[Splicing] Preserve funding_transaction for the later lifecycle of the channel, simple solution

[optout21]

Fixes #3300

In splicing, the funding transaction (not just the ID) is needed during splice negotiation. Funding transaction is kept in field ChannelContext.funding_transaction. However, the way it is currently used is that it is cleared when the tx is broadcast, and this fact is used in some logic.

This change:

• does not clear ChannelContext.funding_transaction when tx is broadcast, but it's preserved (it's never reset)
• adjust on test case (close_on_unfunded_channel)

Note that in some cases the funding transaction may get broadcasted more than once, which is a change in behavior.

Alternative is to preserve behavior, with some extra field, #3317 .

[TheBlueMatt]

We shouldn't keep broadcasting forever, but I think we can trivially solve that by just checking if the funding tx has confirmed before broadcasting?

[jkczyz]

We shouldn't keep broadcasting forever, but I think we can trivially solve that by just checking if the funding tx has confirmed before broadcasting?

Is that what is checked immediately below where the funding tx was previously taken?

https://github.com/lightningdevkit/rust-lightning/blob/ee23649af97e48362f78a844a3b6130cb13104b4/lightning/src/ln/channel.rs#L5492-L5496

Or are there other places we'd need to worry about?

[TheBlueMatt]

Is that what is checked immediately below where the funding tx was previously taken?

Not quite, that will keep broadcasting forever if the channel is 0conf or keep broadcasting after the funding is confirmed at minimum_depth - 1 confirmations, which is a bit weird.

[optout21]

Not quite, that will keep broadcasting forever if the channel is 0conf or keep broadcasting after the funding is confirmed at minimum_depth - 1 confirmations, which is a bit weird.

Indeed, once the funding transaction is confirmed, it makes no sense to rebroadcast, funding_tx_confirmation_height is not checked here.

[TheBlueMatt]

I think with just that change I'd be happy with this PR :)

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[optout21]

Changed the test for preventing re-broadcast to look whether the funding transaction is confirmed or not (instead of looking at ChannelReady state). With this, the test updated had to be reverted, the behavior is the same as before.

[optout21]

Addressed @jkczyz 's comments; not yet squashed

[optout21]

Squashed with no changes