Reject trampoline payments with expired outgoing cltv (#1727)

When we receive a trampoline payment asking us to relay with an
expiry that is already below the current chain height, we know that
this payment will fail when it reaches the next trampoline node.

Instead of waiting for the next trampoline node to reject it, we reject
it immediately.

Author: akumaigorodski

eclair-core/src/main/scala/fr/acinq/eclair/payment/relay/NodeRelay.scala

@@ -89,6 +89,8 @@ object NodeRelay {
       Some(TrampolineFeeInsufficient)
     } else if (upstream.expiryIn - payloadOut.outgoingCltv < nodeParams.expiryDelta) {
       Some(TrampolineExpiryTooSoon)
+    } else if (payloadOut.outgoingCltv <= CltvExpiry(nodeParams.currentBlockHeight)) {
+      Some(TrampolineExpiryTooSoon)
     } else {
       None
     }

eclair-core/src/test/scala/fr/acinq/eclair/payment/relay/NodeRelayerSpec.scala

@@ -256,6 +256,21 @@ class NodeRelayerSpec extends ScalaTestWithActorTestKit(ConfigFactory.load("appl
     register.expectNoMessage(100 millis)
   }
 
+  test("fail to relay when final expiry is below chain height") { f =>
+    import f._
+
+    val expiryIn = CltvExpiry(500000)
+    val expiryOut = CltvExpiry(300000) // not ok (chain heigh = 400000)
+    val p = createValidIncomingPacket(2000000 msat, 2000000 msat, expiryIn, 1000000 msat, expiryOut)
+    nodeRelayer ! NodeRelay.Relay(p)
+
+    val fwd = register.expectMessageType[Register.Forward[CMD_FAIL_HTLC]]
+    assert(fwd.channelId === p.add.channelId)
+    assert(fwd.message === CMD_FAIL_HTLC(p.add.id, Right(TrampolineExpiryTooSoon), commit = true))
+
+    register.expectNoMessage(100 millis)
+  }
+
   test("fail to relay when expiry is too soon (multi-part)") { f =>
     import f._