LiveObjects: suppress no-op diff updates (RTLC14c/RTLM22c) and skip the sync wait for an empty synthetic list (RTO20d4)

[sacOO7]

Note

Stacked on #2284 (liveobjects/rto23c1-get-sync-wait-failure) — this branch bases there because both touch the publishAndApply sync-wait region; review after that PR. Spec companion: ably/specification#515 (stacked on ably/specification#514).

Problem

Three related behaviours diverged from the Objects spec's no-op update clauses, each delivering a spurious or wasteful result:

1. Spurious update events on re-sync of unchanged data (RTLC14c / RTLM22c). When an OBJECT_SYNC re-delivers state identical to what an object already holds (reconnect, re-attach with HAS_OBJECTS), _updateFromDataDiff produced { update: { amount: 0 } } / { update: {} } and _applySync delivered them to subscribers — change events for no change. The spec requires such diffs to be no-op updates (noop: true), which notifyUpdated already suppresses.
2. Spurious empty update on reset of an already-empty root (RTO4b2a). The ATTACHED-without-HAS_OBJECTS reset routes through the same diff (resetToInitialPool → clearObjectsData → notifyUpdated), so an already-empty root emitted { update: {} }. The clarified RTO4b2a specifies suppression (ably-cocoa and ably-java already suppress).
3. publishAndApply parked on a wait with nothing to apply (RTO20d4). When every serial in the PublishResult is null (each skipped per RTO20d1), the synthetic message list is empty, yet the code still entered the RTO20e sync wait — an unnecessary park that would surface a spurious 92008 if the channel dropped meanwhile. (Defensive: all-null serials are not an expected server behaviour today.)

Solution

• The no-op collapse lives in _updateFromDataDiff itself (both LiveCounter and LiveMap), so every caller — the overrideWithObjectState re-sync path, the RTO4b reset, and any future call site — is covered by one change. The overrideWithObjectState sites gain a no-op passthrough (mirroring their existing RTLC6e/RTLM6e terminal-noop returns) so objectMessage is only stamped on real updates.
• The tombstone path is the one mandated exception (the RTLC14c/RTLM22c advisory: the exception "must not be applied when the diff is computed for a tombstone per RTLO4e5"). A tombstone update must never be no-op-marked — notifyUpdated suppresses no-ops before its tombstone branch, so the RTLO4b4c3c listener teardown would silently be skipped for an already-zero counter / already-empty map. tombstone() therefore synthesizes the typed no-change update ({ update: { amount: 0 } } / { update: {} }) via a new protected abstract _createNoChangeUpdate() hook when the diff is a no-op, then stamps tombstone = true. The hook follows the class's existing template-method design (it joins seven existing abstract members) and matches ably-java's synthesis semantics (BaseRealtimeLiveObject.tombstone()); a base-class type-switch was rejected because LiveObject instances carry no type discriminator and a concrete literal is not assignable to the generic TUpdate without casts.
• RTO20d4 guard: publishAndApply returns successfully before the RTO20e wait when the synthetic list is empty.

Tests

Seven UTS unit test ports (spec cases added in ably/specification#514 + #515):

Test ID	Asserts
RTLC14c/zero-delta-diff-is-noop-0	identical-state override → noop: true, no event
RTLM22c/empty-diff-is-noop-0	identical non-tombstoned entries (timeserial ignored per RTLM22b3) → noop: true
RTO20d4/empty-synthetic-list-skips-sync-wait-0	all-null ACK serials while SYNCING → operation resolves without the sync completing
RTLO5/tombstone-zero-value-counter-emits-update-0	tombstoning a zero counter → non-no-op update, tombstone: true, amount: 0
RTLO5/tombstone-empty-map-emits-update-0	tombstoning an all-tombstoned map → non-no-op, tombstone: true, empty payload
RTLO4b4c3c/tombstone-zero-value-counter-tears-down-0	subscriber receives the zero-amount tombstone update and listeners are deregistered
RTO4b2a/reset-of-empty-root-emits-no-update-0	reset of an empty root emits nothing (second-channel liveness control)

Verification

• Full test/uts/objects/unit tier: 332 passing, 0 failing, 0 pending (325 baseline + 7).
• The RTLC14c/RTLM22c tests fail without the production change (noop is undefined pre-fix); the RTO20d4 test times out parked in the RTO20e wait pre-fix.
• Existing tombstone, RTO4b/RTO27, RTLC14/RTLM22 and RTO20d1/RTO20e neighbours all green (the tombstone suite passing proves the teardown exception holds).
• tsc --noEmit and eslint clean on the touched sources.

Summary by CodeRabbit

• Bug Fixes
  • Corrected zero-value counter and empty-map updates during deletion and state replacement.
  • Tombstone events now reach listeners reliably, include deletion details, and properly stop future notifications.
  • Resetting an already-empty root no longer sends unnecessary subscriber updates.
  • Publish operations with no applicable acknowledgements now complete without waiting for synchronization.
• Tests
  • Added coverage for zero-delta updates, tombstones, listener behavior, root resets, and acknowledgement handling.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 096118d4-a581-4407-bbbb-37e28344c527

📥 Commits

Reviewing files that changed from the base of the PR and between f36b64b and 9490a18.

