fix(rate-limit): add Retry-After header to 429 responses and fix TOCTOU race in Redis rate limiter

lib/rate-limit.ts

@@ -69,54 +69,27 @@ export class RateLimiter {
 
     if (url && token) {
       try {
-        const getRes = await fetch(`${url}/pipeline`, {
+        const incrRes = await fetch(`${url}/pipeline`, {
           method: 'POST',
           headers: {
             Authorization: `Bearer ${token}`,
             'Content-Type': 'application/json',
           },
           body: JSON.stringify([
-            ['GET', `ratelimit_class:${ip}`],
-            ['TTL', `ratelimit_class:${ip}`],
+            ['INCR', `ratelimit:${ip}`],
+            ['EXPIRE', `ratelimit:${ip}`, Math.floor(this.windowMs / 1000), 'NX'],
           ]),
         });
 
-        if (getRes.ok) {
-          const getData = await getRes.json();
-          const currentCount = parseInt(getData[0].result ?? '0', 10);
-          const ttl = getData[1].result as number;
-
-          if (currentCount >= this.limit) {
-            return {
-              success: false,
-              limit: this.limit,
-              remaining: 0,
-              reset: ttl > 0 ? now + ttl * 1000 : now + this.windowMs,
-            };
-          }
-
-          const incrRes = await fetch(`${url}/pipeline`, {
-            method: 'POST',
-            headers: {
-              Authorization: `Bearer ${token}`,
-              'Content-Type': 'application/json',
-            },
-            body: JSON.stringify([
-              ['INCR', `ratelimit_class:${ip}`],
-              ['EXPIRE', `ratelimit_class:${ip}`, Math.floor(this.windowMs / 1000), 'NX'],
-            ]),
-          });
-
-          if (incrRes.ok) {
-            const incrData = await incrRes.json();
-            const count = incrData[0].result as number;
-            return {
-              success: count <= this.limit,
-              limit: this.limit,
-              remaining: Math.max(0, this.limit - count),
-              reset: now + this.windowMs,
-            };
-          }
+        if (incrRes.ok) {
+          const incrData = await incrRes.json();
+          const count = incrData[0].result as number;
+          return {
+            success: count <= this.limit,
+            limit: this.limit,
+            remaining: Math.max(0, this.limit - count),
+            reset: now + this.windowMs,
+          };
         }
       } catch (error) {
         console.error('RateLimiter KV error, falling back to memory:', error);
@@ -193,6 +166,29 @@ export class RateLimiter {
    * console.log(`You have ${left} requests left.`);
    */
   async remaining(ip: string): Promise<number> {
+    const url = process.env.KV_REST_API_URL;
+    const token = process.env.KV_REST_API_TOKEN;
+    if (url && token) {
+      try {
+        const res = await fetch(`${url}/pipeline`, {
+          method: 'POST',
+          headers: {
+            Authorization: `Bearer ${token}`,
+            'Content-Type': 'application/json',
+          },
+          body: JSON.stringify([
+            ['GET', `ratelimit:${ip}`],
+          ]),
+        });
+        if (res.ok) {
+          const data = await res.json();
+          const count = parseInt(data[0].result ?? '0', 10);
+          return Math.max(0, this.limit - count);
+        }
+      } catch {
+        // fall through to local cache
+      }
+    }
     const count = ((await this.cache.get(`ratelimit:${ip}`)) as unknown as number) ?? 0;
     return Math.max(0, this.limit - count);
   }
@@ -303,7 +299,9 @@ export async function rateLimit(
 }
 
 export function getRateLimitHeaders(result: RateLimitResult) {
+  const retryAfter = Math.ceil(Math.max(0, result.reset - Date.now()) / 1000);
   return {
+    'Retry-After': retryAfter.toString(),
     'X-RateLimit-Limit': result.limit.toString(),
     'X-RateLimit-Remaining': result.remaining.toString(),
     'X-RateLimit-Reset': result.reset.toString(),

middleware.ts

@@ -1,6 +1,6 @@
 import { NextResponse } from 'next/server';
 import type { NextRequest } from 'next/server';
-import { rateLimit } from './lib/rate-limit';
+import { rateLimit, getRateLimitHeaders } from './lib/rate-limit';
 import { getClientIp } from './utils/getClientIp';
 
 /**
@@ -32,18 +32,17 @@ export async function middleware(request: NextRequest) {
         status: 429,
         headers: {
           'Content-Type': 'application/json',
-          'X-RateLimit-Limit': result.limit.toString(),
-          'X-RateLimit-Remaining': result.remaining.toString(),
-          'X-RateLimit-Reset': result.reset.toString(),
+          ...getRateLimitHeaders(result),
         },
       }
     );
   }
 
   const response = NextResponse.next();
-  response.headers.set('X-RateLimit-Limit', result.limit.toString());
-  response.headers.set('X-RateLimit-Remaining', result.remaining.toString());
-  response.headers.set('X-RateLimit-Reset', result.reset.toString());
+  const headers = getRateLimitHeaders(result);
+  for (const [key, value] of Object.entries(headers)) {
+    response.headers.set(key, value);
+  }
 
   return response;
 }