LND PeerSwap fee estimation seems broken with Bitcoin Core v27.0rc1

[grubles]

LND as swap-out receiver apparently seems to think the fee rate is >= 1BTC/kvB and cancels the swap. LND as swap-in receiver seems unable to claim onchain signet coins for the same reason.

Fedora 39 ARM64
LND 0.17.4-beta
PeerSwap @ fb7d5f7
Bitcoin Core v27.0rc1
Go 1.21.8

peerswapd logs:

2024/03/16 23:47:22 [DEBUG] [FSM] event:id: a71942d629ee5ce4898f709150b05a37d7acb791ae965487217990d48ca94cd6, Event_OnSwapOutRequestReceived on 
2024/03/16 23:47:22 [INFO] Estimated fee rate is below floor of 275 sat/kw, take floor instead
2024/03/16 23:47:22 [DEBUG] Using a fee rate of 1.10 sat/vb for a total fee of 385
2024/03/16 23:47:26 [DEBUG] [FSM] event:id: a71942d629ee5ce4898f709150b05a37d7acb791ae965487217990d48ca94cd6, Event_ActionSucceeded on State_SwapOutReceiver_CreateSwap
2024/03/16 23:47:26 [INFO] [Swap:a71942d629ee5ce4898f709150b05a37d7acb791ae965487217990d48ca94cd6] swap-out request received: peer: 036e680823a129ac9a2eeb8c43514a5b6cb09ae28e3e698a0e34aaa8865d6cc344 chanId: 182807x4x0 initiator: 036e680823a129ac9a2eeb8c43514a5b6cb09ae28e3e698a0e34aaa8865d6cc344 amount 200000000
2024/03/16 23:47:26 [DEBUG] [FSM] event:id: a71942d629ee5ce4898f709150b05a37d7acb791ae965487217990d48ca94cd6, Event_ActionSucceeded on State_SwapOutReceiver_SendFeeInvoice
2024/03/16 23:47:26 [DEBUG] [PaymentWatcher] Swap: a71942d629ee5ce4898f709150b05a37d7acb791ae965487217990d48ca94cd6: Add new payment watcher for payreq lntbs3850n1pjlvtuwpp5ds3nsvdakndztm56dlh22km79jxfvej273ag230agr79p7lx4tvqdy5wpjk2unnwashqgrzw33jqen9v5srzwpj8qcrw7p50qczqcfhxyungvnyxcerjet9x43k2dpc8yuxvdes8ycn2vrzxq6kzvehvsmkzcmzxuunzct98ymr2dpcxuerzdee8ycxgdpcvdsnjdrrvsmqcqzzsxqzjcsp5axg9rhmjznmkjqeu3xtr9x8tqsuk96qhscdvjkk7vc4ss40l7fss9qyyssqxagewxhzkyfyj2u48gxvs8qexv6ukyrfrecnsl24rz2wtdnkrgeyznhynaskuknw2azt6wr9cut3ds3wn7j7rs8g2ha6qkskcu6jkasp2pd3fv of type "fee"
2024/03/16 23:47:28 [DEBUG] [PaymentWatcher] Swap: a71942d629ee5ce4898f709150b05a37d7acb791ae965487217990d48ca94cd6: Calling paymentCallback
2024/03/16 23:47:28 [DEBUG] [FSM] event:id: a71942d629ee5ce4898f709150b05a37d7acb791ae965487217990d48ca94cd6, Event_OnFeeInvoicePaid on State_SwapOutReceiver_AwaitFeeInvoicePayment
2024/03/16 23:47:28 [DEBUG] [grpc_conn]: grpc_retry attempt: 0, got err: rpc error: code = Unknown desc = unmatched backend error: -8: Fee rates larger than or equal to 1BTC/kvB are not accepted
2024/03/16 23:47:28 [INFO] [FSM] Action failure rpc error: code = Unknown desc = unmatched backend error: -8: Fee rates larger than or equal to 1BTC/kvB are not accepted
2024/03/16 23:47:28 [DEBUG] [FSM] event:id: a71942d629ee5ce4898f709150b05a37d7acb791ae965487217990d48ca94cd6, Event_ActionFailed on State_SwapOutReceiver_BroadcastOpeningTx
2024/03/16 23:47:28 [DEBUG] [FSM] Canceling because of rpc error: code = Unknown desc = unmatched backend error: -8: Fee rates larger than or equal to 1BTC/kvB are not accepted
2024/03/16 23:47:28 [DEBUG] [FSM] event:id: a71942d629ee5ce4898f709150b05a37d7acb791ae965487217990d48ca94cd6, Event_ActionSucceeded on State_SendCancel
2024/03/16 23:47:28 [INFO] [Swap:a71942d629ee5ce4898f709150b05a37d7acb791ae965487217990d48ca94cd6] Swap canceled. Reason: rpc error: code = Unknown desc = unmatched backend error: -8: Fee rates larger than or equal to 1BTC/kvB are not accepted

