#36597 wallet: return broadcast errors from CommitTransaction

Updates wallet commit and transaction broadcast interfaces to return an `Expected<void, TransactionBroadcastError>` instead of `void` or error codes with separate output strings. Introduces `TransactionBroadcastError` pairing a `TransactionError` code with an untranslated message, removes `TransactionError::OK`, and adds a `SUBMISSION_SKIPPED` code for transactions that the wallet chooses not to relay. Existing callers such as RPCs and the Qt widgets interface currently discard the result to preserve existing behavior.

Problem: Frontends relying on `interfaces::Wallet::commitTransaction` cannot know if a transaction was accepted into the mempool because `CWallet::CommitTransaction()` only logged errors and returned `void`. Consequently, interfaces such as the QML GUI report 'Transaction sent' even when the node rejected the transaction.

Evidence of importance:

• PR description: 'The QML GUI shows "Transaction sent" even when the node rejected the transaction, because CWallet::CommitTransaction() only logs the rejection and returns void' (bitcoin-core/gui-qml#1008)

Scope: Modifies public interfaces across `interfaces::Wallet`, `interfaces::Chain`, and `interfaces::Node`, refactoring `BroadcastTransaction` signatures and updating all internal invocation sites.

Risk of a mistake: medium (wallet, gui, rpc_api). A mistake in transaction submission error handling or mempool state tracking in CWallet could cause wallet transactions to incorrectly fail or become desynchronized from the node.

Changed since the previous assessment: Initial description was evaluated without patch data; the full patch confirms the signature refactoring across interfaces and the addition of unit tests in `wallet_interfaces_tests.cpp` and `wallet_tests.cpp`.

Paused · Waiting on author

Open objection from ViniciusCestarii with no author reply for 9 days; review would repeat what has been asked.

Waiting on (from the thread read):

• author: respond to ViniciusCestarii's test coverage observation 2026-10-23
• reviewer: maflcko to review author's explanation of use cases 2026-10-23

Positive

support without stated reasons, no open objection (w0xlt, Naomi-Gift)

Thread notes: Added maflcko's inquiry questioning the rationale for CommitTransaction errors (answered by pseudoramdom) and ViniciusCestarii's mutation test observation indicating missing test coverage in interface tests.

Objections, suggestions, and questions:

Support:

• w0xlt (contributor) (Concept ACK, 2026-10-21): (no reason given) [not substantive]
• Naomi-Gift (none) (Concept ACK, 2026-10-22): (no reason given) [not substantive]

Participants:

• w0xlt (contributor, support, 1 statements 2026-10-21 to 2026-10-21): gave Concept ACK
• Naomi-Gift (none, support, 1 statements 2026-10-22 to 2026-10-22): Concept ACK; asked how QML GUI handles SUBMISSION_SKIPPED
• ViniciusCestarii (contributor, neutral, 1 statements 2026-10-23 to 2026-10-23): pointed out surviving mutant from missing test coverage for MEMPOOL_REJECTED
• maflcko (member, question, 1 statements 2026-10-23 to 2026-10-23): asked why broadcast errors are needed instead of fixing wallet bugs or using async feedback

Enables:

• bitcoin-core/gui-qml update to report broadcast errors to users (gui-qml#1008)
• Follow-up PR propagating errors from commitBumpTransaction

116 lines under test/bench/ci.

• src/wallet/test/wallet_tests.cpp +77/-6
• src/wallet/wallet.cpp +32/-21
• src/node/transaction.cpp +21/-12
• src/wallet/test/wallet_interfaces_tests.cpp +32/-0
• src/rpc/mempool.cpp +12/-16
• src/node/interfaces.cpp +8/-13
• src/interfaces/chain.h +9/-8
• src/node/transaction.h +8/-9
• src/net_processing.cpp +9/-5
• src/node/types.h +13/-1
• src/wallet/wallet.h +10/-3
• src/qt/psbtoperationsdialog.cpp +4/-6
• src/interfaces/node.h +5/-3
• src/interfaces/wallet.h +5/-2
• src/net_processing.h +3/-3
• src/wallet/interfaces.cpp +3/-2
• src/common/messages.cpp +2/-2
• src/wallet/rpc/spend.cpp +2/-2
• src/qt/walletmodel.cpp +1/-1
• src/rpc/util.cpp +2/-0
• src/wallet/feebumper.cpp +1/-1
• src/test/fuzz/kitchen_sink.cpp +1/-0

The short text the ranking pass compares across PRs.

Changes CWallet::CommitTransaction and interfaces::Wallet::commitTransaction to return broadcast errors via util::Expected instead of returning void. This allows interfaces such as the QML GUI to inform users when a transaction has been rejected instead of falsely claiming 'Transaction sent' (bitcoin-core/gui-qml#1008). Existing wallet RPC and Qt widget callers ignore the returned error to maintain existing behavior.

P3 · unblocks #1008

Extends node and wallet interface methods to return structured broadcast errors needed by multiprocess consumers like the QML GUI. The PR description notes that frontends relying on `interfaces::Wallet::commitTransaction` could not detect rejected transactions because the method returned `void`, unblocking gui-qml#1008.

Membership: Modifies public interfaces in src/interfaces/wallet.h, src/interfaces/chain.h, and src/interfaces/node.h to propagate broadcast errors across process boundaries.

Factors: security/stability 0, bug 1, performance 0, user value 2, leverage 2

P3 · bug fix (assessed alone as P2; Lowered from P2: no behavior change for current Core callers; the payoff accrues to downstream gui-qml, like #36685.)

Prevents wallet frontends from falsely displaying 'Transaction sent' when the node rejects a transaction. The PR description highlights that `CWallet::CommitTransaction()` previously discarded rejections by returning `void`, masking broadcast failures from the user.

Membership: Labeled 'Wallet' and modifies CWallet::CommitTransaction() alongside wallet spend and bump fee interfaces.

Factors: security/stability 0, bug 1, performance 0, user value 2, leverage 2