Fix blinded path min_final_expiry_delta check (#2678)

The spec says that we must return an error if:

- cltv_expiry < outgoing_cltv_value
- cltv_expiry < current_block_height + min_final_cltv_expiry_delta

For the second check, we actually tested if:

- cltv_expiry < max(outgoing_cltv_value, current_block_height) + min_final_cltv_expiry_delta

But that check should only verify that our `min_final_expiry_delta`
requirement is fulfilled, which is unrelated to the `outgoing_cltv_value`.

It was redundant with the first check, which already guarantees that
intermediate nodes inside the blinded path cannot cheat by using a higher
`cltv_expiry_delta` than what the recipient intended.

eclair-core/src/main/scala/fr/acinq/eclair/payment/receive/MultiPartHandler.scala

@@ -472,12 +472,7 @@ object MultiPartHandler {
   }
 
   private def validatePaymentCltv(nodeParams: NodeParams, add: UpdateAddHtlc, payload: FinalPayload)(implicit log: LoggingAdapter): Boolean = {
-    val minExpiry = payload match {
-      case _: FinalPayload.Standard => nodeParams.channelConf.minFinalExpiryDelta.toCltvExpiry(nodeParams.currentBlockHeight)
-      // For blinded payments, the min-final-expiry-delta is included in the blinded path instead of being added
-      // explicitly by the sender to their onion payload's expiry.
-      case _: FinalPayload.Blinded => nodeParams.channelConf.minFinalExpiryDelta.toCltvExpiry(nodeParams.currentBlockHeight.max(payload.expiry.blockHeight))
-    }
+    val minExpiry = nodeParams.channelConf.minFinalExpiryDelta.toCltvExpiry(nodeParams.currentBlockHeight)
     if (add.cltvExpiry < minExpiry) {
       log.warning("received payment with expiry too small for amount={} totalAmount={}", add.amountMsat, payload.totalAmount)
       false

eclair-core/src/main/scala/fr/acinq/eclair/payment/send/PaymentInitiator.scala

@@ -281,6 +281,7 @@ object PaymentInitiator {
     def finalExpiry(nodeParams: NodeParams): CltvExpiry = {
       val minFinalCltvExpiryDelta = invoice match {
         case invoice: Bolt11Invoice => invoice.minFinalCltvExpiryDelta
+        // For blinded payments, the min-final-expiry-delta is included in the blinded path instead of being added explicitly by the sender.
         case _: Bolt12Invoice => CltvExpiryDelta(0)
       }
       nodeParams.paymentFinalExpiry.computeFinalExpiry(nodeParams.currentBlockHeight, minFinalCltvExpiryDelta)

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

@@ -558,7 +558,8 @@ class MultiPartHandlerSpec extends TestKitBaseClass with FixtureAnyFunSuiteLike
     val invoice = sender.expectMsgType[CreateInvoiceActor.InvoiceCreated].invoice
     assert(invoice.features.hasFeature(RouteBlinding, Some(Mandatory)))
 
-    val packet = createBlindedPacket(5000 msat, invoice.paymentHash, defaultExpiry, CltvExpiry(nodeParams.currentBlockHeight) + CltvExpiryDelta(1), pathId)
+    // We test the case where the HTLC's cltv_expiry is lower than expected and doesn't meet the min_final_expiry_delta.
+    val packet = createBlindedPacket(5000 msat, invoice.paymentHash, defaultExpiry - CltvExpiryDelta(1), defaultExpiry, pathId)
     sender.send(handlerWithRouteBlinding, packet)
     val receivePayment = offerManager.expectMsgType[OfferManager.ReceivePayment]
     assert(receivePayment.paymentHash == invoice.paymentHash)