Merge bitcoin/bitcoin#24584: wallet: avoid mixing different `OutputTypes` during coin selection

71d1d13627ccd27319f347e2d8167c8fe8a433f4 test: add unit test for AvailableCoins (josibake)
da03cb41a4ce15ebceee7fa4a4fdd2d3602fe284 test: functional test for new coin selection logic (josibake)
438e04845bf3302b7f459a50e88a1b772527f1e6 wallet: run coin selection by `OutputType` (josibake)
77b07072061c59f50c69be29fbcddf0d433e1077 refactor: use CoinsResult struct in SelectCoins (josibake)
2e67291ca3ab2d8f498fa910738ca655fde11c5e refactor: store by OutputType in CoinsResult (josibake)

Pull request description:

  # Concept

  Following https://github.com/bitcoin/bitcoin/pull/23789, Bitcoin Core wallet will now generate a change address that matches the payment address type. This improves privacy by not revealing which of the outputs is the change at the time of the transaction in scenarios where the input address types differ from the payment address type. However, information about the change can be leaked in a later transaction. This proposal attempts to address that concern.

  ## Leaking information in a later transaction

  Consider the following scenario:

  ![mix input types(1)](https://user-images.githubusercontent.com/7444140/158597086-788339b0-c698-4b60-bd45-9ede4cd3a483.png)

  1. Alice has a wallet with bech32 type UTXOs and pays Bob, who gives her a P2SH address
  2. Alice's wallet generates a P2SH change output, preserving her privacy in `txid: a`
  3. Alice then pays Carol, who gives her a bech32 address
  4. Alice's wallet combines the P2SH UTXO with a bech32 UTXO and `txid: b` has two bech32 outputs

  From a chain analysis perspective, it is reasonable to infer that the P2SH input in `txid: b` was the change from `txid: a`. To avoid leaking information in this scenario, Alice's wallet should avoid picking the P2SH output and instead fund the transaction with only bech32 Outputs. If the payment to Carol can be funded with just the P2SH output, it should be preferred over the bech32 outputs as this will convert the P2SH UTXO to bech32 UTXOs via the payment and change outputs of the new transaction.

  **TLDR;** Avoid mixing output types, spend non-default `OutputTypes` when it is economical to do so.

  # Approach

  `AvailableCoins` now populates a struct, which makes it easier to access coins by `OutputType`. Coin selection tries to find a funding solution by each output type and chooses the most economical by waste metric. If a solution can't be found without mixing, coin selection runs over the entire wallet, allowing mixing, which is the same as the current behavior.

  I've also added a functional test (`test/functional/wallet_avoid_mixing_output_types.py`) and unit test (`src/wallet/test/availablecoins_tests.cpp`.

ACKs for top commit:
  achow101:
    re-ACK 71d1d13627ccd27319f347e2d8167c8fe8a433f4
  aureleoules:
    ACK 71d1d13627ccd27319f347e2d8167c8fe8a433f4.
  Xekyo:
    reACK 71d1d13627ccd27319f347e2d8167c8fe8a433f4 via `git range-diff master 6530d19 71d1d13`
  LarryRuane:
    ACK 71d1d13627ccd27319f347e2d8167c8fe8a433f4

Tree-SHA512: 2e0716efdae5adf5479446fabc731ae81d595131d3b8bade98b64ba323d0e0c6d964a67f8c14c89c428998bda47993fa924f3cfca1529e2bd49eaa4e31b7e426

1 files changed, 1 insertions, 0 deletions

diff --git a/src/Makefile.test.include b/src/Makefile.test.include
index d8ff64bc43..f52964b033 100644
--- a/src/Makefile.test.include
+++ b/src/Makefile.test.include
@@ -168,6 +168,7 @@ BITCOIN_TESTS += \
   wallet/test/wallet_crypto_tests.cpp \
   wallet/test/wallet_transaction_tests.cpp \
   wallet/test/coinselector_tests.cpp \
+  wallet/test/availablecoins_tests.cpp \
   wallet/test/init_tests.cpp \
   wallet/test/ismine_tests.cpp \
   wallet/test/scriptpubkeyman_tests.cpp