fix(swift-sdk): make a changeset round linear without giving up atomicity - #4595
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fix(swift-sdk): make a changeset round linear without giving up atomicity#4595romchornyi wants to merge 2 commits into
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…e watermark last A changeset round in `PlatformWalletPersistenceHandler` accumulated every row unsaved until `endChangeset`, so each per-row `fetch()` in `upsertUtxo` / `upsertTransaction` re-scanned the whole pending set — O(rows²) per round. On a mixing-heavy wallet the catch-up round after a backward re-walk (thousands of TXO/transaction upserts) turned into minutes of compute on the persistence queue and looked like a hang; sampling put every sample inside SwiftData's pending-merge hashing. Flush `backgroundContext` every 500 applied rows (`noteRowApplied`) so the pending set stays bounded and the round is linear. Because intermediate saves commit rows before the round ends, the `syncedHeight` watermark can no longer be written when the chain changeset arrives: stage it in `deferredSyncedHeights` and apply it immediately before the round's final save. The watermark certifies "every row at or below this height is durable", so it must be the last thing a round commits — a crash mid-round then leaves idempotent rows without an advanced watermark, and the durable re-walk redelivers the remainder. Staged watermarks are dropped on rollback, like the parked payment rows. Measured on the same wallet: the persisted frontier went from stuck at 2,174,999 for 10+ minutes to advancing continuously; a simulator kill mid-round resumed from the last committed watermark and reached the tip with a store that matched the chain.
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ChangesWallet persistence
Estimated code review effort: 4 (Complex) | ~45 minutes Merge Risk: ⚪ Minimal · up to The changeset persistence path now uses round-scoped lookups while retaining a single final save for each round. No current merge-blocking risk is identified. 🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
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Inline comments:
In
`@packages/swift-sdk/Sources/SwiftDashSDK/PlatformWallet/PlatformWalletPersistenceHandler.swift`:
- Line 178: Update the endChangeset flow around backgroundContext.save() to
preserve atomicChangesets: do not commit the shared changeset context before
every callback succeeds. Coordinate an acknowledged checkpoint with Rust, or
isolate the writes in a transaction that rollback() can undo as one unit,
ensuring callback failure leaves no partial SwiftData changes.
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…a round registry instead of intermediate saves Review (#4595): the intermediate `save()` every 500 rows committed every dirty object in `backgroundContext`, so a callback failing later in the round left those rows committed while `endChangeset` rolled back and reported failure to Rust. That breaks the declared `atomicChangesets` contract, which invitation creation requires — the bit cannot simply be dropped. Replace the intermediate saves with a round-scoped registry. The quadratic cost was never the number of rows but the per-row `fetch()` by txid / outpoint: SwiftData evaluates the predicate against the context's pending changes, hashing the whole unsaved set on every lookup. While a round is open, `lookupTransaction` / `lookupTxo` / `lookupPendingInputs` serve rows this round has fetched or created from dictionaries and fall through to a store-only fetch (`includePendingChanges = false`) on a miss. Every in-round creation of those entities registers itself, so a store-only miss never means "created earlier this round"; rows deleted this round are filtered via `isDeleted`. The registry is reset at `beginChangeset` and after commit or rollback. Outside a round the helpers behave like the plain fetches they replace. One `save()` per round, as before; the deferred-watermark staging is no longer needed and is gone. Same wallet, simulator rescan of the full history with a process kill at height 2,180,000 and a relaunch: persistence kept pace with the scan (~2.0M heights in ~75 s), resumed from the persisted watermark, reached the tip; the store matched the previous run row for row (6,787 transactions, 13,882 TXOs, no duplicates, no stubs, no failed rounds).
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Issue being fixed or feature implemented
A changeset round in
PlatformWalletPersistenceHandleraccumulated every row unsaved untilendChangeset, so each per-rowfetch()inupsertUtxo/upsertTransactionre-scanned the whole pending set — O(rows²) per round. On a mixing-heavy wallet (~6.7k transactions, ~13.8k TXOs) the catch-up round after a backward re-walk turned into 10+ minutes of compute on the persistence queue and looked like a hang. Sampling the process put every sample inside SwiftData's pending-merge hashing underupsertUtxo. This is the SDK-side half of the large-wallet "sync finished but transactions are missing" reports; the app-side gate is dashpay/dashwallet-ios#1112.What was done?
The quadratic cost is not the number of rows but the per-row
fetch()by txid / outpoint: SwiftData evaluates the predicate against the context's pending changes, hashing the whole unsaved set on every lookup. The round stays ONE SwiftData transaction with onesave()inendChangeset(theatomicChangesetscontract is untouched); what changes is how in-round lookups are answered:roundTransactions,roundTxos,roundPendingInputs): rows this round has fetched or created are served from dictionaries keyed by their immutable identity (txid / outpoint).lookupTransaction/lookupTxo/lookupPendingInputsrun a store-only fetch (FetchDescriptor.includePendingChanges = false), which skips the pending-merge and is an index lookup. Every in-round creation of these entities registers itself (registerRoundTransaction/registerRoundTxo/registerRoundPendingInput), so a store-only miss never means "created earlier this round"; rows deleted this round are filtered viaisDeleted.beginChangesetand after commit or rollback inendChangeset. Outside a round the helpers behave exactly like the plain fetches they replace.upsertTransaction,upsertUtxo(incl. the stub-parent path and pending-input adoption),resolveInputOutpoint,removePendingInputs,markUtxoSpent,markUtxoInstantLocked.History: the first revision used intermediate saves every 500 rows; review pointed out that a later callback failure would then leave committed rows behind a rolled-back round, breaking
atomicChangesets(which invitation creation requires). That approach is replaced, not layered on.Note: #4589 touches the same file (swept-transaction handling); the hunks do not overlap, but whichever lands second needs a trivial rebase.
How Has This Been Tested?
No new unit tests (the handler has no round-level harness; adding one is a follow-up). Manual, same seed throughout (~6.7k transactions, ~13.8k TXOs, CoinJoin-heavy):
gettxoutaudit of every unspent row matched the chain.a2ba48d7(fix(dash-spv): make backward coverage durable by rewinding synced_height instead of sweeping in memory rust-dashcore#1002); the handler change itself does not depend on it.Breaking Changes
None. A round is still a single SwiftData transaction. Reviewer note: correctness of the store-only lookups relies on every in-round creation of
PersistentTransaction/PersistentTxo/PersistentPendingInputgoing through the register helpers; the three constructor sites in this file do.Checklist:
For repository code-owners and collaborators only
Summary by CodeRabbit
Performance
Reliability