lnwallet+docs: minrelayfee always above fee floor

The minimum relay fee is always ensured to be above our fee floor except
in the very first min relay fee query to bitcoind. This commit ensures
that the fee floor is respected in this first query.

docs/release-notes/release-notes-0.14.2.md

@@ -35,6 +35,9 @@
 
 * [Fix memory corruption in Mission Control
   Store](https://github.com/lightningnetwork/lnd/pull/6068)
+ 
+* [Ensure that the min relay fee is always clamped by our fee
+  floor](https://github.com/lightningnetwork/lnd/pull/6076)
 
 ## RPC Server
 
@@ -49,6 +52,7 @@
 
 * Andras Banki-Horvath
 * Bjarne Magnussen
+* Elle Mouton
 * Harsha Goli
 * Martin Habovštiak
 * Naveen Srinivasan

lnwallet/chainfee/minfeemanager.go

@@ -32,6 +32,12 @@ func newMinFeeManager(minUpdateInterval time.Duration,
 		return nil, err
 	}
 
+	// Ensure that the minimum fee we use is always clamped by our fee
+	// floor.
+	if minFee < FeePerKwFloor {
+		minFee = FeePerKwFloor
+	}
+
 	return &minFeeManager{
 		minFeePerKW:       minFee,
 		lastUpdatedTime:   time.Now(),

lnwallet/chainfee/minfeemanager_test.go

@@ -23,8 +23,10 @@ func (m *mockChainBackend) fetchFee() (SatPerKWeight, error) {
 func TestMinFeeManager(t *testing.T) {
 	t.Parallel()
 
+	// Initialize the mock backend and let it have a minimum fee rate
+	// below our fee floor.
 	chainBackend := &mockChainBackend{
-		minFee: SatPerKWeight(1000),
+		minFee: FeePerKwFloor - 1,
 	}
 
 	// Initialise the min fee manager. This should call the chain backend
@@ -36,11 +38,14 @@ func TestMinFeeManager(t *testing.T) {
 	require.NoError(t, err)
 	require.Equal(t, 1, chainBackend.callCount)
 
+	// Check that the minimum fee rate is clamped by our fee floor.
+	require.Equal(t, feeManager.minFeePerKW, FeePerKwFloor)
+
 	// 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, minFee, FeePerKwFloor)
 	require.Equal(t, 1, chainBackend.callCount)
 
 	// Fake the passing of time.