feat: 에러 처리 표준화 + BossRaid 낙관적 잠금

에러 표준화:
- pkg/apperror — AppError 타입, 7개 sentinel error
- pkg/middleware/error_handler — Fiber ErrorHandler 통합
- 핸들러에서 AppError 반환 시 구조화된 JSON 자동 응답

BossRaid Race Condition:
- 상태 전이 4곳 낙관적 잠금 (UPDATE WHERE status=?)
- TransitionRoomStatus/TransitionRoomStatusMulti 메서드 추가
- ErrStatusConflict sentinel error

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/bossraid/model.go

@@ -1,11 +1,15 @@
 package bossraid
 
 import (
+	"errors"
 	"time"
 
 	"gorm.io/gorm"
 )
 
+// ErrStatusConflict indicates that a room's status was already changed by another request.
+var ErrStatusConflict = errors.New("방 상태가 이미 변경되었습니다")
+
 type RoomStatus string
 
 const (

internal/bossraid/repository.go

@@ -237,6 +237,56 @@ func (r *Repository) UpdateRoomStatus(sessionName string, status RoomStatus) err
 	return result.Error
 }
 
+// TransitionRoomStatus atomically updates a room's status only if it currently matches expectedStatus.
+// Returns ErrStatusConflict if the row was not in the expected state (optimistic locking).
+func (r *Repository) TransitionRoomStatus(sessionName string, expectedStatus RoomStatus, newStatus RoomStatus, extras map[string]interface{}) error {
+	updates := map[string]interface{}{"status": newStatus}
+	for k, v := range extras {
+		updates[k] = v
+	}
+	result := r.db.Model(&BossRoom{}).
+		Where("session_name = ? AND status = ?", sessionName, expectedStatus).
+		Updates(updates)
+	if result.Error != nil {
+		return result.Error
+	}
+	if result.RowsAffected == 0 {
+		return ErrStatusConflict
+	}
+	return nil
+}
+
+// TransitionRoomStatusMulti atomically updates a room's status only if it currently matches one of the expected statuses.
+// Returns ErrStatusConflict if the row was not in any of the expected states.
+func (r *Repository) TransitionRoomStatusMulti(sessionName string, expectedStatuses []RoomStatus, newStatus RoomStatus, extras map[string]interface{}) error {
+	updates := map[string]interface{}{"status": newStatus}
+	for k, v := range extras {
+		updates[k] = v
+	}
+	result := r.db.Model(&BossRoom{}).
+		Where("session_name = ? AND status IN ?", sessionName, expectedStatuses).
+		Updates(updates)
+	if result.Error != nil {
+		return result.Error
+	}
+	if result.RowsAffected == 0 {
+		return ErrStatusConflict
+	}
+	return nil
+}
+
+// TransitionSlotStatus atomically updates a room slot's status only if it currently matches expectedStatus.
+func (r *Repository) TransitionSlotStatus(sessionName string, expectedStatus SlotStatus, newStatus SlotStatus) error {
+	result := r.db.Model(&RoomSlot{}).
+		Where("session_name = ? AND status = ?", sessionName, expectedStatus).
+		Update("status", newStatus)
+	if result.Error != nil {
+		return result.Error
+	}
+	// Slot transition failures are non-fatal — log but don't block
+	return nil
+}
+
 // GetRoomSlotsByServer returns all room slots for a given server.
 func (r *Repository) GetRoomSlotsByServer(serverID uint) ([]RoomSlot, error) {
 	var slots []RoomSlot

internal/bossraid/service.go

@@ -127,40 +127,27 @@ func (s *Service) RequestEntry(usernames []string, bossID int) (*BossRoom, error
 }
 
 // StartRaid marks a room as in_progress and updates the slot status.
-// Uses row-level locking to prevent concurrent state transitions.
+// Uses optimistic locking (WHERE status = 'waiting') to prevent concurrent state transitions.
 func (s *Service) StartRaid(sessionName string) (*BossRoom, error) {
-	var resultRoom *BossRoom
-	err := s.repo.Transaction(func(txRepo *Repository) error {
-		room, err := txRepo.FindBySessionNameForUpdate(sessionName)
-		if err != nil {
-			return fmt.Errorf("방을 찾을 수 없습니다: %w", err)
-		}
-		if room.Status != StatusWaiting {
-			return fmt.Errorf("시작할 수 없는 상태입니다: %s", room.Status)
-		}
-
-		now := time.Now()
-		room.Status = StatusInProgress
-		room.StartedAt = &now
-
-		if err := txRepo.Update(room); err != nil {
-			return fmt.Errorf("상태 업데이트 실패: %w", err)
-		}
-
-		// Update slot status to in_progress
-		slot, err := txRepo.FindRoomSlotBySession(sessionName)
-		if err == nil {
-			slot.Status = SlotInProgress
-			txRepo.UpdateRoomSlot(slot)
-		}
-
-		resultRoom = room
-		return nil
+	now := time.Now()
+	err := s.repo.TransitionRoomStatus(sessionName, StatusWaiting, StatusInProgress, map[string]interface{}{
+		"started_at": now,
 	})
-	if err != nil {
-		return nil, err
+	if err == ErrStatusConflict {
+		return nil, fmt.Errorf("시작할 수 없는 상태입니다 (이미 변경됨)")
 	}
-	return resultRoom, nil
+	if err != nil {
+		return nil, fmt.Errorf("상태 업데이트 실패: %w", err)
+	}
+
+	// Update slot status to in_progress (non-fatal if fails)
+	s.repo.TransitionSlotStatus(sessionName, SlotWaiting, SlotInProgress)
+
+	room, err := s.repo.FindBySessionName(sessionName)
+	if err != nil {
+		return nil, fmt.Errorf("방을 찾을 수 없습니다: %w", err)
+	}
+	return room, nil
 }
 
