[3/4] - lnwallet/chancloser: add new protofsm based RBF chan closer

[Roasbeef]

Overview

In this PR, we add a new RBF based co-op channel close state machine. This uses the new protofsm package to define the states and transitions for the state machine. The new state machine is then also updated to become a peer.MsgEndpoint which allows us to use the new peer.MsgRouter to handle all message dispatch into the state machine. A series of new wrapper structs (taking care to always accept interfaces to decouple from the concrete types) are created to be able to initialize the environment needed by the new state machine.

This PR doesn't yet include integration within the peer struct. That will come in a follow up PR as we need some additional changes to be able to handle the two possible co-op types that will exist.

The diff might look somewhat large, but over half of it us unit tests, and a large portion of the non-test diff is the new state and event declarations.

State Machine Flow

The new state machine can be modeled with the following diagram:

---
title: Co-Op Close V2
---
stateDiagram-v2
    state CoopCloseV2 {
    [*] --> ChannelActive
    ChannelActive --> ShutdownPending: send_shutdown
    ChannelActive --> ShutdownPending: shutdown_received

	ShutdownPending --> ChannelFlushing: shutdown_received
    ShutdownPending --> ChannelFlushing: shutdown_complete
    
    ChannelFlushing --> ClosingNegotiation: channel_flushed

    state ClosingNegotiation {
		direction TB
        [*] --> LocalCloseStart
        [*] --> RemoteCloseStart
        
        LocalCloseStart --> LocalOfferSent: send_offer
        LocalOfferSent --> ClosePending: local_sig_received
        
        RemoteCloseStart --> ClosePending: offer_received
    }
    
    ClosingNegotiation --> ShutdownPending: send_shutdown
    ClosingNegotiation --> ShutdownPending: shutdown_received
   
    ClosingNegotiation --> CloseFin: txn_confirmation
    }

Remaining TODOs

• [x] unit tests for the state machine
• [ ] commit the above diagram and a msg flow diagram to repo as documentation

Fixes https://github.com/lightningnetwork/lnd/issues/7091

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Pull reviewers stats

Stats of the last 30 days for lnd:

	User	Total reviews	Time to review	Total comments
	guggero 🥇	14 ▀▀▀	1d 3h 52m	26 ▀▀
	yyforyongyu 🥈	9 ▀▀	1d 16h 25m	8 ▀
	ellemouton 🥉	8 ▀▀	4d 8h 7m ▀	25 ▀▀
	ProofOfKeags	6 ▀	1d 16h 3m	42 ▀▀▀▀
	bhandras	5 ▀	4d 10h 42m ▀	5
	Crypt-iQ	3 ▀	1d 21h 30m	8 ▀
	Roasbeef	2	12h 16m	3
	saubyk	2	8h 12m	0
	morehouse	1	14d 21h 46m ▀▀▀▀	0
	ziggie1984	1	10d 15h 59m ▀▀▀	2

[saubyk]

Hi @Roasbeef can you please explain the scenario where you're expecting the state to transition back from ClosingNegotiation to ShutdownPending?

Given that the channel flushing has already happened before the state moved to ClosingNegotiation, what would be the scenario where that step would need to be executed again?

[Roasbeef]

@saubyk good question, so I need to update the diagram slightly, but you're right in that at that point the channel is already flushed. With the code that's checked in, there's a special fast path that'll transition from ChannelActive all the way to ClosingNegotiation if we detect that the channel is already flushed. It'll already be flushed if we're doing an RBF iteration, or if we're reconnecting and want to resume the closing process.

Here's the location of one of the fast path transitions:

• https://github.com/lightningnetwork/lnd/blob/c273bb4755a43e675d4f538c9fdad1b74f713690/lnwallet/chancloser/rbf_coop_transitions.go#L350-L364

So in that case we sent shutdown, received it, then rather than wait in the ChannelFlushing state, we'll emit an internal event to have us go directly to the ChannelNegotiation phase.

Also just to clarify, the way the protocol works is that when either side wants to do a new RBF iteration (RBF their closing txn), they send the shutdown message again. You effectively can drop all state, then act as if you're closing for the first time. Reading between the lines, I think yuo're right in that you could just send the signatures again, but perhaps one side wants to close to a new address, and that information is contained within teh shutdown message.

I kept things like this rather than introducing some new states to keep the amount of total states to a minimum, and re-use more of the existing transition logic. As an example, if they send a shutdown message to us, we still want to validate that they're using the same upfront shutdown address, etc.

[lightninglabs-deploy]

@yyforyongyu: review reminder @crypt-iq: review reminder @morehouse: review reminder @roasbeef, remember to re-request review from reviewers when ready

[lightninglabs-deploy]

@yyforyongyu: review reminder @crypt-iq: review reminder @morehouse: review reminder @roasbeef, remember to re-request review from reviewers when ready