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

@@ -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
+}