lnwallet: add noop HTLC below reserve test

Add an extra test that exposes the behavior of the lightning channel
state machine when one party's balance is close to the configured
reserve. Specifically we check that the expected amount of noop HTLCs
actually send the amount to the remote side, with the rest of them
correctly getting nullified.

Author: GeorgeTsagk

lnwallet/channel_test.go

@@ -11396,3 +11396,92 @@ func TestNoopAddSettle(t *testing.T) {
 	require.Equal(t, aliceBalance, aliceBalanceFinal)
 	require.Equal(t, bobBalance, bobBalanceFinal)
 }
+
+// TestNoopAddBelowReserve tests that the noop HTLCs behave as expected when
+// added over a channel where a party is below their reserve.
+func TestNoopAddBelowReserve(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
+	aliceChan, bobChan, err := CreateTestChannels(t, chanType)
+	require.NoError(t, err, "unable to create test channels")
+
+	aliceBalance := aliceChan.channelState.LocalCommitment.LocalBalance
+	bobBalance := bobChan.channelState.LocalCommitment.LocalBalance
+
+	const (
+		// htlcAmt is the default HTLC amount to be used, epxressed in
+		// milli-satoshis.
+		htlcAmt = lnwire.MilliSatoshi(500_000)
+
+		// numHtlc is the total number of HTLCs to be added/settled over
+		// the channel.
+		numHtlc = 20
+	)
+
+	// Let's create the noop add TLV record to be used in all added HTLCs
+	// over the channel.
+	noopRecord := tlv.NewPrimitiveRecord[NoopAddHtlcType, bool](true)
+	records, err := tlv.RecordsToMap([]tlv.Record{noopRecord.Record()})
+	require.NoError(t, err)
+
+	// Let's set Bob's reserve to whatever his local balance is, minus half
+	// of the total amount to be added by the total HTLCs. This way we can
+	// also verify that the noop-adds will start the nullification only once
+	// Bob is above reserve.
+	reserveTarget := (numHtlc / 2) * htlcAmt
+	bobReserve := bobBalance + reserveTarget
+
+	bobChan.channelState.LocalChanCfg.ChanReserve =
+		bobReserve.ToSatoshis()
+
+	aliceChan.channelState.RemoteChanCfg.ChanReserve =
+		bobReserve.ToSatoshis()
+
+	// Add and settle all the HTLCs over the channel.
+	for i := range numHtlc {
+		htlc, preimage := createHTLC(i, htlcAmt)
+		htlc.CustomRecords = records
+
+		aliceHtlcIndex, err := aliceChan.AddHTLC(htlc, nil)
+		require.NoError(t, err, "alice unable to add htlc")
+		bobHtlcIndex, err := bobChan.ReceiveHTLC(htlc)
+		require.NoError(t, err, "bob unable to receive htlc")
+
+		require.NoError(t, ForceStateTransition(aliceChan, bobChan))
+
+		// We'll have Bob settle the HTLC, then force another state
+		// transition.
+		err = bobChan.SettleHTLC(preimage, bobHtlcIndex, nil, nil, nil)
+		require.NoError(t, err, "bob unable to settle inbound htlc")
+		err = aliceChan.ReceiveHTLCSettle(preimage, aliceHtlcIndex)
+		if err != nil {
+			t.Fatalf("alice unable to accept settle of outbound "+
+				"htlc: %v", err)
+		}
+		require.NoError(t, ForceStateTransition(aliceChan, bobChan))
+	}
+
+	// We need to kick the state transition one last time for the balances
+	// to be updated on both commitments.
+	require.NoError(t, ForceStateTransition(aliceChan, bobChan))
+
+	aliceBalanceFinal := aliceChan.channelState.LocalCommitment.LocalBalance
+	bobBalanceFinal := bobChan.channelState.LocalCommitment.LocalBalance
+
+	// The balances of Alice and Bob must have changed exactly by half the
+	// total number of HTLCs we added over the channel, plus one to get Bob
+	// above the reserve. Bob's final balance should be as much as his
+	// reserve plus one extra default HTLC amount.
+	require.Equal(t, aliceBalance-htlcAmt*(numHtlc/2+1), aliceBalanceFinal)
+	require.Equal(t, bobBalance+htlcAmt*(numHtlc/2+1), bobBalanceFinal)
+	require.Equal(
+		t, bobBalanceFinal.ToSatoshis(),
+		bobChan.LocalChanReserve()+htlcAmt.ToSatoshis(),
+	)
+}