Feature Request: fund channel with all available funds, or a specific output

[Sjors]

Avoids the need for a change address, especially if you want to put all available funds in a single channel.

[gdistasi]

I agree, this is a very useful feature.

[windsok]

Agree. I accidentally sent ~8000 satoshis to a change address when doing my first channel opening. My own fault, but an easier way to force spending all available funds and not producing a change UTXO would be useful.

[Sjors]

With fees ticking up again, it's now spending ~€1.50 to open a channel, this becomes more relevant.

[cdecker]

Marking as good first issue, shouldn't be too hard and it's a good exercise in getting acquainted with the source.

[ZmnSCPxj]

Suggestion for those who want to consider taking this issue: withdraw command supports all input to withdraw all funds; it may be useful to unify the code with withdraw, then support selecting an output by "txid:outnum" form in the unified code (to fund using a specific output). This implies adding a wrapper around wallet_select_coins/wallet_select_all.

[wythe]

I added support to fundchannel to accept "all". For now, I just made json_fund_channel look as close to a copy of json_withdraw as possible. Now, as @ZmnSCPxj suggests, I will unify the code and also support selecting a specific output.

It worked on testnet for me, and I added a test_funding_all python test.

Its all on my fund_all branch. Should I make a pull request and mark as work-in-progress?

[ZmnSCPxj]

Yes, please make pull request, thank you! It could be marked WIP if you intend to also add the specific-output support in the same pull request but we could also do a separate pull request for the specific-output support.

[wythe]

Ok sounds like a plan. Will do.

[robtex]

why limit ourselves to a 1) specific amount, 2) 'all', or 3) a specific manually selected utxo?
i think fundchannel should take a range, like
fundchannel X 10000-16000
and the lightning deamon would find "hey i can use this old 12000 utxo to fund the channel"
or "i can use my 7000+6000 utxo:s and make a 13000 channel"
if it doesn't easily find any suitable utxo[s], go ahead and do it the old way.
i mean, we are funding our own channel, not sending money to someone, thus it doesn't have to be exact. and i'd rather leave some satoshis extra in the channel, than getting it back in a silly exchange utxo.
(disclaimer: i'm pretty sure i have suggested the same thing before, but i can't remember where, might have been lnd related. anyhows, sorry for repeating myself. i usually only do that until i get what i want)

[wythe]

I hear you @robtex. Maybe another option, closest fit:
fundchannel fit 10000 16000

I also am experimenting with providing a list of utxos instead of just a single one.

[cdecker]

I'd rather not make this RPC call any more complex than necessary. We can always add a new RPC command, and then we can externalize the coinselection completely, i.e., pass in a list of outputs that should be selected for the funding, and a value, similar to createrawtransaction in bitcoind.

[wythe]

Yeah that makes sense. Should I even add support for specifying the single utxo?

[cdecker]

Well, that'd be covered by the manualfundchannel case, wouldn't it?

[wythe]

Yes.

[jb55]

This is implemented now, can this be closed?

[Sjors]

Ah, I totally missed that. This was added in #1399 even though the description says it wasn't? Nice!

No new functionality, just a continuation of my work toward completing #665