[grubles]

peerswapd logs with LND as swap-in receiver:

2024/03/17 13:07:38 [INFO] [TxWatcher] Wait for confirmation on swap b131d7c5a788822268ad07c5148a13e637282bc788d41b4455e165e09ab32012: Got 3 confirmations call callback                                                                                                                
2024/03/17 13:07:38 [DEBUG] [FSM] event:id: b131d7c5a788822268ad07c5148a13e637282bc788d41b4455e165e09ab32012, Event_OnTxConfirmed on State_SwapInReceiver_AwaitTxConfirmation                                                                                                           
2024/03/17 13:07:45 [DEBUG] [Messenger] From: 036e680823a129ac9a2eeb8c43514a5b6cb09ae28e3e698a0e34aaa8865d6cc344 got same message for swap: b131d7c5a788822268ad07c5148a13e637282bc788d41b4455e165e09ab32012                                                                            
2024/03/17 13:07:48 [DEBUG] PayInvoiceViaChannel: payment still in flight                                                                   
2024/03/17 13:07:49 [DEBUG] PayInvoiceViaChannel: payment still in flight                                                                   
2024/03/17 13:07:50 [DEBUG] [FSM] event:id: b131d7c5a788822268ad07c5148a13e637282bc788d41b4455e165e09ab32012, Event_ActionSucceeded on State_SwapInReceiver_ValidateTxAndPayClaimInvoice                                                                                                
2024/03/17 13:07:50 [INFO] Estimated fee rate is below floor of 275 sat/kw, take floor instead                                              2024/03/17 13:07:50 [DEBUG] Using a fee rate of 1.10 sat/vb for a total fee of 171                                                          
2024/03/17 13:07:50 [DEBUG] [grpc_conn]: grpc_retry attempt: 0, got err: rpc error: code = Unknown desc = unmatched backend error: -8: Fee rates larger than or equal to 1BTC/kvB are not accepted                                                                                      
2024/03/17 13:07:50 [INFO] Error claiming tx with preimage rpc error: code = Unknown desc = unmatched backend error: -8: Fee rates larger th
an or equal to 1BTC/kvB are not accepted                                                                                                    
2024/03/17 13:07:50 [DEBUG] [FSM] event:id: b131d7c5a788822268ad07c5148a13e637282bc788d41b4455e165e09ab32012, Event_OnRetry on State_SwapInReceiver_ClaimSwap                                                                                                                           
2024/03/17 13:07:50 [INFO] Estimated fee rate is below floor of 275 sat/kw, take floor instead                                              2024/03/17 13:07:50 [DEBUG] Using a fee rate of 1.10 sat/vb for a total fee of 171                                                          
2024/03/17 13:07:50 [DEBUG] [grpc_conn]: grpc_retry attempt: 0, got err: rpc error: code = Unknown desc = unmatched backend error: -8: Fee r
ates larger than or equal to 1BTC/kvB are not accepted                                                                                      
2024/03/17 13:07:50 [INFO] Error claiming tx with preimage rpc error: code = Unknown desc = unmatched backend error: -8: Fee rates larger th
an or equal to 1BTC/kvB are not accepted
2024/03/17 13:07:50 [DEBUG] [FSM] event:id: b131d7c5a788822268ad07c5148a13e637282bc788d41b4455e165e09ab32012, Event_OnRetry on State_SwapInR
eceiver_ClaimSwap
2024/03/17 13:07:50 [INFO] Estimated fee rate is below floor of 275 sat/kw, take floor instead
2024/03/17 13:07:50 [DEBUG] Using a fee rate of 1.10 sat/vb for a total fee of 171
2024/03/17 13:07:50 [DEBUG] [grpc_conn]: grpc_retry attempt: 0, got err: rpc error: code = Unknown desc = unmatched backend error: -8: Fee r
ates larger than or equal to 1BTC/kvB are not accepted
2024/03/17 13:07:50 [INFO] Error claiming tx with preimage rpc error: code = Unknown desc = unmatched backend error: -8: Fee rates larger th
an or equal to 1BTC/kvB are not accepted
2024/03/17 13:07:50 [DEBUG] [FSM] event:id: b131d7c5a788822268ad07c5148a13e637282bc788d41b4455e165e09ab32012, Event_OnRetry on State_SwapInR
eceiver_ClaimSwap