📒 Files selected for processing (1)

• test/uts/objects/unit/live_map.test.ts

🚧 Files skipped from review as they are similar to previous changes (1)

• test/uts/objects/unit/live_map.test.ts

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Walkthrough

LiveObject updates now distinguish unchanged-state no-ops from typed zero-change tombstone updates. LiveCounter and LiveMap implement this behavior. RealtimeObject skips sync waiting when acknowledgements contain only null synthetic serials. Tests cover tombstones, subscriptions, resets, and acknowledgements.

Changes

LiveObject update and tombstone flow

Layer / File(s)	Summary
No-op and tombstone update contracts src/plugins/liveobjects/liveobject.ts, src/plugins/liveobjects/livecounter.ts, src/plugins/liveobjects/livemap.ts	Unchanged counter and map state now returns noop updates. Tombstoning converts noop clears into typed non-noop updates.
Tombstone, subscription, and reset validation test/uts/objects/unit/live_counter.test.ts, test/uts/objects/unit/live_map.test.ts, test/uts/objects/unit/live_object_subscribe.test.ts, test/uts/objects/unit/objects_pool.test.ts	Tests cover zero-delta tombstones, unchanged replacements, listener removal, and empty-root resets.

Realtime publish handling

Layer / File(s)	Summary
Empty synthetic publish handling src/plugins/liveobjects/realtimeobject.ts, test/uts/objects/unit/realtime_object.test.ts	publishAndApply returns without local application or sync waiting when all synthetic serials are null. The integration test verifies that the local value remains unchanged.

Estimated code review effort: 3 (Moderate) | ~25 minutes

Sequence Diagram(s)

sequenceDiagram
  participant Publisher
  participant RealtimeObject
  participant SyncState
  Publisher->>RealtimeObject: publishAndApply()
  RealtimeObject-->>Publisher: ACK with null synthetic serials
  RealtimeObject->>RealtimeObject: skip synthetic message application
  RealtimeObject->>SyncState: skip sync wait
  RealtimeObject-->>Publisher: resolve without SYNCED

Loading

Possibly related PRs

• ably/ably-js#2219: Covers overlapping LiveObject, LiveCounter, and LiveMap no-op and tombstone behavior.
• ably/ably-js#2257: Modifies related LiveObject update paths and tests.
• ably/ably-js#2263: Changes LiveObjectUpdateNoop handling in overlapping update flows.

Poem

I’m a rabbit guarding updates tight,
No-op hops vanish out of sight.
Zero tombstones still pass through,
Maps and counters know what to do.
Null serials rest; sync waits no more.

🚥 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 summarizes the two main changes: suppressing no-op LiveObject diffs and skipping the sync wait for empty synthetic messages.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
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.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch liveobjects/noop-diffs-and-empty-synthetic-list

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[Copilot]

Pull request overview

Aligns the LiveObjects implementation with the Objects spec’s no-op update semantics by suppressing “update” events when state/data is unchanged, while preserving the mandated tombstone (delete) teardown behavior and avoiding an unnecessary sync-wait in publishAndApply when there’s nothing to apply locally.

Changes:

• Collapse empty diffs to { noop: true } for LiveCounter (zero delta) and LiveMap (no changed keys), so re-sync/reset paths do not emit spurious updates.
• Ensure tombstoning still produces a deliverable (non-noop) update even when the diff would otherwise be a no-op, via a new _createNoChangeUpdate() hook.
• Short-circuit publishAndApply to skip the RTO20e sync wait when the synthetic message list is empty (e.g., all-null ACK serials).

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/plugins/liveobjects/realtimeobject.ts	Skips the RTO20e sync wait when publishAndApply has no synthetic messages to apply.
src/plugins/liveobjects/liveobject.ts	Adds tombstone no-op carve-out handling and introduces _createNoChangeUpdate() to synthesize a deliverable tombstone update.
src/plugins/liveobjects/livemap.ts	Returns noop for empty key diffs and provides map-specific _createNoChangeUpdate().
src/plugins/liveobjects/livecounter.ts	Returns noop for zero-delta diffs and provides counter-specific _createNoChangeUpdate().
test/uts/objects/unit/realtime_object.test.ts	Adds coverage for RTO20d4 (empty synthetic list skips sync wait).
test/uts/objects/unit/objects_pool.test.ts	Adds coverage for RTO4b2a (reset of already-empty root emits no update).
test/uts/objects/unit/live_object_subscribe.test.ts	Adds coverage ensuring tombstone teardown still happens even with a zero-delta tombstone update.
test/uts/objects/unit/live_map.test.ts	Adds coverage for RTLM22c no-op diffs and tombstone carve-out on all-tombstoned maps.
test/uts/objects/unit/live_counter.test.ts	Adds coverage for RTLC14c no-op diffs and tombstone carve-out on already-zero counters.

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