 // PlayerReward describes the reward for a single player.
@@ -179,48 +166,46 @@ type RewardResult struct {
 }
 
 // CompleteRaid marks a room as completed and grants rewards via blockchain.
-// Uses a database transaction with row-level locking to prevent double-completion.
+// Uses optimistic locking (WHERE status = 'in_progress') to prevent double-completion.
 func (s *Service) CompleteRaid(sessionName string, rewards []PlayerReward) (*BossRoom, []RewardResult, error) {
-	var resultRoom *BossRoom
 	var resultRewards []RewardResult
 
-	err := s.repo.Transaction(func(txRepo *Repository) error {
-		room, err := txRepo.FindBySessionNameForUpdate(sessionName)
-		if err != nil {
-			return fmt.Errorf("방을 찾을 수 없습니다: %w", err)
-		}
-		if room.Status != StatusInProgress {
-			return fmt.Errorf("완료할 수 없는 상태입니다: %s", room.Status)
-		}
-
-		// Validate reward recipients are room players
-		var players []string
-		if err := json.Unmarshal([]byte(room.Players), &players); err != nil {
-			return fmt.Errorf("플레이어 목록 파싱 실패: %w", err)
-		}
-		playerSet := make(map[string]bool, len(players))
-		for _, p := range players {
-			playerSet[p] = true
-		}
-		for _, r := range rewards {
-			if !playerSet[r.Username] {
-				return fmt.Errorf("보상 대상 %s가 방의 플레이어가 아닙니다", r.Username)
-			}
-		}
-
-		// Mark room completed
-		now := time.Now()
-		room.Status = StatusCompleted
-		room.CompletedAt = &now
-		if err := txRepo.Update(room); err != nil {
-			return fmt.Errorf("상태 업데이트 실패: %w", err)
-		}
-
-		resultRoom = room
-		return nil
-	})
+	// Validate reward recipients are room players before transitioning
+	room, err := s.repo.FindBySessionName(sessionName)
 	if err != nil {
-		return nil, nil, err
+		return nil, nil, fmt.Errorf("방을 찾을 수 없습니다: %w", err)
+	}
+
+	var players []string
+	if err := json.Unmarshal([]byte(room.Players), &players); err != nil {
+		return nil, nil, fmt.Errorf("플레이어 목록 파싱 실패: %w", err)
+	}
+	playerSet := make(map[string]bool, len(players))
+	for _, p := range players {
+		playerSet[p] = true
+	}
+	for _, r := range rewards {
+		if !playerSet[r.Username] {
+			return nil, nil, fmt.Errorf("보상 대상 %s가 방의 플레이어가 아닙니다", r.Username)
+		}
+	}
+
+	// Atomically transition status: in_progress → completed
+	now := time.Now()
+	err = s.repo.TransitionRoomStatus(sessionName, StatusInProgress, StatusCompleted, map[string]interface{}{
+		"completed_at": now,
+	})
+	if err == ErrStatusConflict {
+		return nil, nil, fmt.Errorf("완료할 수 없는 상태입니다 (이미 변경됨)")
+	}
+	if err != nil {
+		return nil, nil, fmt.Errorf("상태 업데이트 실패: %w", err)
+	}
+
+	// Re-fetch the updated room
+	resultRoom, err := s.repo.FindBySessionName(sessionName)
+	if err != nil {
+		return nil, nil, fmt.Errorf("방을 찾을 수 없습니다: %w", err)
 	}
 
 	// Grant rewards outside the transaction to avoid holding the lock during RPC calls
@@ -239,9 +224,9 @@ func (s *Service) CompleteRaid(sessionName string, rewards []PlayerReward) (*Bos
 		}
 	}
 
-	// 보상 실패가 있으면 상태를 reward_failed로 업데이트
+	// 보상 실패가 있으면 상태를 reward_failed로 업데이트 (completed → reward_failed)
 	if hasRewardFailure {
-		if err := s.repo.UpdateRoomStatus(sessionName, StatusRewardFailed); err != nil {
+		if err := s.repo.TransitionRoomStatus(sessionName, StatusCompleted, StatusRewardFailed, nil); err != nil {
 			log.Printf("보상 실패 상태 업데이트 실패: %s: %v", sessionName, err)
 		}
 	}
@@ -266,30 +251,19 @@ func (s *Service) CompleteRaid(sessionName string, rewards []PlayerReward) (*Bos
 }
 
