fix(server): race condition for sync (#14770)

#### PR Dependency Tree


* **PR #14770** 👈

This tree was auto-generated by
[Charcoal](https://github.com/danerwilliams/charcoal)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

## Release Notes

* **New Features**
* Implemented batch processing for calendar synchronization to improve
performance and resource utilization.
* Added distributed locking to prevent concurrent operations in
multi-instance environments.

* **Bug Fixes**
* Improved reliability by preventing duplicate synchronization attempts.

* **Tests**
  * Enhanced test coverage for batch processing and locking mechanisms.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/backend/server/src/plugins/calendar/cron.ts

@@ -1,18 +1,27 @@
 import { Injectable } from '@nestjs/common';
 import { Cron, CronExpression } from '@nestjs/schedule';
+import { chunk } from 'lodash-es';
 
+import { Mutex } from '../../base';
 import { Models } from '../../models';
 import { CalendarService } from './service';
 
+const CALENDAR_POLL_LOCK_KEY = 'calendar:poll-accounts';
+const CALENDAR_POLL_BATCH_SIZE = 10;
+
 @Injectable()
 export class CalendarCronJobs {
   constructor(
     private readonly models: Models,
-    private readonly calendar: CalendarService
+    private readonly calendar: CalendarService,
+    private readonly mutex: Mutex
   ) {}
 
   @Cron(CronExpression.EVERY_MINUTE)
   async pollAccounts() {
+    await using lock = await this.mutex.acquire(CALENDAR_POLL_LOCK_KEY);
+    if (!lock) return;
+
     const subscriptions =
       await this.models.calendarSubscription.listAllWithAccountForSync();
 
@@ -46,16 +55,18 @@ export class CalendarCronJobs {
     }
 
     const now = Date.now();
-    await Promise.allSettled(
-      Array.from(accountDueAt.entries()).map(([accountId, info]) => {
-        if (
+    const dueAccountIds = Array.from(accountDueAt.entries())
+      .filter(
+        ([, info]) =>
           !info.lastSyncAt ||
           now - info.lastSyncAt.getTime() >= info.refreshInterval * 60 * 1000
-        ) {
-          return this.calendar.syncAccount(accountId);
-        }
-        return Promise.resolve();
-      })
-    );
+      )
+      .map(([accountId]) => accountId);
+
+    for (const accountIds of chunk(dueAccountIds, CALENDAR_POLL_BATCH_SIZE)) {
+      await Promise.allSettled(
+        accountIds.map(accountId => this.calendar.syncAccount(accountId))
+      );
+    }
   }
 }