[grubles]

Something strange is happening with LND + Core v27. The swap is canceled due to the abnormally high fee rate, but somehow the transaction was broadcasted and ended up being confirmed in a block? And checking the tx on an explorer, the feerate appears normal.

2024-03-19 15:20:31.321 [INF] LNWL: a536eb55ccf685466baf6d2bc9bbc10ec260d69e3d5245133157fa578d2f0745: broadcast failed because of: unmatched
 backend error: -8: Fee rates larger than or equal to 1BTC/kvB are not accepted                                                             
2024-03-19 15:20:31.325 [INF] LNWL: Removed invalid transaction: a536eb55ccf685466baf6d2bc9bbc10ec260d69e3d5245133157fa578d2f0745 

2024-03-19 15:21:16.909 [INF] LNWL: Marking unconfirmed transaction a536eb55ccf685466baf6d2bc9bbc10ec260d69e3d5245133157fa578d2f0745 mined in block 187469

   {
      "id":  "e6dd6ef1346f08c31cd29f4cd4ce6ba82a7a61dc22d905ddcf20ed8545482b7f",
      "created_at":  "1710875843",
      "asset":  "btc",
      "type":  "swap-out",
      "role":  "receiver",
      "state":  "State_SwapCanceled",
      "initiator_node_id":  "036e680823a129ac9a2eeb8c43514a5b6cb09ae28e3e698a0e34aaa8865d6cc344",
      "peer_node_id":  "036e680823a129ac9a2eeb8c43514a5b6cb09ae28e3e698a0e34aaa8865d6cc344",
      "amount":  "100000000",
      "channel_id":  "182807x4x0",
      "opening_tx_id":  "",
      "claim_tx_id":  "",
      "cancel_message":  "rpc error: code = Unknown desc = unmatched backend error: -8: Fee rates larger than or equal to 1BTC/kvB are not accepted",
      "lnd_chan_id":  "200998422139109376"
    }

[YusukeShimizu]

It may be an existing bug in LND found due to changes in bitcoin core.
I will investigate more and try to confirm.

Possible cause

In lnd, maxfeerate is set to 1e8 / 10(sat/kb) as a parameter when calling SendRawTransaction.
But BTC/kvB is the expected value, so the units would be wrong.

Reason for error in v27.0

bitcoin/bitcoin#29434 adds a sanity check for maxfeerate and does not accept values above 1BTC/kvB.
The error message in that case is

Fee rates larger than or equal to 1BTC/kvB are not accepted

[maflcko]

I presume this was fixed in btcsuite/btcd#2142 upstream?

[YusukeShimizu]

It appears so.