swaps: use longer final_cltv_delta for client-normal-swap

This gives more time for the client to come back online.

see https://github.com/spesmilo/electrum/issues/8940

- re note on submarine_swaps.py#L53:
  lnpeer.Peer.maybe_fulfill_htlc only checks against MIN_FINAL_CLTV_DELTA_ACCEPTED(=144),
  so this increased cltv_delta is not enforced when receiving the htlc on ln.
  It is put in the invoice, so the sender is supposed to honour it ofc.
  It would be nice to enforce it (make the check in maybe_fulfill_htlc dependent on
  what was in the invoice).

electrum/lnpeer.py

@@ -2119,6 +2119,7 @@ class Peer(Logger):
         else:
             raise Exception(f"unexpected {mpp_resolution=}")
 
+        # TODO check against actual min_final_cltv_expiry_delta from invoice (and give 2-3 blocks of leeway?)
         if local_height + MIN_FINAL_CLTV_DELTA_ACCEPTED > htlc.cltv_abs:
             if not already_forwarded:
                 log_fail_reason(f"htlc.cltv_abs is unreasonably close")

electrum/lnworker.py

@@ -2146,9 +2146,10 @@ class LNWallet(LNWorker):
             payment_hash: bytes,
             amount_msat: Optional[int],
             message: str,
-            expiry: int,
+            expiry: int,  # expiration of invoice (in seconds, relative)
             fallback_address: Optional[str],
             channels: Optional[Sequence[Channel]] = None,
+            min_final_cltv_expiry_delta: Optional[int] = None,
     ) -> Tuple[LnAddr, str]:
         assert isinstance(payment_hash, bytes), f"expected bytes, but got {type(payment_hash)}"
 
@@ -2169,12 +2170,14 @@ class LNWallet(LNWorker):
         amount_btc = amount_msat/Decimal(COIN*1000) if amount_msat else None
         if expiry == 0:
             expiry = LN_EXPIRY_NEVER
+        if min_final_cltv_expiry_delta is None:
+            min_final_cltv_expiry_delta = MIN_FINAL_CLTV_DELTA_FOR_INVOICE
         lnaddr = LnAddr(
             paymenthash=payment_hash,
             amount=amount_btc,
             tags=[
                 ('d', message),
-                ('c', MIN_FINAL_CLTV_DELTA_FOR_INVOICE),
+                ('c', min_final_cltv_expiry_delta),
                 ('x', expiry),
                 ('9', invoice_features),
                 ('f', fallback_address),

electrum/submarine_swaps.py

@@ -50,9 +50,11 @@ LOCKUP_FEE_SIZE = 153 # assuming 1 output, 2 outputs
 MIN_LOCKTIME_DELTA = 60
 LOCKTIME_DELTA_REFUND = 70
 MAX_LOCKTIME_DELTA = 100
+MIN_FINAL_CLTV_DELTA_FOR_CLIENT = 3 * 144  # note: put in invoice, but is not enforced by receiver in lnpeer.py
 assert MIN_LOCKTIME_DELTA <= LOCKTIME_DELTA_REFUND <= MAX_LOCKTIME_DELTA
 assert MAX_LOCKTIME_DELTA < lnutil.MIN_FINAL_CLTV_DELTA_ACCEPTED
 assert MAX_LOCKTIME_DELTA < lnutil.MIN_FINAL_CLTV_DELTA_FOR_INVOICE
+assert MAX_LOCKTIME_DELTA < MIN_FINAL_CLTV_DELTA_FOR_CLIENT
 
 
 # The script of the reverse swaps has one extra check in it to verify
@@ -443,6 +445,7 @@ class SwapManager(Logger):
             our_privkey: bytes,
             prepay: bool,
             channels: Optional[Sequence['Channel']] = None,
+            min_final_cltv_expiry_delta: Optional[int] = None,
     ) -> Tuple[SwapData, str, str]:
         """creates a hold invoice"""
         if prepay:
@@ -458,6 +461,7 @@ class SwapManager(Logger):
             expiry=300,
             fallback_address=None,
             channels=channels,
+            min_final_cltv_expiry_delta=min_final_cltv_expiry_delta,
         )
         # add payment info to lnworker
         self.lnworker.add_payment_info_for_hold_invoice(payment_hash, invoice_amount_sat)
@@ -471,6 +475,7 @@ class SwapManager(Logger):
                 expiry=300,
                 fallback_address=None,
                 channels=channels,
+                min_final_cltv_expiry_delta=min_final_cltv_expiry_delta,
             )
             self.lnworker.bundle_payments([payment_hash, prepay_hash])
             self.prepayments[prepay_hash] = payment_hash
@@ -666,6 +671,13 @@ class SwapManager(Logger):
             our_privkey=refund_privkey,
             prepay=False,
             channels=channels,
+            # When the client is doing a normal swap, we create a ln-invoice with larger than usual final_cltv_delta.
+            # If the user goes offline after broadcasting the funding tx (but before it is mined and
+            # the server claims it), they need to come back online before the held ln-htlc expires (see #8940).
+            # If the held ln-htlc expires, and the funding tx got confirmed, the server will have claimed the onchain
+            # funds, and the ln-htlc will be timed out onchain (and channel force-closed). i.e. the user loses the swap
+            # amount. Increasing the final_cltv_delta the user puts in the invoice extends this critical window.
+            min_final_cltv_expiry_delta=MIN_FINAL_CLTV_DELTA_FOR_CLIENT,
         )
         return swap, invoice