fix: prevent race condition in KeyValueStore.getAutoSavedValue() (#2193)

This was painful to discover the hard way :( Code to test this: ```js const store = await Actor.openKeyValueStore("test", { forceCloud: true }); const key = "something"; const res = await Promise.all([ store.getAutoSavedValue(key, { foo: [] }), new Promise((r) => setTimeout(r, Math.random() * 1000)).then(() => store.getAutoSavedValue(key, { foo: [] }) ), ]); res[0].foo.push("test"); console.log(res); ``` Run this a few times and at some point you will get two different values in the console.log. (Note that it's using `forceCloud` to make `this.GetValue()` run for a little longer.)

Author: mvolfik

packages/core/src/storages/key_value_store.ts

@@ -227,6 +227,15 @@ export class KeyValueStore {
  }
 
  const value = await this.getValue<T>(key, defaultValue);
+
+ // The await above could have run in parallel with another call to this function. If the other call finished more quickly,
+ // the value will in cache at this point, and returning the new fetched value would introduce two different instances of
+ // the auto-saved object, and only the latter one would be persisted.
+ // Therefore we re-check the cache here, and if such race condition happened, we drop the fetched value and return the cached one.
+ if (this.cache.has(key)) {
+ return this.cache.get(key) as T;
+ }
+
  this.cache.set(key, value!);
  this.ensurePersistStateEvent();