 // FailRaid marks a room as failed and resets the slot.
-// Uses row-level locking to prevent concurrent state transitions.
+// Uses optimistic locking (WHERE status IN ('waiting','in_progress')) to prevent concurrent state transitions.
 func (s *Service) FailRaid(sessionName string) (*BossRoom, error) {
-	var resultRoom *BossRoom
-	err := s.repo.Transaction(func(txRepo *Repository) error {
-		room, err := txRepo.FindBySessionNameForUpdate(sessionName)
-		if err != nil {
-			return fmt.Errorf("방을 찾을 수 없습니다: %w", err)
-		}
-		if room.Status != StatusWaiting && room.Status != StatusInProgress {
-			return fmt.Errorf("실패 처리할 수 없는 상태입니다: %s", room.Status)
-		}
-
-		now := time.Now()
-		room.Status = StatusFailed
-		room.CompletedAt = &now
-
-		if err := txRepo.Update(room); err != nil {
-			return fmt.Errorf("상태 업데이트 실패: %w", err)
-		}
-		resultRoom = room
-		return nil
-	})
+	now := time.Now()
+	err := s.repo.TransitionRoomStatusMulti(sessionName,
+		[]RoomStatus{StatusWaiting, StatusInProgress},
+		StatusFailed,
+		map[string]interface{}{"completed_at": now},
+	)
+	if err == ErrStatusConflict {
+		return nil, fmt.Errorf("실패 처리할 수 없는 상태입니다 (이미 변경됨)")
+	}
 	if err != nil {
-		return nil, err
+		return nil, fmt.Errorf("상태 업데이트 실패: %w", err)
 	}
 
 	// Reset slot to idle so it can accept new raids
@@ -297,7 +271,11 @@ func (s *Service) FailRaid(sessionName string) (*BossRoom, error) {
 		log.Printf("슬롯 리셋 실패 (fail): %s: %v", sessionName, err)
 	}
 
-	return resultRoom, nil
+	room, err := s.repo.FindBySessionName(sessionName)
+	if err != nil {
+		return nil, fmt.Errorf("방을 찾을 수 없습니다: %w", err)
+	}
+	return room, nil
 }
 
 // GetRoom returns a room by session name.

main.go

@@ -145,6 +145,7 @@ func main() {
 	app := fiber.New(fiber.Config{
 		StreamRequestBody: true,
 		BodyLimit:         4 * 1024 * 1024 * 1024, // 4GB
+		ErrorHandler:      middleware.ErrorHandler,
 	})
 	app.Use(middleware.RequestID)
 	app.Use(middleware.Metrics)

pkg/apperror/apperror.go

@@ -0,0 +1,33 @@
+package apperror
+
+import "fmt"
+
+// AppError is a structured application error with an HTTP status code.
+type AppError struct {
+	Code    string `json:"error"`
+	Message string `json:"message"`
+	Status  int    `json:"-"`
+}
+
+func (e *AppError) Error() string { return e.Message }
+
+// New creates a new AppError.
+func New(code string, message string, status int) *AppError {
+	return &AppError{Code: code, Message: message, Status: status}
+}
+
+// Wrap creates a new AppError that wraps a cause error.
+func Wrap(code string, message string, status int, cause error) *AppError {
+	return &AppError{Code: code, Message: fmt.Sprintf("%s: %v", message, cause), Status: status}
+}
+
+// Common errors
+var (
+	ErrUnauthorized = &AppError{Code: "unauthorized", Message: "인증이 필요합니다", Status: 401}
+	ErrForbidden    = &AppError{Code: "forbidden", Message: "권한이 없습니다", Status: 403}
+	ErrNotFound     = &AppError{Code: "not_found", Message: "리소스를 찾을 수 없습니다", Status: 404}
+	ErrConflict     = &AppError{Code: "conflict", Message: "이미 존재합니다", Status: 409}
+	ErrBadRequest   = &AppError{Code: "bad_request", Message: "잘못된 요청입니다", Status: 400}
+	ErrInternal     = &AppError{Code: "internal_error", Message: "서버 오류가 발생했습니다", Status: 500}
+	ErrRateLimited  = &AppError{Code: "rate_limited", Message: "요청이 너무 많습니다", Status: 429}
+)

pkg/middleware/error_handler.go

@@ -0,0 +1,31 @@
+package middleware
+
+import (
+	"errors"
+
+	"a301_server/pkg/apperror"
+
+	"github.com/gofiber/fiber/v2"
+)
+
+// ErrorHandler is a Fiber error handler that returns structured JSON for AppError.
+func ErrorHandler(c *fiber.Ctx, err error) error {
+	var appErr *apperror.AppError
+	if errors.As(err, &appErr) {
+		return c.Status(appErr.Status).JSON(appErr)
+	}
+
+	// Default Fiber error handling
+	var fiberErr *fiber.Error
+	if errors.As(err, &fiberErr) {
+		return c.Status(fiberErr.Code).JSON(fiber.Map{
+			"error":   "server_error",
+			"message": fiberErr.Message,
+		})
+	}
+
+	return c.Status(500).JSON(fiber.Map{
+		"error":   "internal_error",
+		"message": "서버 오류가 발생했습니다",
+	})
+}