Fix a cache race condition in local backed session storage (#16177)

* fix: cache race condition

* fix: typo and order

* feat: tweak log message

apps/browser/src/platform/services/local-backed-session-storage.service.ts

@@ -73,8 +73,29 @@ export class LocalBackedSessionStorageService
 
     const value = await this.getLocalSessionValue(await this.sessionKey.get(), key);
 
+    if (this.cache[key] === undefined && value !== undefined) {
+      // Cache is still empty and we just got a value from local/session storage, cache it.
       this.cache[key] = value;
       return value as T;
+    } else if (this.cache[key] === undefined && value === undefined) {
+      // Cache is still empty and we got nothing from local/session storage, no need to modify cache.
+      return value as T;
+    } else if (this.cache[key] !== undefined && value !== undefined) {
+      // Conflict, somebody wrote to the cache while we were reading from storage
+      // but we also got a value from storage. We assume the cache is more up to date
+      // and use that value.
+      this.logService.warning(
+        `Conflict while reading from local session storage, both cache and storage have values. Key: ${key}. Using cached value.`,
+      );
+      return this.cache[key] as T;
+    } else if (this.cache[key] !== undefined && value === undefined) {
+      // Cache was filled after the local/session storage read completed. We got null
+      // from the storage read, but we have a value from the cache, use that.
+      this.logService.warning(
+        `Conflict while reading from local session storage, cache has value but storage does not. Key: ${key}. Using cached value.`,
+      );
+      return this.cache[key] as T;
+    }
   }
 
   async has(key: string): Promise<boolean> {