lnwallet: add noop HTLC test

Adds a simple test to check the noop HTLC logic of the lightning
channel.

Author: GeorgeTsagk

lnwallet/channel_test.go

@@ -11339,3 +11339,60 @@ func TestCreateCooperativeCloseTx(t *testing.T) {
 		})
 	}
 }
+
+// TestNoopAddSettle tests that adding and settling an HTLC with no-op, no
+// balances are actually affected.
+func TestNoopAddSettle(t *testing.T) {
+	t.Parallel()
+
+	// Create a test channel which will be used for the duration of this
+	// unittest. The channel will be funded evenly with Alice having 5 BTC,
+	// and Bob having 5 BTC.
+	chanType := channeldb.SimpleTaprootFeatureBit |
+		channeldb.AnchorOutputsBit | channeldb.ZeroHtlcTxFeeBit |
+		channeldb.SingleFunderTweaklessBit | channeldb.TapscriptRootBit
+	aliceChannel, bobChannel, err := CreateTestChannels(
+		t, chanType,
+	)
+	require.NoError(t, err, "unable to create test channels")
+
+	const htlcAmt = 10_000
+	htlc, preimage := createHTLC(0, htlcAmt)
+	noopRecord := tlv.NewPrimitiveRecord[tlv.TlvType65544, bool](true)
+
+	records, err := tlv.RecordsToMap([]tlv.Record{noopRecord.Record()})
+	require.NoError(t, err)
+	htlc.CustomRecords = records
+
+	aliceBalance := aliceChannel.channelState.LocalCommitment.LocalBalance
+	bobBalance := bobChannel.channelState.LocalCommitment.LocalBalance
+
+	// Have Alice add the HTLC, then lock it in with a new state transition.
+	aliceHtlcIndex, err := aliceChannel.AddHTLC(htlc, nil)
+	require.NoError(t, err, "alice unable to add htlc")
+	bobHtlcIndex, err := bobChannel.ReceiveHTLC(htlc)
+	require.NoError(t, err, "bob unable to receive htlc")
+	if err := ForceStateTransition(aliceChannel, bobChannel); err != nil {
+		t.Fatalf("Can't update the channel state: %v", err)
+	}
+
+	// We'll have Bob settle the HTLC, then force another state transition.
+	err = bobChannel.SettleHTLC(preimage, bobHtlcIndex, nil, nil, nil)
+	require.NoError(t, err, "bob unable to settle inbound htlc")
+	err = aliceChannel.ReceiveHTLCSettle(preimage, aliceHtlcIndex)
+	if err != nil {
+		t.Fatalf("alice unable to accept settle of outbound "+
+			"htlc: %v", err)
+	}
+	if err := ForceStateTransition(aliceChannel, bobChannel); err != nil {
+		t.Fatalf("Can't update the channel state: %v", err)
+	}
+
+	aliceBalanceFinal := aliceChannel.channelState.LocalCommitment.LocalBalance //nolint:ll
+	bobBalanceFinal := bobChannel.channelState.LocalCommitment.LocalBalance
+
+	// The balances of Alice and Bob should be the exact same and shouldn't
+	// have changed.
+	require.Equal(t, aliceBalance, aliceBalanceFinal)
+	require.Equal(t, bobBalance, bobBalanceFinal)
+}