lnwallet: reject UpdateFee messages if it puts initiator below the water

In this commit, we ensure that we reject any UpdateFee messages if
after applying the update, the initiator doesn’t have enough funds to
actually pay for the new commitment state.

A test has been added to exercise this new behavior.

lnwallet/channel_test.go

@@ -3376,6 +3376,36 @@ func TestChanSyncOweRevocationAndCommitForceTransition(t *testing.T) {
 	}
 }
 
+// TestFeeUpdateRejectInsaneFee tests that if the initiator tries to attach a
+// fee that would put them below their current reserve, then it's rejected by
+// the state machine.
+func TestFeeUpdateRejectInsaneFee(t *testing.T) {
+	t.Parallel()
+
+	// Create a test channel which will be used for the duration of this
+	// unittest. The channel will be funded evenly with Alice having 5 BTC,
+	// and Bob having 5 BTC.
+	aliceChannel, bobChannel, cleanUp, err := createTestChannels(1)
+	if err != nil {
+		t.Fatalf("unable to create test channels: %v", err)
+	}
+	defer cleanUp()
+
+	// Next, we'll try to add a fee rate to Alice which is 1,000,000x her
+	// starting fee rate.
+	startingFeeRate := aliceChannel.channelState.LocalCommitment.FeePerKw
+	newFeeRate := startingFeeRate * 1000000
+
+	// Both Alice and Bob should reject this new fee rate as it it far too
+	// large.
+	if err := aliceChannel.UpdateFee(newFeeRate); err == nil {
+		t.Fatalf("alice should've rejected fee update")
+	}
+	if err := bobChannel.ReceiveUpdateFee(newFeeRate); err == nil {
+		t.Fatalf("bob should've rejected fee update")
+	}
+}
+
 // TestChannelRetransmissionFeeUpdate tests that the initiator will include any
 // pending fee updates if it needs to retransmit signatures.
 func TestChannelRetransmissionFeeUpdate(t *testing.T) {