From c01699853d36cf7c8ce1187ff4a7137dcadd90f8 Mon Sep 17 00:00:00 2001
From: Elle Mouton <elle.mouton@gmail.com>
Date: Fri, 25 Jun 2021 16:19:34 +0200
Subject: [PATCH] chainfee: add minFeeManager

This commit adds a minFeeManager which holds a copy of minFeePerKW
and updates this fee every few calls.
---
 lnwallet/chainfee/minfeemanager.go      | 74 +++++++++++++++++++++++++
 lnwallet/chainfee/minfeemanager_test.go | 54 ++++++++++++++++++
 2 files changed, 128 insertions(+)
 create mode 100644 lnwallet/chainfee/minfeemanager.go
 create mode 100644 lnwallet/chainfee/minfeemanager_test.go

diff --git a/lnwallet/chainfee/minfeemanager.go b/lnwallet/chainfee/minfeemanager.go
new file mode 100644
index 00000000..490b020e
--- /dev/null
+++ b/lnwallet/chainfee/minfeemanager.go
@@ -0,0 +1,74 @@
+package chainfee
+
+import (
+	"sync"
+	"time"
+)
+
+// minFeeManager is used to store and update the minimum fee that is required
+// by a transaction to be accepted to the mempool. The minFeeManager ensures
+// that the backend used to fetch the fee is not queried too regularly.
+type minFeeManager struct {
+	mu                sync.Mutex
+	minFeePerKW       SatPerKWeight
+	lastUpdatedTime   time.Time
+	minUpdateInterval time.Duration
+	fetchFeeFunc      fetchFee
+}
+
+// fetchFee represents a function that can be used to fetch a fee.
+type fetchFee func() (SatPerKWeight, error)
+
+// newMinFeeManager creates a new minFeeManager and uses the
+// given fetchMinFee function to set the minFeePerKW of the minFeeManager.
+// This function requires the fetchMinFee function to succeed.
+func newMinFeeManager(minUpdateInterval time.Duration,
+	fetchMinFee fetchFee) (*minFeeManager, error) {
+
+	minFee, err := fetchMinFee()
+	if err != nil {
+		return nil, err
+	}
+
+	return &minFeeManager{
+		minFeePerKW:       minFee,
+		lastUpdatedTime:   time.Now(),
+		minUpdateInterval: minUpdateInterval,
+		fetchFeeFunc:      fetchMinFee,
+	}, nil
+}
+
+// fetchMinFee returns the stored minFeePerKW if it has been updated recently
+// or if the call to the chain backend fails. Otherwise, it sets the stored
+// minFeePerKW to the fee returned from the backend and floors it based on
+// our fee floor.
+func (m *minFeeManager) fetchMinFee() SatPerKWeight {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	if time.Since(m.lastUpdatedTime) < m.minUpdateInterval {
+		return m.minFeePerKW
+	}
+
+	newMinFee, err := m.fetchFeeFunc()
+	if err != nil {
+		log.Errorf("Unable to fetch updated min fee from chain "+
+			"backend. Using last known min fee instead: %v", err)
+
+		return m.minFeePerKW
+	}
+
+	// By default, we'll use the backend node's minimum fee as the
+	// minimum fee rate we'll propose for transactions. However, if this
+	// happens to be lower than our fee floor, we'll enforce that instead.
+	m.minFeePerKW = newMinFee
+	if m.minFeePerKW < FeePerKwFloor {
+		m.minFeePerKW = FeePerKwFloor
+	}
+	m.lastUpdatedTime = time.Now()
+
+	log.Debugf("Using minimum fee rate of %v sat/kw",
+		int64(m.minFeePerKW))
+
+	return m.minFeePerKW
+}
diff --git a/lnwallet/chainfee/minfeemanager_test.go b/lnwallet/chainfee/minfeemanager_test.go
new file mode 100644
index 00000000..0209f1a2
--- /dev/null
+++ b/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)
+}