serialize event count in store function call (#1065)

if multiple events are processed concurrently, all event process
request might read the initial store size and write to the store,
potentially exceeding the store size limit. Serializing the
store size read should fix this. Once the size is loaded in memory,
further event process read should just read the in memory value

Author: raju-opti

lib/event_processor/batch_event_processor.ts

@@ -74,6 +74,7 @@ export class BatchEventProcessor extends BaseService implements EventProcessor {
   private batchSize: number;
   private eventStore?: Store<EventWithId>;
   private eventCountInStore: Maybe<number> = undefined;
+  private eventCountWaitPromise: Promise<unknown> = Promise.resolve();
   private maxEventsInStore: number = MAX_EVENTS_IN_STORE;
   private dispatchRepeater: Repeater;
   private failedEventRepeater?: Repeater;
@@ -264,15 +265,22 @@ export class BatchEventProcessor extends BaseService implements EventProcessor {
     }
   }
 
-  private async findEventCountInStore(): Promise<void> {
+  private async readEventCountInStore(store: Store<EventWithId>): Promise<void> {
+    try {
+      const keys = await store.getKeys();
+      this.eventCountInStore = keys.length;
+    } catch (e) {
+      this.logger?.error(e);
+    }
+  }
+
+  private async findEventCountInStore(): Promise<unknown> {
     if (this.eventStore && this.eventCountInStore === undefined) {
-      try {
-        const keys = await this.eventStore.getKeys();
-        this.eventCountInStore = keys.length;
-      } catch (e) {
-        this.logger?.error(e);
-      }
+      const store = this.eventStore;
+      this.eventCountWaitPromise = this.eventCountWaitPromise.then(() => this.readEventCountInStore(store));
+      return this.eventCountWaitPromise;
     }
+    return Promise.resolve();
   }
 
   private async storeEvent(eventWithId: EventWithId): Promise<void> {