacket from a Packet, a Packet with missing mandatory fields is
+// rejected, and extra global Unknown fields are permitted.
+func TestGlobalUnknownFields(t *testing.T) {
+	// Make a random packet.
+	pkg := RandPacket(t, false, false)
+
+	// An encoded valid packet should have exactly three global Unknown
+	// fields.
+	packet, err := pkg.EncodeAsPsbt()
+	require.NoError(t, err)
+	require.Len(t, packet.Unknowns, 3)
+
+	// Specifically, the isVirtual marker, HRP, and Version must be present.
+	requiredKeys := [][]byte{
+		PsbtKeyTypeGlobalTapIsVirtualTx,
+		PsbtKeyTypeGlobalTapChainParamsHRP,
+		PsbtKeyTypeGlobalTapPsbtVersion,
+	}
+	for _, key := range requiredKeys {
+		_, err := findCustomFieldsByKeyPrefix(packet.Unknowns, key)
+		require.NoError(t, err)
+	}
+
+	// Decoding a VPacket from this minimal Packet must succeed.
+	_, err = NewFromPsbt(packet)
+	require.NoError(t, err)
+
+	var packetBuf bytes.Buffer
+	err = packet.Serialize(&packetBuf)
+	require.NoError(t, err)
+
+	cloneBuffer := func(b *bytes.Buffer) *bytes.Buffer {
+		return bytes.NewBuffer(bytes.Clone(b.Bytes()))
+	}
+
+	// If we remove a mandatory VPacket field from the Packet, decoding
+	// must fail.
+	invalidPacketBytes := cloneBuffer(&packetBuf)
+	invalidPacket, err := psbt.NewFromRawBytes(invalidPacketBytes, false)
+	require.NoError(t, err)
+
+	invalidPacket.Unknowns = invalidPacket.Unknowns[1:]
+	_, err = NewFromPsbt(invalidPacket)
+	require.Error(t, err)
+
+	// If we add a global Unknown field to the valid Packet, decoding must
+	// still succeed.
+	extraPacketBytes := cloneBuffer(&packetBuf)
+	extraPacket, err := psbt.NewFromRawBytes(extraPacketBytes, false)
+	require.NoError(t, err)
+
+	// The VPacket global Unknown keys start at 0x70, so we'll use a key
+	// value very far from that.
+	extraUnknown := &psbt.Unknown{
+		Key:   []byte{0xaa},
+		Value: []byte("really_cool_unknown_value"),
+	}
+	extraPacket.Unknowns = append(extraPacket.Unknowns, extraUnknown)
+
+	// The decoded VPacket should not contain the extra Unknown field, but
+	// the decoder should succeed.
+	_, err = NewFromPsbt(extraPacket)
+	require.NoError(t, err)
+}
+
 // TestEncodingDecoding tests the decoding of a virtual packet from raw bytes.
 func TestEncodingDecoding(t *testing.T) {
 	t.Parallel()