Merge pull request #2288 from ably/liveobjects/noop-diffs-and-empty-synthetic-list

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

Author: sacOO7

src/plugins/liveobjects/livecounter.ts

@@ -235,6 +235,12 @@ export class LiveCounter extends LiveObject<LiveCounterData, LiveCounterUpdate>
 
     // update will contain the diff between previous value and new value from object state
     const update = this._updateFromDataDiff(previousDataRef, this._dataRef);
+    // RTLC14c - _updateFromDataDiff collapses a zero-delta diff (unchanged counter data) to a noop.
+    // pass it straight through without stamping the object message, mirroring the terminal noop
+    // return above (RTLC6e).
+    if (this._isNoopUpdate(update)) {
+      return update;
+    }
     update.objectMessage = objectMessage;
 
     return update;
@@ -253,11 +259,27 @@ export class LiveCounter extends LiveObject<LiveCounterData, LiveCounterUpdate>
     return { data: 0 };
   }
 
-  protected _updateFromDataDiff(prevDataRef: LiveCounterData, newDataRef: LiveCounterData): LiveCounterUpdate {
+  protected _updateFromDataDiff(
+    prevDataRef: LiveCounterData,
+    newDataRef: LiveCounterData,
+  ): LiveCounterUpdate | LiveObjectUpdateNoop {
     const counterDiff = newDataRef.data - prevDataRef.data;
+    // RTLC14c - as an exception to RTLC14b: if newData equals previousData (the computed delta is 0)
+    // the counter data did not change, so instead of returning an update return a LiveCounterUpdate
+    // object with noop set to true (RTLO4b4b), as in RTLC9h. This exception must not be applied when
+    // the diff is computed for a tombstone per RTLO4e5; LiveObject.tombstone re-synthesizes a
+    // non-noop update via _createNoChangeUpdate() so the RTLO4b4c3c listener teardown still fires.
+    if (counterDiff === 0) {
+      return { noop: true };
+    }
     return { update: { amount: counterDiff }, _type: 'LiveCounterUpdate' };
   }
 
+  protected _createNoChangeUpdate(): LiveCounterUpdate {
+    // RTLO4e5 tombstone carve-out (RTLC14c) - a zero-delta no-change update for an already-zero counter
+    return { update: { amount: 0 }, _type: 'LiveCounterUpdate' };
+  }
+
   protected _mergeInitialDataFromCreateOperation(
     objectOperation: ObjectOperation<ObjectData>,
     msg: ObjectMessage,

src/plugins/liveobjects/livemap.ts

@@ -462,6 +462,12 @@ export class LiveMap<T extends Record<string, Value> = Record<string, Value>>
 
     // update will contain the diff between previous value and new value from object state
     const update = this._updateFromDataDiff(previousDataRef, this._dataRef);
+    // RTLM22c - _updateFromDataDiff collapses an empty key-diff (no map key changed) to a noop.
+    // pass it straight through without stamping the object message, mirroring the terminal noop
+    // return above (RTLM6e).
+    if (this._isNoopUpdate(update)) {
+      return update;
+    }
     update.objectMessage = objectMessage;
 
     return update;
@@ -488,7 +494,7 @@ export class LiveMap<T extends Record<string, Value> = Record<string, Value>>
    *
    * @internal
    */
-  clearData(): LiveMapUpdate<T> {
+  clearData(): LiveMapUpdate<T> | LiveObjectUpdateNoop {
     // Remove all parent references for objects this map was referencing
     for (const [key, entry] of this._dataRef.data.entries()) {
       if (entry.data && 'objectId' in entry.data) {
@@ -600,7 +606,10 @@ export class LiveMap<T extends Record<string, Value> = Record<string, Value>>
     return { data: new Map<string, LiveMapEntry>() };
   }
 
-  protected _updateFromDataDiff(prevDataRef: LiveMapData, newDataRef: LiveMapData): LiveMapUpdate<T> {
+  protected _updateFromDataDiff(
+    prevDataRef: LiveMapData,
+    newDataRef: LiveMapData,
+  ): LiveMapUpdate<T> | LiveObjectUpdateNoop {
     const update: LiveMapUpdate<T> = { update: {}, _type: 'LiveMapUpdate' };
 
     for (const [key, currentEntry] of prevDataRef.data.entries()) {
@@ -653,9 +662,24 @@ export class LiveMap<T extends Record<string, Value> = Record<string, Value>>
       }
     }
 
