feat: discounted fees for confidential transactions

[delta1]

ELIP 200: Discounted fees for Confidential Transactions

This is a mempool policy only change to enable discounted fees for Confidential Transactions.

Two new configuration options are added to toggle accepting and creating these discounted CTs. accept defaults to true for liquidv1 only, false for other chains. create defaults to false on all chains for now.

At a feerate of 1 sat/vb for a 2 input, 3 output transaction:

• explicit transaction has a fee of 326 sats
• confidential transaction has a fee of 410 sats (without discount: 2575 sats)

This change is optional and backwards compatible for nodes and wallets.

This PR also alters block template transaction ordering, such that transactions are ordered by their feerate according to discountvsize. This means that discounted CT won't be selected last due to their lower real feerate according to vsize.

[tiero]

Please can you clearly add to the description if this change impacts wallets and other Elements implementation?

[psgreco]

Please can you clearly add to the description if this change impacts wallets and other Elements implementation?

The idea for this change is that it will be optional, and more beneficial to users, so if wallets keep doing the same thing they were doing, it will still be valid, they will just be overpaying a bit based on the new standard.

[delta1]

yes this is optional and backwards compatible

spec is ELIP 200: https://github.com/ElementsProject/ELIPs/blob/main/elip-0200.mediawiki

[delta1]

Force pushed with the change to calculate the discount per confidential output

[delta1]

Force pushed to include commit which changes the block template ordering

[roconnor-blockstream]

My review is incomplete, but this is my first pass.

[apoelstra]

Done reviewing 41abd40

[delta1]

@roconnor-blockstream @apoelstra thanks for your reviews, addressed your comments

[roconnor-blockstream]

I am no longer able to find issues with this PR.

I didn't review the testing.

[delta1]

"silent conversion between integer types of different signedness"

gotta love C++ :)

[delta1]

@apoelstra can I squash this into 1 commit and rebase on master?

[apoelstra]

@delta1 yeah, that'd be good. Though I would prefer if you could squash this into four or five commits. For example

• one that introduces the new config args, but don't use them to do anything
• one that introduces the new discount functions but doesn't use it
• one that hooks it up to the mempool logic
• one that hooks it up to the network logic
• one that hooks it up to the wallet logic

[apoelstra]

utACK 83a7fe5

I can't test this locally because, even on master, even with -j1, two functional tests are failing in my Nix environment (interface_bitcoin_cli --descriptors and mempool_persist). But I confirmed that everything appears to build on the tip of this PR, and the code changes look good (and the total diff (nearly) matches the previous 20-commit thing).

[roconnor-blockstream]

I have nothing to add.

[delta1]

pushed to update liquidv1 default for accept_discount_ct to false

git range-diff 7ff33d45df3a07b421cd7bacc63e9f34fef72fcb..3ebc35359f1304ff9f3a2a5286a154287794e23f 7ff33d45df3a07b421cd7bacc63e9f34fef72fcb..83a7fe5442d9af612183b20d988d367a74ce11de
1:  c1661b251a ! 1:  c2692d9907 discount: introduce config args
    @@ src/chainparams.cpp: public:
      
              multi_data_permitted = true;
     +        create_discount_ct = args.GetBoolArg("-creatediscountct", false);
    -+        accept_discount_ct = args.GetBoolArg("-acceptdiscountct", false);
    ++        accept_discount_ct = args.GetBoolArg("-acceptdiscountct", true);
      
              parentGenesisBlockHash = uint256S("000000000019d6689c085ae165831e934ff763ae46a2a6c172b3f1b60a8ce26f");
              const bool parent_genesis_is_null = parentGenesisBlockHash == uint256();
2:  9cbd3e0f05 = 2:  283d245bd9 discount: add policy/discount.h
3:  b7b2288a74 = 3:  76d42f5056 discount: implement mempool logic
4:  c85f0d4fda = 4:  146b1acc71 discount: implement net processing
5:  4807f3ef1f = 5:  c7dae295ba discount: change miner tx ordering
6:  52962c16c9 = 6:  b30b79d653 discount: allow wallet to create discount txs
7:  3ebc35359f = 7:  83a7fe5442 discount: add functional tests

[apoelstra]

utACK 3ebc353