fix: make rate limit increment atomic

backend/go.mod

@@ -14,6 +14,11 @@ require (
 	gorm.io/gorm v1.31.1
 )
 
+require (
+	github.com/alicebob/miniredis/v2 v2.37.0 // indirect
+	github.com/yuin/gopher-lua v1.1.1 // indirect
+)
+
 require (
 	filippo.io/edwards25519 v1.1.0 // indirect
 	github.com/bytedance/sonic v1.14.0 // indirect

backend/go.sum

@@ -1,5 +1,7 @@
 filippo.io/edwards25519 v1.1.0 h1:FNf4tywRC1HmFuKW5xopWpigGjJKiJSV0Cqo0cJWDaA=
 filippo.io/edwards25519 v1.1.0/go.mod h1:BxyFTGdWcka3PhytdK4V28tE5sGfRvvvRV7EaN4VDT4=
+github.com/alicebob/miniredis/v2 v2.37.0 h1:RheObYW32G1aiJIj81XVt78ZHJpHonHLHW7OLIshq68=
+github.com/alicebob/miniredis/v2 v2.37.0/go.mod h1:TcL7YfarKPGDAthEtl5NBeHZfeUQj6OXMm/+iu5cLMM=
 github.com/bsm/ginkgo/v2 v2.12.0 h1:Ny8MWAHyOepLGlLKYmXG4IEkioBysk6GpaRTLC8zwWs=
 github.com/bsm/ginkgo/v2 v2.12.0/go.mod h1:SwYbGRRDovPVboqFv0tPTcG1sN61LM1Z4ARdbAV9g4c=
 github.com/bsm/gomega v1.27.10 h1:yeMWxP2pV2fG3FgAODIY8EiRE3dy0aeFYt4l7wh6yKA=
@@ -85,6 +87,8 @@ github.com/twitchyliquid64/golang-asm v0.15.1 h1:SU5vSMR7hnwNxj24w34ZyCi/FmDZTkS
 github.com/twitchyliquid64/golang-asm v0.15.1/go.mod h1:a1lVb/DtPvCB8fslRZhAngC2+aY1QWCk3Cedj/Gdt08=
 github.com/ugorji/go/codec v1.3.0 h1:Qd2W2sQawAfG8XSvzwhBeoGq71zXOC/Q1E9y/wUcsUA=
 github.com/ugorji/go/codec v1.3.0/go.mod h1:pRBVtBSKl77K30Bv8R2P+cLSGaTtex6fsA2Wjqmfxj4=
+github.com/yuin/gopher-lua v1.1.1 h1:kYKnWBjvbNP4XLT3+bPEwAXJx262OhaHDWDVOPjL46M=
+github.com/yuin/gopher-lua v1.1.1/go.mod h1:GBR0iDaNXjAgGg9zfCvksxSRnQx76gclCIb7kdAd1Pw=
 go.uber.org/goleak v1.3.0 h1:2K3zAYmnTNqV73imy9J1T3WC+gmCePx2hEGkimedGto=
 go.uber.org/goleak v1.3.0/go.mod h1:CoHD4mav9JJNrW/WLlf7HGZPjdw8EucARQHekz1X6bE=
 go.uber.org/mock v0.5.0 h1:KAMbZvZPyBPWgD14IrIQ38QCyjwpvVVV6K/bHl1IwQU=

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)
+	}
+}