fix(dash-spv): count acceptance signals that arrived before the broadcast

[romchornyi]

Problem

broadcast_transaction_and_wait reports Uncertain for transactions the network demonstrably accepted, whenever the acceptance evidence arrived before the broadcast was registered.

The acceptance heuristic in MempoolManager is echo-based: start_broadcast sends the transaction to a subset of peers, withholds it from a holdout set, and waits for a holdout peer to announce the txid back via inv. process_instant_send and the confirmation path can promote a broadcast too — but only one that is already tracked in self.broadcasts.

That leaves a blind spot. When the identical signed transaction reaches the network by another route first, its InstantSend lock (or its relay to us from a peer) lands before start_broadcast ever runs, and no later signal repeats it: a peer does not re-announce a transaction it is only now being sent. The broadcast sits Pending until acceptance_timeout and is reported Uncertain.

This is routine rather than exotic, because BIP70 hands the merchant the signed bytes and takes its acknowledgement before the wallet broadcasts. A field case from Dash Wallet iOS on mainnet:

20:18:57.247  New wallet transaction detected txid=e730e18f… context=mempool
20:18:57.325  InstantLock signature verified for txid e730e18f…      ← 0.7 s before our dispatch
20:18:58.019  Broadcast e730e18f… to 2/3 peers (1 withheld for acceptance detection)
20:19:58.054  Broadcast e730e18f… outcome uncertain: no acceptance signal within 60s
20:34:58      TransactionBroadcastResult(… Accepted { relayed_by: 0 })  ← only once a block confirmed it

The caller (rs-platform-wallet's SpvBroadcaster, 30 s cap) turned that into MaybeSent, and the wallet discarded a paid gift-card order on the strength of it — the customer paid, the merchant fulfilled the order, and the app kept no record of the card.

Fix

start_broadcast now consults what is already known before registering a pending broadcast, via a new preexisting_acceptance:

• a lock waiting in pending_is_locks — the IS lock arrived before the transaction did;
• a mempool entry already flagged is_instant_send;
• a mempool entry at all, which at this point can only have come from a peer relaying it to us (the local injection happens after this call), i.e. the same proof of propagation the echo threshold waits for.

Any of those emits Accepted { relayed_by: 0 } immediately and marks the tracked state Accepted. The transaction is still sent to peers: knowing that one peer has it says nothing about the rest.

start_broadcast therefore returns Vec<SyncEvent> instead of (), and handle_tx forwards it — without that the waiter in broadcast_transaction_and_wait, which subscribes before dispatching, would never see the verdict.

Behavior notes

• Strictly a widening of what counts as acceptance. No signal that was previously conclusive changes meaning, and Pending/Uncertain/echo handling is untouched for broadcasts with no prior evidence.
• relayed_by: 0 already denoted "proven by something other than an echo" (IS lock, confirmation); this reuses it rather than adding a variant.
• A transaction the local node has never seen behaves exactly as before.

Testing

Four new tests in sync::mempool::manager, all failing before the change:

• an IS lock processed before start_broadcast yields Accepted { relayed_by: 0 } and leaves the tracked status Accepted;
• a mempool entry relayed by a peer before the broadcast does the same;
• with no prior evidence the broadcast stays Pending and emits nothing (guards against the seed firing on our own injection);
• handle_tx on the local sentinel address forwards the verdict, covering the propagation path the waiter depends on.

Full dash-spv library suite: 567 passed, 0 failed. cargo fmt clean, cargo clippy -p dash-spv --lib reports nothing.

Summary by CodeRabbit

• Bug Fixes
  • Improved transaction broadcast tracking and reporting.
  • Transactions with existing acceptance evidence are now recognized immediately.
  • Broadcasts remain pending when acceptance cannot yet be confirmed.
  • Acceptance events are consistently propagated, including for irrelevant or already tracked transactions.
• Tests
  • Added coverage for prior acceptance evidence, pending broadcasts, irrelevant peer relays, and transaction event propagation.

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📝 Walkthrough

Walkthrough

Local transaction handling now returns broadcast events. start_broadcast detects prior acceptance evidence and emits an immediate accepted result. Broadcasts without prior evidence remain pending. Tests cover both paths.

Changes

Broadcast acceptance

Layer / File(s)	Summary
Propagate local broadcast events dash-spv/src/sync/mempool/manager.rs	handle_tx collects events from start_broadcast and preserves them for duplicate and irrelevant transactions.
Recognize prior acceptance dash-spv/src/sync/mempool/manager.rs	start_broadcast checks pending InstantSend locks, locked or peer-relayed mempool entries, and seen_txids. It stores broadcasts as accepted or pending and emits an immediate accepted result when applicable. Tests cover these states and local event propagation.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: ⚪ Minimal · up to bacc4

The change broadens transaction acceptance detection for evidence received before broadcast while preserving existing behavior; no actionable merge-blocking risk remains after normal checks and review.

Sequence Diagram(s)

sequenceDiagram
  participant LocalTransaction
  participant MempoolManager
  participant AcceptanceEvidence
  participant SyncEvent
  LocalTransaction->>MempoolManager: handle_tx
  MempoolManager->>AcceptanceEvidence: start_broadcast and check prior evidence
  AcceptanceEvidence-->>MempoolManager: accepted or no evidence
  MempoolManager-->>SyncEvent: emit accepted result or keep pending
  MempoolManager-->>LocalTransaction: return accumulated events

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🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes counting acceptance signals received before broadcast registration.
Docstring Coverage	✅ Passed	Docstring check was indeterminate for this PR — some files could not be analyzed in time. Not blocking.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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Actionable comments posted: 1

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Inline comments:
In `@dash-spv/src/sync/mempool/manager.rs`:
- Around line 438-443: The peer-relay evidence is lost for irrelevant
transactions because they return before entering self.transactions, allowing
later local dispatch to reach Uncertain. Update the remote-receipt path before
the relevance return to record txids in a bounded, expiring peer-relay cache,
and update preexisting_acceptance to consult that cache alongside the existing
transaction checks. Add coverage for remote irrelevant receipt followed by local
dispatch.

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• dash-spv/src/sync/mempool/manager.rs

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[codecov]

Codecov Report

❌ Patch coverage is 99.21875% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 76.94%. Comparing base (b66db39) to head (bacc45f).

Files with missing lines	Patch %	Lines
dash-spv/src/sync/mempool/manager.rs	99.21%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #982      +/-   ##
==========================================
+ Coverage   76.91%   76.94%   +0.03%     
==========================================
  Files         329      329              
  Lines       82897    83019     +122     
==========================================
+ Hits        63757    63876     +119     
- Misses      19140    19143       +3     

Flag	Coverage Δ
core	78.25% <ø> (ø)
ffi	50.87% <ø> (ø)
rpc	20.00% <ø> (ø)
spv	92.09% <99.21%> (+0.03%)	⬆️
wallet	79.14% <ø> (ø)

Files with missing lines	Coverage Δ
dash-spv/src/sync/mempool/manager.rs	98.18% <99.21%> (+0.07%)	⬆️

... and 5 files with indirect coverage changes