Merge #16766: wallet: Make IsTrusted scan parents recursively

4671fc3d9e669da8b8781f0cbefee43cb9acd527 Expand on wallet_balance.py comment from https://github.com/bitcoin/bitcoin/pull/16766\#issuecomment-527563982 (Jeremy Rubin)
91f3073f08aff395dd813296bf99fd8ccc81bb27 Update release notes to mention changes to IsTrusted and impact on wallet (Jeremy Rubin)
8f174ef112199aa4e98d756039855cc561687c2e Systematize style of IsTrusted single line if (Jeremy Rubin)
b49dcbedf79613f0e0f61bfd742ed265213ed280 update variable naming conventions for IsTrusted (Jeremy Rubin)
5ffe0d144923f365cb1c2fad181eca15d1668692 Update comment in test/functional/wallet_balance.py (Jeremy Rubin)
a550c58267f50c59c2eea1d46edaa5019a8ad5d8 Update wallet_balance.py test to reflect new behavior (Jeremy Rubin)
5dd7da4ccd1354f09e2d00bab29288db0d5665d0 Reuse trustedParents in looped calls to IsTrusted (Jeremy Rubin)
595f09d6de7f1b94428cdd1310777aa6a4c584e5 Cache tx Trust per-call to avoid DoS (Jeremy Rubin)
dce032ce294fe0d531770f540b1de00dc1d13f4b Make IsTrusted scan parents recursively (Jeremy Rubin)

Pull request description:

  This slightly modifies the behavior of IsTrusted to recursively check the parents of a transaction. Otherwise, it's possible that a parent is not IsTrusted but a child is. If a parent is not trusted, then a child should not be either.

  This recursive scan can be a little expensive, so ~it might be beneficial to have a way of caching IsTrusted state, but this is a little complex because various conditions can change between calls to IsTrusted (e.g., re-org).~ I added a cache which works per call/across calls, but does not store the results semi-permanently. Which reduces DoS risk of this change. There is no risk of untrusted parents causing a resource exploitation, as we immediately return once that is detected.

  This is a change that came up as a bug-fix esque change while working on OP_SECURETHEBAG. You can see the branch where this change is important here: https://github.com/bitcoin/bitcoin/compare/master...JeremyRubin:stb-with-rpc?expand=1. Essentially, without this change, we can be tricked into accepting an OP_SECURETHEBAG output because we don't properly check the parents. As this was a change which, on its own, was not dependent on OP_SECURETHEBAG, I broke it out as I felt the change stands on its own by fixing a long standing wallet bug.

  The test wallet_balance.py has been corrected to meet the new behavior. The below comment, reproduced, explains what the issue is and the edge cases that can arise before this change.

          # Before `test_balance()`, we have had two nodes with a balance of 50
          # each and then we:
          #
          # 1) Sent 40 from node A to node B with fee 0.01
          # 2) Sent 60 from node B to node A with fee 0.01
          #
          # Then we check the balances:
          #
          # 1) As is
          # 2) With transaction 2 from above with 2x the fee
          #
          # Prior to #16766, in this situation, the node would immediately report
          # a balance of 30 on node B as unconfirmed and trusted.
          #
          # After #16766, we show that balance as unconfirmed.
          #
          # The balance is indeed "trusted" and "confirmed" insofar as removing
          # the mempool transactions would return at least that much money. But
          # the algorithm after #16766 marks it as unconfirmed because the 'taint'
          # tracking of transaction trust for summing balances doesn't consider
          # which inputs belong to a user. In this case, the change output in
          # question could be "destroyed" by replace the 1st transaction above.
          #
          # The post #16766 behavior is correct; we shouldn't be treating those
          # funds as confirmed. If you want to rely on that specific UTXO existing
          # which has given you that balance, you cannot, as a third party
          # spending the other input would destroy that unconfirmed.
          #
          # For example, if the test transactions were:
          #
          # 1) Sent 40 from node A to node B with fee 0.01
          # 2) Sent 10 from node B to node A with fee 0.01
          #
          # Then our node would report a confirmed balance of 40 + 50 - 10 = 80
          # BTC, which is more than would be available if transaction 1 were
          # replaced.

  The release notes have been updated to note the new behavior.

ACKs for top commit:
  ariard:
    Code Review ACK 4671fc3, maybe extend DoS protection in a follow-up PR.
  fjahr:
    Code review ACK 4671fc3d9e669da8b8781f0cbefee43cb9acd527
  ryanofsky:
    Code review ACK 4671fc3d9e669da8b8781f0cbefee43cb9acd527. Changes since last review: 2 new commits adding suggested release note and python test comment, also a clean rebase with no changes to the earlier commits. The PR description is more comprehensive now, too. Looks good!
  promag:
    Code review ACK 4671fc3d9e669da8b8781f0cbefee43cb9acd527.

Tree-SHA512: 6b183ff425304fef49724290053514cb2770f4a2350dcb83660ef24af5c54f7c4c2c345b0f62bba60eb2d2f70625ee61a7fab76a7f491bb5a84be5c4cc86b92f

1 files changed, 1 insertions, 0 deletions

diff --git a/doc/release-notes.md b/doc/release-notes.md
index ea82962e75..a47c8802b0 100644
--- a/doc/release-notes.md
+++ b/doc/release-notes.md
@@ -85,6 +85,7 @@ Wallet
 ------
 
 - The wallet now by default uses bech32 addresses when using RPC, and creates native segwit change outputs.
+- The way that output trust was computed has been fixed in #16766, which impacts confirmed/unconfirmed balance status and coin selection.
 
 Low-level changes
 =================