wallet: re-activate “AmountWithFeeExceedsBalance” error #34299

1. 
   stratospher commented at 9:06 am on January 15, 2026: contributor

   picks up #25269.

   AmountWithFeeExceedsBalance error inside WalletModel::prepareTransaction is never thrown and remains an unused code path. This is because createTransaction does not retrieve the fee when the process fails. The fee return arg is set only at the end of the process, when the transaction is successfully created. Therefore, if the transaction creation fails, the fee is not available inside WalletModel::prepareTransaction to trigger the AmountWithFeeExceedsBalance error.

   This PR re-implements the feature inside CreateTransactionInternal and adds test coverage for it.

   on master	on PR

   the unreachable code path is removed in https://github.com/bitcoin-core/gui/pull/807 which requires this PR.

2. wallet: fix, make 'total_effective_amount' optional actually optional

   this is not needed for the remaining commits but good to fix
   and came up in #25269 review.
   
   Co-authored-by: furszy <matiasfurszyfer@protonmail.com>

   c467325aaf
3. wallet: clarify PreSelectedInputs total_amount

   using total_amount variable to store either the actual total
   amount or effective total amount based on whether SFFO is enabled
   is confusing. keep this SFFO-based functionality in a separate
   function GetTotal() and store both total_amount and effective
   total amount separately.
   
   without this when SFFO is not there, available_balance in the
   next commit would yield inconsistent result:
   - effective amount of preset_inputs summed with
   - total amount of available_coins.

   af3f3bfd57
4. wallet: introduce "tx amount exceeds balance when fees are included" error

   This was previously implemented at the GUI level but we never hit that
   code path.
   
   Co-authored-by: furszy <matiasfurszyfer@protonmail.com>

   f66d5bc099
5. DrahtBot added the label Wallet on Jan 15, 2026
6. 
   DrahtBot commented at 9:06 am on January 15, 2026: contributor

   The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

   Code Coverage & Benchmarks

   For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/34299.

   Reviews

   See the guideline for information on the review process. A summary of reviews will appear here.

7. 
   furszy commented at 2:48 pm on January 15, 2026: member

   Thanks for picking it up!
8. 
   furszy commented at 2:16 am on January 21, 2026: member

   q: in af3f3bfd57bf8869030153ecf3695f07ae7dd805, wouldn’t be better to replace PreSelectedInputs for CoinsResult? (and then remove PreSelectedInputs).

   The outputs we add through FetchSelectedInputs can be of type UNKNOWN during the CoinsResult.Add call (their type doesn’t matter, they must be selected anyway).

Contributors
stratospher DrahtBot furszy

Labels
Wallet