+    // RTLM22c - as an exception to RTLM22b: if the computed update contains no changed keys (it is
+    // empty) no map key actually changed, so instead of returning an update return a LiveMapUpdate
+    // object with noop set to true (RTLO4b4b), as in RTLM16b. This exception must not be applied when
+    // the diff is computed for a tombstone per RTLO4e5; LiveObject.tombstone re-synthesizes a
+    // non-noop update via _createNoChangeUpdate() so the RTLO4b4c3c listener teardown still fires.
+    if (Object.keys(update.update).length === 0) {
+      return { noop: true };
+    }
+
     return update;
   }
 
+  protected _createNoChangeUpdate(): LiveMapUpdate<T> {
+    // RTLO4e5 tombstone carve-out (RTLM22c) - an empty no-change update for a map with no
+    // non-tombstoned entries
+    return { update: {}, _type: 'LiveMapUpdate' };
+  }
+
   protected _mergeInitialDataFromCreateOperation(
     objectOperation: ObjectOperation<ObjectData>,
     msg: ObjectMessage,

src/plugins/liveobjects/liveobject.ts

@@ -134,11 +134,18 @@ export abstract class LiveObject<
       'LiveObject.tombstone()',
       `objectId=${this.getObjectId()}`,
     ); // RTLO4e3
-    const update = this.clearData(); // RTLO4e4
-    update.objectMessage = objectMessage;
-    update.tombstone = true;
-
-    return update;
+    // RTLO4e5 - compute the diff between the pre-clear data and the zero value. Per the RTLC14c /
+    // RTLM22c tombstone carve-out, that noop exception "must not be applied when the diff is
+    // computed for a tombstone": tombstoning an already-empty object yields a noop diff, but the
+    // resulting tombstone update (RTLO4b4e) must still be delivered so it drives the RTLO4b4c3c
+    // listener teardown. So when the diff collapses to a noop, synthesize the typed no-change
+    // update instead, leaving a real (non-noop) update to stamp.
+    const diff = this.clearData(); // RTLO4e4
+    const update: TUpdate = this._isNoopUpdate(diff) ? this._createNoChangeUpdate() : diff;
+    update.objectMessage = objectMessage; // RTLO4e7
+    update.tombstone = true; // RTLO4e6
+
+    return update; // RTLO4e8
   }
 
   /**
@@ -158,7 +165,7 @@ export abstract class LiveObject<
   /**
    * @internal
    */
-  clearData(): TUpdate {
+  clearData(): TUpdate | LiveObjectUpdateNoop {
     const previousDataRef = this._dataRef;
     this._dataRef = this._getZeroValueData();
     return this._updateFromDataDiff(previousDataRef, this._dataRef);
@@ -350,7 +357,7 @@ export abstract class LiveObject<
     }
   }
 
-  private _isNoopUpdate(update: TUpdate | LiveObjectUpdateNoop): update is LiveObjectUpdateNoop {
+  protected _isNoopUpdate(update: TUpdate | LiveObjectUpdateNoop): update is LiveObjectUpdateNoop {
     return (update as LiveObjectUpdateNoop).noop === true;
   }
 
@@ -383,8 +390,16 @@ export abstract class LiveObject<
   protected abstract _getZeroValueData(): TData;
   /**
    * Calculate the update object based on the current LiveObject data and incoming new data.
+   *
+   * Returns a noop update when the data is unchanged (RTLC14c / RTLM22c).
+   */
+  protected abstract _updateFromDataDiff(prevDataRef: TData, newDataRef: TData): TUpdate | LiveObjectUpdateNoop;
+  /**
+   * Returns a typed update that represents "no change" (e.g. a counter delta of 0, or an empty
+   * map key-diff), used by {@link LiveObject.tombstone} to synthesize a deliverable tombstone
+   * update when the tombstone diff itself collapsed to a noop per the RTLC14c / RTLM22c carve-out.
    */
-  protected abstract _updateFromDataDiff(prevDataRef: TData, newDataRef: TData): TUpdate;
+  protected abstract _createNoChangeUpdate(): TUpdate;
   /**
    * Merges the initial data from the create operation into the LiveObject.
    *

