fix: make rate limit increment atomic

backend/internal/middleware/redis/redis.go

@@ -70,6 +70,14 @@ else
 end
 `)
 
+var incrementWithExpireScript = redis.NewScript(`
+local count = redis.call("INCR", KEYS[1])
+if count == 1 then
+  redis.call("PEXPIRE", KEYS[1], ARGV[1])
+end
+return count
+`)
+
 func (c *Client) Unlock(ctx context.Context, key string, token string) error {
 	if c == nil || c.rdb == nil {
 		return nil
@@ -82,15 +90,10 @@ func (c *Client) IncrementWithExpire(ctx context.Context, key string, expire tim
 	if c == nil || c.rdb == nil {
 		return 0, nil
 	}
-	count, err := c.rdb.Incr(ctx, key).Result()
-	if err != nil {
-		return 0, err
-	}
-	if count == 1 {
-		err = c.rdb.Expire(ctx, key, expire).Err()
-		if err != nil {
-			return 0, err
-		}
-	}
-	return count, nil
-}
+	return incrementWithExpireScript.Run(
+		ctx,
+		c.rdb,
+		[]string{key},
+		expire.Milliseconds(),
+	).Int64()
+}

backend/internal/middleware/redis/redis_test.go

@@ -0,0 +1,56 @@
+package redis
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	miniredis "github.com/alicebob/miniredis/v2"
+	goredis "github.com/redis/go-redis/v9"
+)
+
+func TestIncrementWithExpireSetsTTLWithoutExtendingWindow(t *testing.T) {
+	mr, err := miniredis.Run()
+	if err != nil {
+		t.Fatalf("start miniredis: %v", err)
+	}
+	defer mr.Close()
+
+	client := &Client{
+		rdb: goredis.NewClient(&goredis.Options{Addr: mr.Addr()}),
+	}
+	defer client.Close()
+
+	ctx := context.Background()
+	key := "feedsystem:ratelimit:test"
+	expire := 30 * time.Second
+
+	count, err := client.IncrementWithExpire(ctx, key, expire)
+	if err != nil {
+		t.Fatalf("first increment: %v", err)
+	}
+	if count != 1 {
+		t.Fatalf("expected count 1, got %d", count)
+	}
+
+	firstTTL := mr.TTL(key)
+	if firstTTL <= 0 || firstTTL > expire {
+		t.Fatalf("expected ttl in (0, %s], got %s", expire, firstTTL)
+	}
+
+	mr.FastForward(5 * time.Second)
+	ttlBeforeSecond := mr.TTL(key)
+
+	count, err = client.IncrementWithExpire(ctx, key, expire)
+	if err != nil {
+		t.Fatalf("second increment: %v", err)
+	}
+	if count != 2 {
+		t.Fatalf("expected count 2, got %d", count)
+	}
+
+	ttlAfterSecond := mr.TTL(key)
+	if ttlAfterSecond != ttlBeforeSecond {
+		t.Fatalf("expected ttl to stay at %s, got %s", ttlBeforeSecond, ttlAfterSecond)
+	}
+}