[bug]: channel open transaction fails to broadcast because lnd is unaware of own mempool transaction after rescan

[antonilol]

Background

2 issues, but one is caused by the other

• when a channel is opened and the transaction fails to broadcast, lnd will keep the channel in the pending channels, but it can know it wont ever confirm
• when a wallet is restored, the mempool is not scanned

Your environment

• version of lnd: lnd version 0.15.99-beta commit=tlv/v1.1.0-104-gef8cf73d4 (today's master)
• which operating system (uname -a on *Nix) archlinux
• version of btcd, bitcoind, or other backend bitcoin core v24.0.1
• any other relevant environment details

Steps to reproduce

1. create a new wallet
2. send coins to it
3. wait for 1 confirmation
4. spend the coins (at 1 sat/vB)
5. delete the wallet
6. restore the wallet
7. if the spend of step 4 confirmed start over
8. open a channel (at 1 sat/vB)

Expected behaviour

mempool is scanned after restore, the spending transaction is seen in the mempool and lnd will not try to reuse it
if for some reason a channel open fails, lnd should handle the error

Actual behaviour

lnd tried to open a channel with a utxo that was already spent in the mempool and did not provide enough fee for rbf, and thus got rejected by bitcoind

[yyforyongyu]

Related and should be mitigated by #6400

[antonilol]

will it also rescan the mempool when restoring a wallet?

[yyforyongyu]

will it also rescan the mempool when restoring a wallet?

Nope that PR specifically deals with the "fail to broadcast" part.

[Roasbeef]

will it also rescan the mempool when restoring a wallet?

If something is in the mempool by the time the rescan if finished, then the regular notifications set up afterwards should pick up that spend.

Also that open channel step at the end won't succeed unless you're trying to spend unconfirmed coins. By default, we'll only spend confirmed coins. I think if you just try to send unconfirmed coins, then this issue is resolved?

[antonilol]

afterwards

that's the problem, while the transaction is in the mempool lnd does not see it, and will try to spend wallet outputs that have been spent in the mempool

should pick up that spend

after the spend confirmed it did pick it up

Also that open channel step at the end won't succeed unless you're trying to spend unconfirmed coins. By default, we'll only spend confirmed coins. I think if you just try to send unconfirmed coins, then this issue is resolved?

the wallet saw a confirmed output in the wallet that has been spent, but it did not know it was already spent because lnd doesnt scan the mempool after wallet recovery rescan. to spend unconfirmed coins it has to know of the unconfirmed outputs in the first place

[Roasbeef]

that's the problem, while the transaction is in the mempool lnd does not see it, and will try to spend wallet outputs that have been spent in the mempool

I see what you're getting at: if the rescan finishes before the txn hits the mempool, it won't pick it up until things are confirmed. This is because we get notifications for new transactions in the mempool, but not for transactions that are already in the mempool.

So then a fix here would be to grab the latest mempool to continue to rescan after we hit the chain tip. However this won't work for neutrino. FWIW, after the confirmation, then things would pick up as normal.

[antonilol]

before the txn hits the mempool

did you mean 'before it confirms' here?

So then a fix here would be to grab the latest mempool to continue to rescan after we hit the chain tip. However this won't work for neutrino. FWIW, after the confirmation, then things would pick up as normal.

yeah that's what i said under the 'expected behavior', for neutrino users a warning on utxos where lnd doesn't know if they are spent would be useful