src/plugins/liveobjects/realtimeobject.ts

@@ -396,6 +396,12 @@ export class RealtimeObject {
       );
     }
 
+    // RTO20d4 - if the synthetic messages list is empty (e.g. every serial was null and skipped per
+    // RTO20d1) there is nothing to apply locally, so complete without performing the RTO20e sync wait.
+    if (syntheticMessages.length === 0) {
+      return;
+    }
+
     // RTO20e - Wait for sync to complete if not synced
     if (this._state !== ObjectsState.synced) {
       this._client.Logger.logAction(

test/uts/objects/unit/live_counter.test.ts

@@ -511,6 +511,43 @@ describe('uts/objects/unit/live_counter', function () {
     expect(update.objectMessage).to.equal(msg);
   });
 
+  // UTS: objects/unit/RTLO5/tombstone-zero-value-counter-emits-update-0
+  // Complements object-delete-tombstones-0 (which tombstones a populated counter). Here the
+  // counter data is already 0, so the tombstone diff (previousData 0, newData 0) is a zero delta.
+  // Per the RTLC14c tombstone carve-out (RTLO4e5) this update must NOT be marked as a no-op — it
+  // must still be delivered so the RTLO4b4c3c listener teardown runs.
+  it('RTLO5 - OBJECT_DELETE on an already-zero counter emits a non-noop tombstone update', async function () {
+    const { channel, client } = await setupSyncedChannel('test-RTLO5-zero');
+
+    const counter = createZeroCounter(channel, 'counter:abc@1000');
+    const capture = captureNotifyUpdated(counter);
+    (counter as any)._dataRef.data = 0;
+    (counter as any)._siteTimeserials = { site1: '00' };
+
+    const msg = makeObjectMessage(client, {
+      serial: '01',
+      siteCode: 'site1',
+      serialTimestamp: 1700000000000,
+      operation: {
+        action: OBJ_OP.OBJECT_DELETE,
+        objectId: 'counter:abc@1000',
+        objectDelete: {},
+      },
+    });
+
+    const result = counter.applyOperation(msg.operation!, msg, ObjectsOperationSource.channel);
+
+    expect(counter.isTombstoned()).to.equal(true);
+    expect((counter as any)._dataRef.data).to.equal(0);
+    expect(result).to.equal(true);
+    const update = capture.getUpdate();
+    // RTLC14c carve-out: the zero-delta tombstone update is NOT a no-op
+    expect((update as any).noop).to.not.equal(true);
+    expect(update.tombstone).to.equal(true);
+    expect(update.update.amount).to.equal(0);
+    expect(update.objectMessage).to.equal(msg);
+  });
+
   // =========================================================================
   // RTLC7e - Operations on tombstoned counter are rejected
   // =========================================================================
@@ -798,6 +835,29 @@ describe('uts/objects/unit/live_counter', function () {
     expect((update as any).objectMessage).to.equal(stateMsg);
   });
 
+  // UTS: objects/unit/RTLC14c/zero-delta-diff-is-noop-0
+  it('RTLC14c - Zero-delta diff is a no-op', async function () {
+    const { channel, client } = await setupSyncedChannel('test-RTLC14c');
+
+    const counter = createZeroCounter(channel, 'counter:abc@1000');
+    (counter as any)._dataRef.data = 100;
+
+    const stateMsg = makeObjectMessage(client, {
+      object: {
+        objectId: 'counter:abc@1000',
+        siteTimeserials: { site1: '01' },
+        tombstone: false,
+        counter: { count: 100 },
+      },
+    });
+
+    const update = counter.overrideWithObjectState(stateMsg);
+
+    // RTLC14c - the computed delta is 0, so the diff collapses to a no-op update
+    expect((update as any).noop).to.equal(true);
+    expect((counter as any)._dataRef.data).to.equal(100);
+  });
+
   // =========================================================================
   // RTLC8, RTLC16 - COUNTER_CREATE then COUNTER_INC accumulates
   // =========================================================================

