htlcswitch: return FEE_INSUFFICIENT before checking balance

htlcswitch/link.go

@@ -2551,17 +2551,8 @@ func (l *channelLink) CheckHtlcForward(payHash [32]byte,
 	policy := l.cfg.FwrdingPolicy
 	l.RUnlock()
 
-	// First check whether the outgoing htlc satisfies the channel policy.
+	// Using the amount of the incoming HTLC, we'll calculate the expected
-	err := l.canSendHtlc(
+	// fee this incoming HTLC must carry in order to satisfy the
-		policy, payHash, amtToForward, outgoingTimeout, heightNow,
-		originalScid,
-	)
-	if err != nil {
-		return err
-	}
-
-	// Next, using the amount of the incoming HTLC, we'll calculate the
-	// expected fee this incoming HTLC must carry in order to satisfy the
 	// constraints of the outgoing link.
 	expectedFee := ExpectedFee(policy, amtToForward)
 
@@ -2569,7 +2560,8 @@ func (l *channelLink) CheckHtlcForward(payHash [32]byte,
 	// this HTLC as it didn't provide a sufficient amount of fees, or the
 	// values have been tampered with, or the send used incorrect/dated
 	// information to construct the forwarding information for this hop. In
-	// any case, we'll cancel this HTLC.
+	// any case, we'll cancel this HTLC. We're checking for this case first
+	// to leak as little information as possible.
 	actualFee := incomingHtlcAmt - amtToForward
 	if incomingHtlcAmt < amtToForward || actualFee < expectedFee {
 		l.log.Warnf("outgoing htlc(%x) has insufficient fee: "+
@@ -2585,6 +2577,15 @@ func (l *channelLink) CheckHtlcForward(payHash [32]byte,
 		return NewLinkError(failure)
 	}
 
+	// Check whether the outgoing htlc satisfies the channel policy.
+	err := l.canSendHtlc(
+		policy, payHash, amtToForward, outgoingTimeout, heightNow,
+		originalScid,
+	)
+	if err != nil {
+		return err
+	}
+
 	// Finally, we'll ensure that the time-lock on the outgoing HTLC meets
 	// the following constraint: the incoming time-lock minus our time-lock
 	// delta should equal the outgoing time lock. Otherwise, whether the

htlcswitch/link_test.go

@@ -5643,6 +5643,19 @@ func TestCheckHtlcForward(t *testing.T) {
 		}
 	})
 
+	// Test that insufficient fee error takes preference over insufficient
+	// channel balance.
+	t.Run("insufficient fee error preference", func(t *testing.T) {
+		t.Parallel()
+
+		result := link.CheckHtlcForward(
+			hash, 100005, 100000, 200,
+			150, 0, lnwire.ShortChannelID{},
+		)
+		_, ok := result.WireMessage().(*lnwire.FailFeeInsufficient)
+		require.True(t, ok, "expected FailFeeInsufficient failure code")
+	})
+
 	t.Run("expiry too soon", func(t *testing.T) {
 		result := link.CheckHtlcForward(hash, 1500, 1000,
 			200, 150, 190, lnwire.ShortChannelID{})

htlcswitch/switch_test.go

@@ -5339,6 +5339,7 @@ func testSwitchHandlePacketForward(t *testing.T, zeroConf, private,
 	ogPacket := &htlcPacket{
 		incomingChanID: bobLink.ShortChanID(),
 		incomingHTLCID: 0,
+		incomingAmount: 1000,
 		obfuscator:     NewMockObfuscator(),
 		htlc: &lnwire.UpdateAddHTLC{
 			PaymentHash: rhash,