Check when relay fee is insufficient (#634)

We should return a `FeeInsufficient` error when an incoming htlc doesn't
pay us what we require in our latest `channel_update`.

Note that the spec encourages us to being a bit more lax than that (BOLT
7):

> SHOULD accept HTLCs that pay an older fee, for some reasonable time
after sending channel_update.

>    Note: this allows for any propagation delay.

eclair-core/src/main/scala/fr/acinq/eclair/payment/Relayer.scala

@@ -18,12 +18,13 @@ package fr.acinq.eclair.payment
 
 import akka.actor.{Actor, ActorLogging, ActorRef, Props, Status}
 import fr.acinq.bitcoin.{BinaryData, Crypto, MilliSatoshi}
+import fr.acinq.eclair.nodeFee
 import fr.acinq.eclair.channel._
 import fr.acinq.eclair.crypto.Sphinx
 import fr.acinq.eclair.payment.PaymentLifecycle.{PaymentFailed, PaymentSucceeded}
 import fr.acinq.eclair.router.Announcements
 import fr.acinq.eclair.wire._
-import fr.acinq.eclair.{Globals, NodeParams, ShortChannelId}
+import fr.acinq.eclair.{NodeParams, ShortChannelId}
 import scodec.bits.BitVector
 import scodec.{Attempt, DecodeResult}
 
@@ -104,6 +105,8 @@ class Relayer(nodeParams: NodeParams, register: ActorRef, paymentHandler: ActorR
               Left(CMD_FAIL_HTLC(add.id, Right(AmountBelowMinimum(add.amountMsat, channelUpdate)), commit = true))
             case Some(channelUpdate) if add.expiry != perHopPayload.outgoingCltvValue + channelUpdate.cltvExpiryDelta =>
               Left(CMD_FAIL_HTLC(add.id, Right(IncorrectCltvExpiry(add.expiry, channelUpdate)), commit = true))
+            case Some(channelUpdate) if (add.amountMsat - perHopPayload.amtToForward) < nodeFee(channelUpdate.feeBaseMsat, channelUpdate.feeProportionalMillionths, perHopPayload.amtToForward) =>
+              Left(CMD_FAIL_HTLC(add.id, Right(FeeInsufficient(add.amountMsat, channelUpdate)), commit = true))
             case _ =>
               Right(CMD_ADD_HTLC(perHopPayload.amtToForward, add.paymentHash, perHopPayload.outgoingCltvValue, nextPacket.serialize, upstream_opt = Some(add), commit = true))
           }

eclair-core/src/test/scala/fr/acinq/eclair/payment/RelayerSpec.scala

@@ -30,6 +30,7 @@ import fr.acinq.eclair.wire._
 import fr.acinq.eclair.{Globals, TestConstants, TestkitBaseClass}
 import org.junit.runner.RunWith
 import org.scalatest.junit.JUnitRunner
+import scodec.bits.BitVector
 
 import scala.concurrent.duration._
 
@@ -234,6 +235,25 @@ class RelayerSpec extends TestkitBaseClass {
     paymentHandler.expectNoMsg(100 millis)
   }
 
+  test("fail to relay an htlc-add when relay fee isn't sufficient") { case (relayer, register, paymentHandler) =>
+    val sender = TestProbe()
+
+    val hops1 = hops.updated(1, hops(1).copy(lastUpdate = hops(1).lastUpdate.copy(feeBaseMsat = hops(1).lastUpdate.feeBaseMsat / 2)))
+    val (cmd, _) = buildCommand(finalAmountMsat, finalExpiry, paymentHash, hops1)
+    // and then manually build an htlc
+    val add_ab = UpdateAddHtlc(channelId = channelId_ab, id = 123456, cmd.amountMsat, cmd.paymentHash, cmd.expiry, cmd.onion)
+    relayer ! LocalChannelUpdate(null, channelId_bc, channelUpdate_bc.shortChannelId, c, None, channelUpdate_bc)
+
+    sender.send(relayer, ForwardAdd(add_ab))
+
+    val fail = register.expectMsgType[Register.Forward[CMD_FAIL_HTLC]].message
+    assert(fail.id === add_ab.id)
+    assert(fail.reason == Right(FeeInsufficient(add_ab.amountMsat, channelUpdate_bc)))
+
+    register.expectNoMsg(100 millis)
+    paymentHandler.expectNoMsg(100 millis)
+  }
+
   test("fail an htlc-add at the final node when amount has been modified by second-to-last node") { case (relayer, register, paymentHandler) =>
     val sender = TestProbe()