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

@@ -753,6 +753,56 @@ describe('uts/objects/unit/live_map', function () {
     expect(update.objectMessage).to.equal(msg);
   });
 
+  // UTS: objects/unit/RTLO5/tombstone-empty-map-emits-update-0
+  // Complements object-delete-tombstones-map-0 (which tombstones a map with live entries). Here
+  // every entry is already tombstoned, so the map has no non-tombstoned entries and the tombstone
+  // diff (per RTLM22b, which considers only non-tombstoned entries) contains no changed keys. Per
+  // the RTLM22c tombstone carve-out (RTLO4e5) this empty update must NOT be marked as a no-op — it
+  // must still be delivered so the RTLO4b4c3c listener teardown runs. Uses a non-root map: an
+  // OBJECT_DELETE targeting root is rejected per RTLO4e10.
+  it('RTLO5 - OBJECT_DELETE on an all-tombstoned map emits a non-noop tombstone update', async function () {
+    const { channel, client } = await setupSyncedChannel('test-RTLO5-empty');
+
+    const map = createZeroMap(channel, 'map:test@1000');
+    const capture = captureNotifyUpdated(map);
+    getDataMap(map).set('name', {
+      data: { string: 'Alice' },
+      timeserial: '01',
+      tombstone: true,
+      tombstonedAt: 1600000000000,
+    });
+    getDataMap(map).set('age', {
+      data: { number: 30 },
+      timeserial: '01',
+      tombstone: true,
+      tombstonedAt: 1600000000000,
+    });
+    (map as any)._siteTimeserials = { site1: '00' };
+
+    const msg = makeObjectMessage(client, {
+      serial: '01',
+      siteCode: 'site1',
+      serialTimestamp: 1700000000000,
+      operation: {
+        action: OBJ_OP.OBJECT_DELETE,
+        objectId: 'map:test@1000',
+        objectDelete: {},
+      },
+    });
+
+    const result = map.applyOperation(msg.operation!, msg, ObjectsOperationSource.channel);
+
+    expect(map.isTombstoned()).to.equal(true);
+    expect(getDataMap(map).size).to.equal(0); // data cleared
+    expect(result).to.equal(true);
+    const update = capture.getUpdate();
+    // RTLM22c carve-out: the empty tombstone update is NOT a no-op
+    expect((update as any).noop).to.not.equal(true);
+    expect(update.tombstone).to.equal(true);
+    expect(update.update).to.deep.equal({});
+    expect(update.objectMessage).to.equal(msg);
+  });
+
   // =====================================================================
   // RTLO4e10 - OBJECT_DELETE targeting root is rejected
   // =====================================================================
@@ -1036,6 +1086,41 @@ describe('uts/objects/unit/live_map', function () {
     expect(diff).to.not.have.property('now_dead');
   });
 
+  // UTS: objects/unit/RTLM22c/empty-diff-is-noop-0
+  it('RTLM22c - empty diff is a no-op', async function () {
+    const { channel, client } = await setupSyncedChannel('test-RTLM22c');
+
+    const map = createZeroMap(channel, 'root');
+    getDataMap(map).set('name', {
+      data: { string: 'alice' },
+      timeserial: '01',
+      tombstone: false,
+      tombstonedAt: undefined,
+    });
+
+    // The non-tombstoned entries before and after are identical under the RTLM22b
+    // comparison rules (same key, same data; only timeserial differs, which is not compared).
+    const stateMsg = makeObjectMessage(client, {
+      object: {
+        objectId: 'root',
+        siteTimeserials: { site1: '02' },
+        tombstone: false,
+        map: {
+          semantics: MAP_SEMANTICS_LWW,
+          entries: {
+            name: { data: { string: 'alice' }, timeserial: '02' },
+          },
+        },
+      },
+    });
+
+    const update = map.overrideWithObjectState(stateMsg);
+
+    // RTLM22c - the computed update contains no changed keys, so the diff collapses to a no-op
+    expect((update as any).noop).to.equal(true);
+    expect(getDataMap(map).get('name')!.data).to.deep.equal({ string: 'alice' });
+  });
+
   // =====================================================================
   // RTLM15d4 - Unsupported action is discarded
   // =====================================================================