multi: allow min-depth of zero for non-zero conf channels

[ellemouton]

If our peer indicates to us that they are OK with a zero conf channel (ie, they trust us) by setting min-depth to 0 in accept_channel, then as long as they support SCID aliases, we should just upgrade the channel to ZeroConf and continue the funding flow.

EDIT: updated to just allow the peer to set min-depth to 0. We dont upgrade the channel type.

Fixes #8783

NOTE: we already allow ourselves to do this when our peer sends us an OpenChannel without a zero-conf channel type:

lnd/funding/manager.go

Lines 1570 to 1583 in fed7609

	if !zeroConf && acceptorResp.ZeroConf {
	if !scidFeatureVal {
	// Fail the funding flow.
	flowErr := fmt.Errorf("scid-alias feature " +
	"must be negotiated for zero-conf")
	log.Errorf("Cancelling funding flow for "+
	"zero-conf channel %v: %v", cid,
	flowErr)
	f.failFundingFlow(peer, cid, flowErr)
	return
	}
	// Set zeroConf to true to enable the zero-conf flow.
	zeroConf = true

So all this PR does is allow our peer to do this too :)

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[Crypt-iQ]

👍

[ellemouton]

hmmm ok from this comment on the original issue, it sounds like we dont actually need to go and change this to a zero conf channel. We can just allow them to send a min-depth of 0 (even though this is not a zero conf channel) and then we just continue as normal (and only send channel_ready when we see fit).

so the diff can actually be way smaller

cc @Crypt-iQ - wdyt?

[Crypt-iQ]

hmmm ok from this comment on the original issue, it sounds like we dont actually need to go and change this to a zero conf channel. We can just allow them to send a min-depth of 0 (even though this is not a zero conf channel) and then we just continue as normal (and only send channel_ready when we see fit).

so the diff can actually be way smaller

cc @Crypt-iQ - wdyt?

yup that seems fine

[ellemouton]

Ok I've made the update discussed above.

It's hard to write a test for this case cause we dont really support this between two LND nodes opening a non-zero conf channel cause we want to register for confirmations and this is not allowed with a depth of 0.

So to test this you can apply this patch which allows our channel acceptor to set a MinDepth of 0 & also makes sure we still register for 1 conf if we are on the receiver side of this operation. Then to actually test the flow, you can run the scid_alias_channel_update/public_no-chan-type_update test which uses the channel acceptor to open a non zero conf channel

[lightninglabs-deploy]

@Crypt-iQ: review reminder

[yyforyongyu]

Needs a rebase, otherwise LGTM🌊