lnwallet: enforce fee floor on max fee allocation

Without this, it was possible for a combination of our balance and max
fee allocation to result in a fee rate below the fee floor causing the
remote party to reject the update and close the channel.

lnwallet/channel.go

@@ -5,6 +5,7 @@ import (
 	"container/list"
 	"crypto/sha256"
 	"fmt"
+	"math"
 	"sort"
 	"sync"
 
@@ -6239,7 +6240,10 @@ func (lc *LightningChannel) MaxFeeRate(maxAllocation float64) SatPerKWeight {
 		balance.ToSatoshis() + lc.channelState.LocalCommitment.CommitFee,
 	)
 	maxFee := feeBalance * maxAllocation
-	return SatPerKWeight(maxFee / (float64(weight) / 1000))
+
+	// Ensure the fee rate doesn't dip below the fee floor.
+	maxFeeRate := maxFee / (float64(weight) / 1000)
+	return SatPerKWeight(math.Max(maxFeeRate, float64(FeePerKwFloor)))
 }
 
 // RemoteNextRevocation returns the channelState's RemoteNextRevocation.

lnwallet/channel_test.go

@@ -6611,3 +6611,30 @@ func TestForceCloseBorkedState(t *testing.T) {
 		t.Fatalf("append remove chain tail should have failed")
 	}
 }
+
+// TestChannelMaxFeeRate ensures we correctly compute a channel initiator's max
+// fee rate based on an allocation and its available balance. It should never
+// dip below the established fee floor.
+func TestChannelMaxFeeRate(t *testing.T) {
+	t.Parallel()
+
+	aliceChannel, _, cleanUp, err := CreateTestChannels(true)
+	if err != nil {
+		t.Fatalf("unable to create test channels: %v", err)
+	}
+	defer cleanUp()
+
+	assertMaxFeeRate := func(maxAlloc float64, expFeeRate SatPerKWeight) {
+		maxFeeRate := aliceChannel.MaxFeeRate(maxAlloc)
+		if maxFeeRate != expFeeRate {
+			t.Fatalf("expected max fee rate of %v with max "+
+				"allocation of %v, got %v", expFeeRate,
+				maxAlloc, maxFeeRate)
+		}
+	}
+
+	assertMaxFeeRate(1.0, 690607734)
+	assertMaxFeeRate(0.001, 690607)
+	assertMaxFeeRate(0.000001, 690)
+	assertMaxFeeRate(0.0000001, FeePerKwFloor)
+}