funding: use p2tr by default for batch_open_channel

[Torakushi]

fixes #7601

In LND v0.15.1, batch_open_channel used p2tr by default for change output. However, in a later version, a breaking change has been fixed in FundPSBT to make the change type configurable (default to p2wkh). As batch_open_channel uses FundPsbt, we need to put back p2tr as default for change output for this specific case

[Torakushi]

@ziggie1984

[ziggie1984]

Thanks for this quick PR, left some comments :)

[ziggie1984]

I am not sure if the change-output is always the last output though, maybe just find the change by excluding the amounts? Its maybe constant for this scenario but not generally speaking because its sorted I think in the btcwallet package because there the funding takes place.

Could you explain your thought behind your formula ?

[Torakushi]

This formula doesn't take the last output. Here is the explanation!

If you have M channels points and 1 change-output, then the indexes go from 0 to M.
We know the OutputIndex of every channel point, so we just need to find the missing int between 0 and M.

To do that, we know how to calculate the sum of the M consecutive int (aka if there is no missing int):

Ideal_Sum= M*(M+1)/2

In our case, we know the sum of all the channel output indexes, so, to find out the missing int, we just need to take the difference:

Ideal_Sum- Sum(Channels outpoint indexes)

In our case M=3, so you have the formula in the code (Ideal_Sum=6)

With an example: if channel indexes are 0,1,3 then change output index is 6-(0+1+3)=2

[ziggie1984]

Thanks for your explanation, nice again something learned :). Not sure if this ideal sum thing is common knowledge but I would suggest a nice comment maybe ?

[Torakushi]

no problem! :) will add it!

[ziggie1984]

nit: your comment describes the code I would have prefered something like this:

Suggestion:

//For calculating the change output index we use the formula for consecutive sum of integers n(n+1)/2. In our case n=3. All the channel point indices are known so we just calculate the difference to get the change output index`

[guggero]

Pretty neat indeed.

[ziggie1984]

Thanks for the PR, LGTM

[ziggie1984]

nit: your comment describes the code I would have prefered something like this:

Suggestion:

//For calculating the change output index we use the formula for consecutive sum of integers n(n+1)/2. In our case n=3. All the channel point indices are known so we just calculate the difference to get the change output index`

[guggero]

Thanks for the fix, LGTM 🎉

[guggero]

nit: move to above the struct initialization to avoid line too long.

	firstReq := b.channels[0].fundingReq
	feeRateSatPerKVByte := firstReq.FundingFeePerKw.FeePerKVByte()
	changeType := walletrpc.ChangeAddressType_CHANGE_ADDRESS_TYPE_P2TR
	fundPsbtReq := &walletrpc.FundPsbtRequest{
		Template: &walletrpc.FundPsbtRequest_Raw{
			Raw: txTemplate,
		},
		Fees: &walletrpc.FundPsbtRequest_SatPerVbyte{
			SatPerVbyte: uint64(feeRateSatPerKVByte) / 1000,
		},
		MinConfs:         firstReq.MinConfs,
		SpendUnconfirmed: firstReq.MinConfs == 0,
		ChangeType:       changeType,
	}

[guggero]

Can use txscript.ParsePkScript instead which returns a PkScript that has a Class() method.

[guggero]

Pretty neat indeed.

[Roasbeef]

LGTM 🛵