htlcswitch: extend Mailbox iface with dust, fee methods

This commit extends the Mailbox interface with the SetDustClosure,
SetFeeRate, and DustPackets methods. This enables the mailbox to
report the dust exposure to the Switch when the Switch decides whether
to forward a dust packet. The dust is counted from the time an Add is
introduced via AddPacket until it is removed via AckPacket. This can
lead to some packets being counted twice before they are signed for,
but this is a trade-off between accuracy and simplicity.

htlcswitch/mailbox_test.go

@@ -6,9 +6,13 @@ import (
 	"testing"
 	"time"
 
+	"github.com/btcsuite/btcutil"
 	"github.com/davecgh/go-spew/spew"
+	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/clock"
+	"github.com/lightningnetwork/lnd/lnwallet/chainfee"
 	"github.com/lightningnetwork/lnd/lnwire"
+	"github.com/stretchr/testify/require"
 )
 
 const testExpiry = time.Minute
@@ -536,6 +540,126 @@ func TestMailBoxDuplicateAddPacket(t *testing.T) {
 	})
 }
 
+// TestMailBoxDustHandling tests that DustPackets returns the expected values
+// for the local and remote dust sum after calling SetFeeRate and
+// SetDustClosure.
+func TestMailBoxDustHandling(t *testing.T) {
+	t.Run("tweakless mailbox dust", func(t *testing.T) {
+		testMailBoxDust(t, channeldb.SingleFunderTweaklessBit)
+	})
+	t.Run("zero htlc fee anchors mailbox dust", func(t *testing.T) {
+		testMailBoxDust(t, channeldb.SingleFunderTweaklessBit|
+			channeldb.AnchorOutputsBit|
+			channeldb.ZeroHtlcTxFeeBit,
+		)
+	})
+}
+
+func testMailBoxDust(t *testing.T, chantype channeldb.ChannelType) {
+	t.Parallel()
+
+	ctx := newMailboxContext(t, time.Now(), testExpiry)
+	defer ctx.mailbox.Stop()
+
+	_, _, aliceID, bobID := genIDs()
+
+	// It should not be the case that the MailBox has packets before the
+	// feeRate or dustClosure is set. This is because the mailbox is always
+	// created *with* its associated link and attached via AttachMailbox,
+	// where these parameters will be set. Even though the lifetime is
+	// longer than the link, the setting will persist across multiple link
+	// creations.
+	ctx.mailbox.SetFeeRate(chainfee.SatPerKWeight(253))
+
+	localDustLimit := btcutil.Amount(400)
+	remoteDustLimit := btcutil.Amount(500)
+	isDust := dustHelper(chantype, localDustLimit, remoteDustLimit)
+	ctx.mailbox.SetDustClosure(isDust)
+
+	// The first packet will be dust according to the remote dust limit,
+	// but not the local. We set a different amount if this is a zero fee
+	// htlc channel type.
+	firstAmt := lnwire.MilliSatoshi(600_000)
+
+	if chantype.ZeroHtlcTxFee() {
+		firstAmt = lnwire.MilliSatoshi(450_000)
+	}
+
+	firstPkt := &htlcPacket{
+		outgoingChanID: aliceID,
+		outgoingHTLCID: 0,
+		incomingChanID: bobID,
+		incomingHTLCID: 0,
+		amount:         firstAmt,
+		htlc: &lnwire.UpdateAddHTLC{
+			ID: uint64(0),
+		},
+	}
+
+	err := ctx.mailbox.AddPacket(firstPkt)
+	require.NoError(t, err)
+
+	// Assert that the local sum is 0, and the remote sum accounts for this
+	// added packet.
+	localSum, remoteSum := ctx.mailbox.DustPackets()
+	require.Equal(t, lnwire.MilliSatoshi(0), localSum)
+	require.Equal(t, firstAmt, remoteSum)
+
+	// The next packet will be dust according to both limits.
+	secondAmt := lnwire.MilliSatoshi(300_000)
+	secondPkt := &htlcPacket{
+		outgoingChanID: aliceID,
+		outgoingHTLCID: 1,
+		incomingChanID: bobID,
+		incomingHTLCID: 1,
+		amount:         secondAmt,
+		htlc: &lnwire.UpdateAddHTLC{
+			ID: uint64(1),
+		},
+	}
+
+	err = ctx.mailbox.AddPacket(secondPkt)
+	require.NoError(t, err)
+
+	// Assert that both the local and remote sums have increased by the
+	// second amount.
+	localSum, remoteSum = ctx.mailbox.DustPackets()
+	require.Equal(t, secondAmt, localSum)
+	require.Equal(t, firstAmt+secondAmt, remoteSum)
+
+	// Now we pull both packets off of the queue.
+	for i := 0; i < 2; i++ {
+		select {
+		case <-ctx.mailbox.PacketOutBox():
+		case <-time.After(50 * time.Millisecond):
+			ctx.t.Fatalf("did not receive packet in time")
+		}
+	}
+
+	// Assert that the sums haven't changed.
+	localSum, remoteSum = ctx.mailbox.DustPackets()
+	require.Equal(t, secondAmt, localSum)
+	require.Equal(t, firstAmt+secondAmt, remoteSum)
+
+	// Remove the first packet from the mailbox.
+	removed := ctx.mailbox.AckPacket(firstPkt.inKey())
+	require.True(t, removed)
+
+	// Assert that the remote sum does not include the firstAmt.
+	localSum, remoteSum = ctx.mailbox.DustPackets()
+	require.Equal(t, secondAmt, localSum)
+	require.Equal(t, secondAmt, remoteSum)
+
+	// Remove the second packet from the mailbox.
+	removed = ctx.mailbox.AckPacket(secondPkt.inKey())
+	require.True(t, removed)
+
+	// Assert that both sums are equal to 0.
+	localSum, remoteSum = ctx.mailbox.DustPackets()
+	require.Equal(t, lnwire.MilliSatoshi(0), localSum)
+	require.Equal(t, lnwire.MilliSatoshi(0), remoteSum)
+}
+
 // TestMailOrchestrator asserts that the orchestrator properly buffers packets
 // for channels that haven't been made live, such that they are delivered
 // immediately after BindLiveShortChanID. It also tests that packets are delivered