From fa43420ab9c1204ba72557bc5dbde7f9bc3770bf Mon Sep 17 00:00:00 2001
From: Fredrik Strand Oseberg <fredrik.no@gmail.com>
Date: Fri, 14 Feb 2025 09:30:14 +0100
Subject: [PATCH] fix: mutating delta events (#9303)

Fix a bug where delta events would be mutated due to Object.assign.
---
 .../delta/delta-cache.test.ts                 | 63 +++++++++++++++++++
 .../delta/delta-cache.ts                      | 15 +++--
 2 files changed, 72 insertions(+), 6 deletions(-)

diff --git a/src/lib/features/client-feature-toggles/delta/delta-cache.test.ts b/src/lib/features/client-feature-toggles/delta/delta-cache.test.ts
index 07a84c96da..e39ba087b7 100644
--- a/src/lib/features/client-feature-toggles/delta/delta-cache.test.ts
+++ b/src/lib/features/client-feature-toggles/delta/delta-cache.test.ts
@@ -183,4 +183,67 @@ describe('RevisionCache', () => {
             ]),
         );
     });
+
+    it('should not mutate previous feature-updated events when new events with the same feature name are added', () => {
+        const baseEvent: DeltaHydrationEvent = {
+            eventId: 1,
+            features: [
+                {
+                    name: 'base-flag',
+                    type: 'release',
+                    enabled: true,
+                    project: 'streaming-deltas',
+                    stale: false,
+                    strategies: [],
+                    variants: [],
+                    description: null,
+                    impressionData: false,
+                },
+            ],
+            type: 'hydration',
+            segments: [],
+        };
+
+        const deltaCache = new DeltaCache(baseEvent, 10);
+
+        const initialFeatureEvent: DeltaEvent = {
+            eventId: 129,
+            type: DELTA_EVENT_TYPES.FEATURE_UPDATED,
+            feature: {
+                impressionData: false,
+                enabled: false,
+                name: 'streaming-test',
+                description: null,
+                project: 'streaming-deltas',
+                stale: false,
+                type: 'release',
+                variants: [],
+                strategies: [],
+            },
+        };
+        // This tests is to verify that the initialFeatureEvent is not mutated when a new event with the same feature name is added
+        // the following dirty way to clone this object is to avoid mutation on the comparison object. Because the object is passed by reference
+        // we would be comparing the same object with itself which would cause the expect check to always pass because the comparison would
+        // also change to match the object being compared.
+        deltaCache.addEvents([JSON.parse(JSON.stringify(initialFeatureEvent))]);
+
+        const updatedFeatureEvent: DeltaEvent = {
+            eventId: 130,
+            type: DELTA_EVENT_TYPES.FEATURE_UPDATED,
+            feature: {
+                impressionData: false,
+                enabled: true,
+                name: 'streaming-test',
+                description: null,
+                project: 'streaming-deltas',
+                stale: false,
+                type: 'release',
+                variants: [],
+                strategies: [{ name: 'new-strategy', parameters: {} }],
+            },
+        };
+        deltaCache.addEvents([updatedFeatureEvent]);
+        // @ts-ignore
+        expect(deltaCache.events[1]).toStrictEqual(initialFeatureEvent);
+    });
 });
diff --git a/src/lib/features/client-feature-toggles/delta/delta-cache.ts b/src/lib/features/client-feature-toggles/delta/delta-cache.ts
index abaa2d300c..98c9f955d4 100644
--- a/src/lib/features/client-feature-toggles/delta/delta-cache.ts
+++ b/src/lib/features/client-feature-toggles/delta/delta-cache.ts
@@ -55,11 +55,13 @@ export class DeltaCache {
         for (const appliedEvent of events) {
             switch (appliedEvent.type) {
                 case DELTA_EVENT_TYPES.FEATURE_UPDATED: {
-                    const featureToUpdate = this.hydrationEvent.features.find(
+                    const featureIndex = this.hydrationEvent.features.findIndex(
                         (feature) => feature.name === appliedEvent.feature.name,
                     );
-                    if (featureToUpdate) {
-                        Object.assign(featureToUpdate, appliedEvent.feature);
+
+                    if (featureIndex > -1) {
+                        this.hydrationEvent.features[featureIndex] =
+                            appliedEvent.feature;
                     } else {
                         this.hydrationEvent.features.push(appliedEvent.feature);
                     }
@@ -74,11 +76,12 @@ export class DeltaCache {
                     break;
                 }
                 case DELTA_EVENT_TYPES.SEGMENT_UPDATED: {
-                    const segmentToUpdate = this.hydrationEvent.segments.find(
+                    const segmentIndex = this.hydrationEvent.segments.findIndex(
                         (segment) => segment.id === appliedEvent.segment.id,
                     );
-                    if (segmentToUpdate) {
-                        Object.assign(segmentToUpdate, appliedEvent.segment);
+                    if (segmentIndex > -1) {
+                        this.hydrationEvent.segments[segmentIndex] =
+                            appliedEvent.segment;
                     } else {
                         this.hydrationEvent.segments.push(appliedEvent.segment);
                     }