backport: partial bitcoin#25507; fix selection of fully mixed coins, disable BnB #6600

UdjinM6 · 2025-02-26T21:20:41Z

Issue being fixed or feature implemented

Coin selection is a bit broken on develop rn, there are few issues:

we should never use BnB because it has no idea about mixing;
the target value should not be altered in cases when the tx fee is subtracted from outputs or no change is created at all.

Issues were introduced via recent wallet backports, so it's on develop only, no released versions are affected.

What was done?

How Has This Been Tested?

Run ./src/test/test_dash -t spend_tests and ./test/functional/wallet_basic.py

develop + 5544542 - > failure
This branch -> success

Breaking Changes

Checklist:

I have performed a self-review of my own code
I have commented my code, particularly in hard-to-understand areas
I have added or updated relevant unit/integration/functional/e2e tests
I have made corresponding changes to the documentation
I have assigned this pull request to a milestone (for repository code-owners and collaborators only)

…f subtracting fees from output 140d942 wallet: don't add change fee to target if subtracting fees from output (S3RK) Pull request description: Change fee is payed by the recipient, so we don't need to add it to our target for coin selection. ACKs for top commit: achow101: ACK 140d942 ishaanam: ACK 140d942 furszy: Code review ACK 140d942 Tree-SHA512: b5efd0264c47ecee9204a3fd039bad24c69f9e614c6e1d9bb240ee5be6356b175aa074f3be123e6cfb8becd4d7bd1028eebe18801662cc69d19413d8d5a9dd5c

…ixed coins

coderabbitai · 2025-02-26T21:22:24Z

Walkthrough

The changes modify the coin selection logic used in wallet transactions. In the updated implementation, the BnB coin selection method is effectively disabled by placing it behind an always-false condition. Instead, a new variable, target_with_change, is introduced and initialized with the original target value. This variable is then adjusted to include a change fee when specific conditions are met. The adjusted target value is subsequently passed to the knapsack solver, replacing the previous logic that directly added the change fee to the target. No changes have been made to any exported or public interfaces.

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src/wallet/spend.cpp (1 hunks)

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🔇 Additional comments (4)

src/wallet/spend.cpp (4)

376-379: Disabled BnB selection algorithm

The PR intentionally disables the BnB (Branch and Bound) coin selection algorithm by adding a false && condition, effectively preventing this code path from executing. This is appropriate since the BnB algorithm does not account for coin mixing, as mentioned in the PR objectives.

383-383: Added new target_with_change variable

This new variable properly separates the original target value from the potentially adjusted value that accounts for change fees.

386-389: Added conditional logic for including change fee

This change correctly implements the logic to only add the change fee to the target value when we're not subtracting fees from outputs and not working with fully mixed coins. This addresses the issue mentioned in the PR objectives where the target value was incorrectly altered when transaction fees were deducted from outputs or when no change was produced.

392-392: Updated KnapsackSolver to use the new target_with_change variable

The KnapsackSolver now uses the conditionally adjusted target value instead of always adding the change fee. This ensures that the target value calculation is correct for all scenarios, particularly when working with fully mixed coins or when subtracting fees from outputs.

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knst · 2025-02-27T16:04:21Z

src/wallet/spend.cpp

@@ -373,7 +373,7 @@ bool CWallet::AttemptSelection(const CAmount& nTargetValue, const CoinEligibilit
    std::vector<OutputGroup> positive_groups = GroupOutputs(coins, coin_selection_params, eligibility_filter, true /* positive_only */);
    std::set<CInputCoin> bnb_coins;
    CAmount bnb_value;
-    if (SelectCoinsBnB(positive_groups, nTargetValue, coin_selection_params.m_cost_of_change, bnb_coins, bnb_value)) {
+    if (false && SelectCoinsBnB(positive_groups, nTargetValue, coin_selection_params.m_cost_of_change, bnb_coins, bnb_value)) {


should it be always disabled or only in case of CJ?

yes, we should avoid spending cj-related coins via regular send

UdjinM6 added this to the 23 milestone Feb 26, 2025

achow101 and others added 3 commits February 27, 2025 00:21

fix: do not add change fee to the target value when selecting fully m…

5882363

…ixed coins

fix: disable BnB coin selection algo

0c32cf4

UdjinM6 force-pushed the fix_wallet_backports branch from 5544542 to 0c32cf4 Compare February 26, 2025 21:21

UdjinM6 changed the title ~~backport: bitcoin#25507; fix selection of fully mixed coins, disable BnB~~ backport: partial bitcoin#25507; fix selection of fully mixed coins, disable BnB Feb 26, 2025

knst reviewed Feb 27, 2025

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UdjinM6 requested review from PastaPastaPasta and kwvg March 4, 2025 16:28

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backport: partial bitcoin#25507; fix selection of fully mixed coins, disable BnB #6600

backport: partial bitcoin#25507; fix selection of fully mixed coins, disable BnB #6600

UdjinM6 commented Feb 26, 2025

coderabbitai bot commented Feb 26, 2025 •

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backport: partial bitcoin#25507; fix selection of fully mixed coins, disable BnB #6600

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backport: partial bitcoin#25507; fix selection of fully mixed coins, disable BnB #6600

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UdjinM6 commented Feb 26, 2025

Issue being fixed or feature implemented

What was done?

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coderabbitai bot commented Feb 26, 2025 • edited Loading

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coderabbitai bot commented Feb 26, 2025 •

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