Subject: [bitcoindev] TRUC and P2A for CTV fee management

From: "'stutxo' via Bitcoin Development Mailing List" <[email-masked]>
Date: Tue, 17 Dec 2024 16:25:43 -0800 (PST)

Hi everyone,

I am trying to learn more about op_ctv (or its true name, op_securethebag). 
One thing I keep hearing is that estimating fees are potentially an issue 
when spending CTV transactions. 

jamesob mentioned fees in his simple_ctv_valut 
<https://github.com/jamesob/simple-ctv-vault>

*Because coins may remain vaulted for long periods of time, the unvault 
process is sensitive to changes in the fee market. Because use of OP_CTV 
requires precommiting to a tree of all possible specific outputs and the 
number of inputs, we cannot use RBF to dynamically adjust feerate of 
unvaulting transactions.*
and rustyrussell on nostr also mentioned fees being a problem 
<https://primal.net/e/note1ulw6tr8hrqz092wy02fcyshn3fgt309kc5w95mhl7j0shj7c9zxsu8a2mw>
 
*Optimised sponsors for solving the "but how do I add fees" problem in a 
way that doesn't drive miner centralisation.*

With v3 transactions available in bitcoin 28.0 
<https://bitcoinops.org/en/bitcoin-core-28-wallet-integration-guide/>there 
are a bunch of new techniques that have been enabled that we can use to 
hopefully solve these issues

As long as you have an output for 240 sats paying to a P2A address, such as 
tb1pfees9rn5nz on signet, you or anyone else will be able to bump the fees 
using CPFP on the anchor output. 

I have some examples of these transactions here on signet

CTV spend transaction with zero fees:
https://mempool.space/signet/tx/32f4f4e6165e7f8df9b9a762e11a6ca7f16087713e0e3e42352021e6bf3800e3

P2A CPFP transaction:
https://mempool.space/signet/tx/9a3582f03b0ac39cff8ed024cf8f38e4fc4a1ee2ff216badf041bf4572c0d03b

Code used is here:
https://github.com/stutxo/simple_ctv

Is there anything I am missing here? What are the downsides of this method? 
Is this how most ctv scripts spends would work?

Thanks!
stu

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