fix hodlinvoice deadlock

[ziggie1984]

Fixes https://github.com/lightningnetwork/lnd/issues/8803

Depends on https://github.com/lightningnetwork/lnd/pull/8826

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[ziggie1984]

So this is still a draft and no tests added yet, tho I want your spotlight on the following topics before continuing:

See also https://github.com/lightningnetwork/lnd/issues/8803#issuecomment-2308899898 for the overall design.

1. The design of the hodlHTLCDisp in parcticular the following question in regards of the subscriber:

I had to change the EventReceiver because I wanted to be flexible in using a particular ID for the subsriber, so I went with the short channel ID, so that when an HTLC gets resolved I can quickly look up the relevant subscriber just by using the shortchanID which is part of the HTLC (resolution) info.

Another approach would be that we always register the correct subscriber to an Hodl HTLC meaning that instead of:

hodlMap fn.Set[models.CircuitKey]

we would need to change the set to a map with the subscribers as a value

hodlMap map[models.CircuitKey]fn.EventReceiver

Then we would need to add the subscriber in the invoices.NotifyExitHop call. Then when resolving we would based on the circuit key get the receiver/subscriber we want to send this resolution to

If we do it the latter way, I think there would be no need for the introduction of an interface of the EventReceiver.

2. So the case which caused (at least from my understanding) the deadlock in the issue would now be fixed because when resolving an HTLC we

• first check if we still have the subscriber available (in case the channellink is already gone)
• and secondly we check if a corresponding HTLC is in the hodlmap of the notifier, if thats not the case we will not send a nofification out.

3. Is the idea of using an interface for the htlcnotifier in the invoiceregistry struct the right direction especially when thinking of unit tests or does it increase complexity unnecessary?

type hodlHTLCNotifier interface {
    fn.EventPublisher[HtlcResolution, bool]

    publishSubscriberEvent(res HtlcResolution) error

    recordHTLC(htlc models.CircuitKey) error

    Start() error

    Stop() error
}

[ProofOfKeags]

I had to change the EventReceiver because I wanted to be flexible in using a particular ID for the subsriber, so I went with the short channel ID, so that when an HTLC gets resolved I can quickly look up the relevant subscriber just by using the shortchanID which is part of the HTLC (resolution) info.

Unless I'm missing something, I don't see how the introduction of the interface actually helps things here. I see no mention of the ShortChannelID in the updated Impl or in a new implementation.

Another approach would be that we always register the correct subscriber to an Hodl HTLC meaning that instead of:

This seems like the way I would try to handle it.

Overall I don't know that I can endorse the HTLCNotifier interface, mostly because I don't see what it is solving. The main issue at hand here is that the invoice registry has a lot of its data structures being accessed concurrently from different threads. In other parts of LND we've started to take the approach of moving everything into the main event loop of the component. So rather than having methods on the invoice registry that directly modify data structures, locked or otherwise. Instead, we describe the actions we want to occur and then send those over some channel into the main event loop. This guarantees sequential processing of all data owned by the invoice registry which will then allow us to remove locking infrastructure which should drastically reduce the deadlock risk.