multi: cap anchors feerate at configurable maximum

This commit caps the update fee the initiator will send when the anchors
channel type is used. We do not limit anything on the receiver side.

10 sat/vbyte is the current default max fee rate we use. This should be
enough to ensure propagation before anchoring down the commitment
transaction.

lnwallet/channel_test.go

@@ -7996,6 +7996,19 @@ func TestForceCloseBorkedState(t *testing.T) {
 func TestChannelMaxFeeRate(t *testing.T) {
 	t.Parallel()
 
+	assertMaxFeeRate := func(c *LightningChannel,
+		maxAlloc float64, anchorMax, expFeeRate chainfee.SatPerKWeight) {
+
+		t.Helper()
+
+		maxFeeRate := c.MaxFeeRate(maxAlloc, anchorMax)
+		if maxFeeRate != expFeeRate {
+			t.Fatalf("expected max fee rate of %v with max "+
+				"allocation of %v, got %v", expFeeRate,
+				maxAlloc, maxFeeRate)
+		}
+	}
+
 	aliceChannel, _, cleanUp, err := CreateTestChannels(
 		channeldb.SingleFunderTweaklessBit,
 	)
@@ -8004,21 +8017,32 @@ func TestChannelMaxFeeRate(t *testing.T) {
 	}
 	defer cleanUp()
 
-	assertMaxFeeRate := func(maxAlloc float64,
-		expFeeRate chainfee.SatPerKWeight) {
+	assertMaxFeeRate(aliceChannel, 1.0, 0, 690607734)
+	assertMaxFeeRate(aliceChannel, 0.001, 0, 690607)
+	assertMaxFeeRate(aliceChannel, 0.000001, 0, 690)
+	assertMaxFeeRate(aliceChannel, 0.0000001, 0, chainfee.FeePerKwFloor)
 
-		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)
-		}
+	// Check that anchor channels are capped at their max fee rate.
+	anchorChannel, _, cleanUp, err := CreateTestChannels(
+		channeldb.SingleFunderTweaklessBit | channeldb.AnchorOutputsBit,
+	)
+	if err != nil {
+		t.Fatalf("unable to create test channels: %v", err)
 	}
+	defer cleanUp()
 
-	assertMaxFeeRate(1.0, 690607734)
-	assertMaxFeeRate(0.001, 690607)
-	assertMaxFeeRate(0.000001, 690)
-	assertMaxFeeRate(0.0000001, chainfee.FeePerKwFloor)
+	// Anchor commitments are heavier, hence will the same allocation lead
+	// to slightly lower fee rates.
+	assertMaxFeeRate(
+		anchorChannel, 1.0, chainfee.FeePerKwFloor,
+		chainfee.FeePerKwFloor,
+	)
+	assertMaxFeeRate(anchorChannel, 0.001, 1000000, 444839)
+	assertMaxFeeRate(anchorChannel, 0.001, 300000, 300000)
+	assertMaxFeeRate(anchorChannel, 0.000001, 700, 444)
+	assertMaxFeeRate(
+		anchorChannel, 0.0000001, 1000000, chainfee.FeePerKwFloor,
+	)
 }
 
 // TestChannelFeeRateFloor asserts that valid commitments can be proposed and