Include min_final_cltv_expiry_delta in blinded paths

If a payer uses the current block height as the expiry for the payments
they send to us, we will reject it because we need to have a few blocks
to potentially force-close and get our funds back safely.

With blinded paths, we don't need to rely on payers to add a safety
margin: we can directly encode it in the blinded path's total expiry
delta.

src/commonMain/kotlin/fr/acinq/lightning/payment/OfferManager.kt

@@ -159,18 +159,23 @@ class OfferManager(val nodeParams: NodeParams, val walletParams: WalletParams, v
                 }
                 val pathId = OfferPaymentMetadata.V1(ByteVector32(decrypted.pathId), amount, preimage, request.payerId, truncatedPayerNote, request.quantity, currentTimestampMillis()).toPathId(nodeParams.nodePrivateKey)
                 val recipientPayload = RouteBlindingEncryptedData(TlvStream(RouteBlindingEncryptedDataTlv.PathId(pathId))).write().toByteVector()
+                val cltvExpiryDelta = remoteChannelUpdates.maxOfOrNull { it.cltvExpiryDelta } ?: walletParams.invoiceDefaultRoutingFees.cltvExpiryDelta
                 val paymentInfo = OfferTypes.PaymentInfo(
                     feeBase = remoteChannelUpdates.maxOfOrNull { it.feeBaseMsat } ?: walletParams.invoiceDefaultRoutingFees.feeBase,
                     feeProportionalMillionths = remoteChannelUpdates.maxOfOrNull { it.feeProportionalMillionths } ?: walletParams.invoiceDefaultRoutingFees.feeProportional,
-                    cltvExpiryDelta = remoteChannelUpdates.maxOfOrNull { it.cltvExpiryDelta } ?: walletParams.invoiceDefaultRoutingFees.cltvExpiryDelta,
+                    // We include our min_final_cltv_expiry_delta in the path, but we *don't* include it in the payment_relay field
+                    // for our trampoline node (below). This ensures that we will receive payments with at least this final expiry delta.
+                    // This ensures that even when payers haven't received the latest block(s) or don't include a safety margin in the
+                    // expiry they use, we can still safely receive their payment.
+                    cltvExpiryDelta = cltvExpiryDelta + nodeParams.minFinalCltvExpiryDelta,
                     minHtlc = remoteChannelUpdates.minOfOrNull { it.htlcMinimumMsat } ?: 1.msat,
                     maxHtlc = amount,
                     allowedFeatures = Features.empty
                 )
                 val remoteNodePayload = RouteBlindingEncryptedData(
                     TlvStream(
                         RouteBlindingEncryptedDataTlv.OutgoingNodeId(EncodedNodeId.WithPublicKey.Wallet(nodeParams.nodeId)),
-                        RouteBlindingEncryptedDataTlv.PaymentRelay(paymentInfo.cltvExpiryDelta, paymentInfo.feeProportionalMillionths, paymentInfo.feeBase),
+                        RouteBlindingEncryptedDataTlv.PaymentRelay(cltvExpiryDelta, paymentInfo.feeProportionalMillionths, paymentInfo.feeBase),
                         RouteBlindingEncryptedDataTlv.PaymentConstraints((paymentInfo.cltvExpiryDelta + nodeParams.maxFinalCltvExpiryDelta).toCltvExpiry(currentBlockHeight.toLong()), paymentInfo.minHtlc)
                     )
                 ).write().toByteVector()

src/commonTest/kotlin/fr/acinq/lightning/payment/OfferManagerTestsCommon.kt

@@ -85,6 +85,10 @@ class OfferManagerTestsCommon : LightningTestSuite() {
         assertIs<OnionMessageAction.PayInvoice>(payInvoice)
         assertEquals(OfferInvoiceReceived(payOffer, payInvoice.invoice), bobOfferManager.eventsFlow.first())
         assertEquals(payOffer, payInvoice.payOffer)
+        assertEquals(1, payInvoice.invoice.blindedPaths.size)
+        val path = payInvoice.invoice.blindedPaths.first()
+        assertEquals(EncodedNodeId(aliceTrampolineKey.publicKey()), path.route.route.introductionNodeId)
+        assertEquals(aliceOfferManager.nodeParams.expiryDeltaBlocks + aliceOfferManager.nodeParams.minFinalCltvExpiryDelta, path.paymentInfo.cltvExpiryDelta)
     }
 
     @Test
@@ -113,6 +117,10 @@ class OfferManagerTestsCommon : LightningTestSuite() {
         assertIs<OnionMessageAction.PayInvoice>(payInvoice)
         assertEquals(OfferInvoiceReceived(payOffer, payInvoice.invoice), bobOfferManager.eventsFlow.first())
         assertEquals(payOffer, payInvoice.payOffer)
+        assertEquals(1, payInvoice.invoice.blindedPaths.size)
+        val path = payInvoice.invoice.blindedPaths.first()
+        assertEquals(EncodedNodeId(aliceTrampolineKey.publicKey()), path.route.route.introductionNodeId)
+        assertEquals(aliceOfferManager.nodeParams.expiryDeltaBlocks + aliceOfferManager.nodeParams.minFinalCltvExpiryDelta, path.paymentInfo.cltvExpiryDelta)
     }
 
     @Test