fix: Use WeakMap in DerivedStateProvider to separate user state caches (#12866)

Bug fix for PM-15914 where switching users would incorrectly share cached
derived states. The `DerivedStateProvider` now uses a `WeakMap` to maintain
separate caches for each user's state `Observable`.

- Modifies `DefaultDerivedStateProvider` to use `WeakMap` for caching
- Each user's state `Observable` gets its own definition cache
- Added test to verify correct behavior during user switching
- Allows proper garbage collection of unused state caches

This fixes issues where:
- Users would see other users' cached states after switching accounts
- Derived states weren't properly isolated between users
- Cache keys didn't distinguish between different user states

libs/common/src/platform/state/implementations/default-derived-state.provider.ts

@@ -8,7 +8,14 @@ import { DerivedStateProvider } from "../derived-state.provider";
 import { DefaultDerivedState } from "./default-derived-state";
 
 export class DefaultDerivedStateProvider implements DerivedStateProvider {
-  private cache: Record<string, DerivedState<unknown>> = {};
+  /**
+   * The cache uses a WeakMap to maintain separate derived states per user.
+   * Each user's state Observable acts as a unique key, without needing to
+   * pass around `userId`. Also, when a user's state Observable is cleaned up
+   * (like during an account swap) their cache is automatically garbage
+   * collected.
+   */
+  private cache = new WeakMap<Observable<unknown>, Record<string, DerivedState<unknown>>>();
 
   constructor() {}
 
@@ -17,8 +24,14 @@ export class DefaultDerivedStateProvider implements DerivedStateProvider {
     deriveDefinition: DeriveDefinition<TFrom, TTo, TDeps>,
     dependencies: TDeps,
   ): DerivedState<TTo> {
+    let stateCache = this.cache.get(parentState$);
+    if (!stateCache) {
+      stateCache = {};
+      this.cache.set(parentState$, stateCache);
+    }
+
     const cacheKey = deriveDefinition.buildCacheKey();
-    const existingDerivedState = this.cache[cacheKey];
+    const existingDerivedState = stateCache[cacheKey];
     if (existingDerivedState != null) {
       // I have to cast out of the unknown generic but this should be safe if rules
       // around domain token are made
@@ -26,7 +39,7 @@ export class DefaultDerivedStateProvider implements DerivedStateProvider {
     }
 
     const newDerivedState = this.buildDerivedState(parentState$, deriveDefinition, dependencies);
-    this.cache[cacheKey] = newDerivedState;
+    stateCache[cacheKey] = newDerivedState;
     return newDerivedState;
   }
 

libs/common/src/platform/state/implementations/default-derived-state.spec.ts

@@ -9,6 +9,7 @@ import { DeriveDefinition } from "../derive-definition";
 import { StateDefinition } from "../state-definition";
 
 import { DefaultDerivedState } from "./default-derived-state";
+import { DefaultDerivedStateProvider } from "./default-derived-state.provider";
 
 let callCount = 0;
 const cleanupDelayMs = 10;
@@ -182,4 +183,29 @@ describe("DefaultDerivedState", () => {
       expect(await firstValueFrom(observable)).toEqual(new Date(newDate));
     });
   });
+
+  describe("account switching", () => {
+    let provider: DefaultDerivedStateProvider;
+
+    beforeEach(() => {
+      provider = new DefaultDerivedStateProvider();
+    });
+
+    it("should provide a dedicated cache for each account", async () => {
+      const user1State$ = new Subject<string>();
+      const user1Derived = provider.get(user1State$, deriveDefinition, deps);
+      const user1Emissions = trackEmissions(user1Derived.state$);
+
+      const user2State$ = new Subject<string>();
+      const user2Derived = provider.get(user2State$, deriveDefinition, deps);
+      const user2Emissions = trackEmissions(user2Derived.state$);
+
+      user1State$.next("2015-12-30");
+      user2State$.next("2020-12-29");
+      await awaitAsync();
+
+      expect(user1Emissions).toEqual([new Date("2015-12-30")]);
+      expect(user2Emissions).toEqual([new Date("2020-12-29")]);
+    });
+  });
 });