chainfee: add minFeeManager

This commit adds a minFeeManager which holds a copy of minFeePerKW
and updates this fee every few calls.

lnwallet/chainfee/minfeemanager_test.go

@@ -0,0 +1,54 @@
+package chainfee
+
+import (
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+)
+
+type mockChainBackend struct {
+	minFee    SatPerKWeight
+	callCount int
+}
+
+func (m *mockChainBackend) fetchFee() (SatPerKWeight, error) {
+	m.callCount++
+	return m.minFee, nil
+}
+
+// TestMinFeeManager tests that the minFeeManager returns an up to date min fee
+// by querying the chain backend and that it returns a cached fee if the chain
+// backend was recently queried.
+func TestMinFeeManager(t *testing.T) {
+	t.Parallel()
+
+	chainBackend := &mockChainBackend{
+		minFee: SatPerKWeight(1000),
+	}
+
+	// Initialise the min fee manager. This should call the chain backend
+	// once.
+	feeManager, err := newMinFeeManager(
+		100*time.Millisecond,
+		chainBackend.fetchFee,
+	)
+	require.NoError(t, err)
+	require.Equal(t, 1, chainBackend.callCount)
+
+	// If the fee is requested again, the stored fee should be returned
+	// and the chain backend should not be queried.
+	chainBackend.minFee = SatPerKWeight(2000)
+	minFee := feeManager.fetchMinFee()
+	require.Equal(t, minFee, SatPerKWeight(1000))
+	require.Equal(t, 1, chainBackend.callCount)
+
+	// Fake the passing of time.
+	feeManager.lastUpdatedTime = time.Now().Add(-200 * time.Millisecond)
+
+	// If the fee is queried again after the backoff period has passed
+	// then the chain backend should be queried again for the min fee.
+	minFee = feeManager.fetchMinFee()
+	require.Equal(t, SatPerKWeight(2000), minFee)
+	require.Equal(t, 2, chainBackend.callCount)
+}