fix: prevent duplicate notification emails via atomic Redis SET NX (#346)

The warning and limit-reached notification emails were being sent
multiple times because of a race condition: concurrent workers could
both read the Redis cooldown key as empty (GET), both send emails,
then both set the key (SETEX). Replaced the non-atomic GET + SETEX
pattern with a single atomic SET ... NX EX that claims the cooldown
slot before any emails are sent. Also increased cooldown from 6 hours
to 24 hours so each notification is sent at most once per day.

https://claude.ai/code/session_01VBYXi5e64Vtq1cXWsfTYTw

Co-authored-by: Claude <noreply@anthropic.com>

Author: KMKoushik

apps/web/src/server/service/team-service.ts

@@ -356,7 +356,7 @@ export class TeamService {
   }
 
   /**
-   * Notify all team users that email limit has been reached, at most once per 6 hours.
+   * Notify all team users that email limit has been reached, at most once per day.
    */
   static async maybeNotifyEmailLimitReached(
     teamId: number,
@@ -390,13 +390,15 @@ export class TeamService {
 
     const redis = getRedis();
     const cacheKey = `limit:notify:${teamId}:${reason}`;
-    const alreadySent = await redis.get(cacheKey);
-    if (alreadySent) {
+    // Atomic SET NX to prevent race conditions: only one concurrent caller
+    // can acquire the cooldown key. TTL = 24 hours (one notification per day).
+    const acquired = await redis.set(cacheKey, "1", "EX", 24 * 60 * 60, "NX");
+    if (acquired !== "OK") {
       logger.info(
         { teamId, cacheKey },
         "[TeamService]: Skipping notify — cooldown active",
       );
-      return; // within cooldown window
+      return; // another request already claimed this window
     }
 
     const team = await TeamService.getTeamCached(teamId);
@@ -447,17 +449,11 @@ export class TeamService {
       throw err;
     }
 
-    // Set cooldown for 6 hours
-    await redis.setex(cacheKey, 6 * 60 * 60, "1");
-    logger.info(
-      { teamId, cacheKey },
-      "[TeamService]: Set limit reached notification cooldown",
-    );
   }
 
   /**
    * Notify all team users that they're nearing their email limit.
-   * Rate limited via Redis to avoid spamming; sends at most once per 6 hours per reason.
+   * Rate limited via Redis to avoid spamming; sends at most once per day per reason.
    */
   static async sendWarningEmail(
     teamId: number,
@@ -492,13 +488,15 @@ export class TeamService {
 
     const redis = getRedis();
     const cacheKey = `limit:warning:${teamId}:${reason}`;
-    const alreadySent = await redis.get(cacheKey);
-    if (alreadySent) {
+    // Atomic SET NX to prevent race conditions: only one concurrent caller
+    // can acquire the cooldown key. TTL = 24 hours (one notification per day).
+    const acquired = await redis.set(cacheKey, "1", "EX", 24 * 60 * 60, "NX");
+    if (acquired !== "OK") {
       logger.info(
         { teamId, cacheKey },
         "[TeamService]: Skipping warning — cooldown active",
       );
-      return; // within cooldown window
+      return; // another request already claimed this window
     }
 
     const team = await TeamService.getTeamCached(teamId);
@@ -558,12 +556,6 @@ export class TeamService {
       throw err;
     }
 
-    // Set cooldown for 6 hours
-    await redis.setex(cacheKey, 6 * 60 * 60, "1");
-    logger.info(
-      { teamId, cacheKey },
-      "[TeamService]: Set warning notification cooldown",
-    );
   }
 }