htlcswitch: within link, with each new block, check to see if commit fee should change

In this commit we add a new case to the main select statement within a
channel link. This select statement will serve as a Sipping Bird which
will check the network fee rate (as returned by the fee estimator) and
compare that to the fee on the commitment transaction. Using the
shouldAdjustCommitFee function, we determine if we should update the
commitment fee. If so, then we’ll send an UpdateFee message and also
trigger a new commitment update.

We also add a new unit test: TestChannelLinkUpdateCommitFee to ensure
that we update the fee accordingly if the fee increases or decreases by
a large portion.

htlcswitch/mock.go

@@ -22,8 +22,44 @@ import (
 	"github.com/roasbeef/btcd/chaincfg/chainhash"
 	"github.com/roasbeef/btcd/txscript"
 	"github.com/roasbeef/btcd/wire"
+	"github.com/roasbeef/btcutil"
 )
 
+type mockFeeEstimator struct {
+	byteFeeIn   chan btcutil.Amount
+	weightFeeIn chan btcutil.Amount
+
+	quit chan struct{}
+}
+
+func (m *mockFeeEstimator) EstimateFeePerByte(numBlocks uint32) (btcutil.Amount, error) {
+	select {
+	case feeRate := <-m.byteFeeIn:
+		return feeRate, nil
+	case <-m.quit:
+		return 0, fmt.Errorf("exiting")
+	}
+}
+
+func (m *mockFeeEstimator) EstimateFeePerWeight(numBlocks uint32) (btcutil.Amount, error) {
+	select {
+	case feeRate := <-m.weightFeeIn:
+		return feeRate, nil
+	case <-m.quit:
+		return 0, fmt.Errorf("exiting")
+	}
+}
+
+func (m *mockFeeEstimator) Start() error {
+	return nil
+}
+func (m *mockFeeEstimator) Stop() error {
+	close(m.quit)
+	return nil
+}
+
+var _ lnwallet.FeeEstimator = (*mockFeeEstimator)(nil)
+
 type mockServer struct {
 	started  int32
 	shutdown int32
@@ -304,6 +340,8 @@ func (s *mockServer) readHandler(message lnwire.Message) error {
 		return nil
 	case *lnwire.ChannelReestablish:
 		targetChan = msg.ChanID
+	case *lnwire.UpdateFee:
+		targetChan = msg.ChanID
 	default:
 		return fmt.Errorf("unknown message type: %T", msg)
 	}