feat(delivery): enforce instance name uniqueness at task creation

Wrap task creation in a transaction that locks the business line row (SELECT ... FOR UPDATE) so concurrent requests cannot both pass the availability check. A name is considered occupied by in-flight or retained tasks (pending through registering, finished, register_failed, canceling, rollback_pending/rolling_back/rollback_failed) and by active deployment results; it becomes reusable only after rollback or explicit cleanup releases it.

server/internal/service/delivery.go

@@ -552,7 +552,18 @@ func (s *DeliveryService) CreateTask(ctx context.Context, userID uint64, isAdmin
 		PayloadHash:      hex.EncodeToString(digest[:]),
 		IdempotencyKey:   idempotencyKey,
 	}
-	if err := s.db.WithContext(ctx).Create(&task).Error; err != nil {
+	if err := s.db.WithContext(ctx).Transaction(func(tx *gorm.DB) error {
+		// Serialize name allocation per business line so concurrent requests cannot
+		// pass the availability check and create the same instance identity.
+		var businessLine model.BusinessLine
+		if err := tx.Clauses(clause.Locking{Strength: "UPDATE"}).First(&businessLine, input.BusinessLineID).Error; err != nil {
+			return err
+		}
+		if err := ensureInstanceNameAvailable(tx, input.BusinessLineID, task.Component, input.InstanceName); err != nil {
+			return err
+		}
+		return tx.Create(&task).Error
+	}); err != nil {
 		if lookupErr := s.db.WithContext(ctx).Where("idempotency_key = ?", idempotencyKey).First(&existing).Error; lookupErr == nil {
 			return &existing, true, nil
 		}
@@ -567,6 +578,45 @@ func (s *DeliveryService) CreateTask(ctx context.Context, userID uint64, isAdmin
 	return &task, false, nil
 }
 
+// ensureInstanceNameAvailable treats in-flight tasks, retained failed deployments,
+// and active results as name reservations. A name is reusable only after rollback
+// or explicit cleanup has released the corresponding task/result.
+func ensureInstanceNameAvailable(tx *gorm.DB, businessLineID uint64, component, instanceName string) error {
+	occupiedTaskStatuses := []string{
+		model.TaskPending,
+		model.TaskValidating,
+		model.TaskDispatching,
+		model.TaskRunning,
+		model.TaskRegistering,
+		model.TaskFinished,
+		model.TaskRegisterFailed,
+		model.TaskCanceling,
+		model.TaskRollbackPending,
+		model.TaskRollingBack,
+		model.TaskRollbackFailed,
+	}
+	var taskCount int64
+	if err := tx.Model(&model.DeliveryTask{}).
+		Where("business_line_id = ? AND component = ? AND instance_name = ? AND status IN ?", businessLineID, component, instanceName, occupiedTaskStatuses).
+		Count(&taskCount).Error; err != nil {
+		return err
+	}
+	if taskCount > 0 {
+		return fmt.Errorf("instance_name %q is already in use by an active or pending delivery", instanceName)
+	}
+
+	var resultCount int64
+	if err := tx.Model(&model.DeploymentResult{}).
+		Where("business_line_id = ? AND component = ? AND instance_name = ? AND status = ?", businessLineID, component, instanceName, "active").
+		Count(&resultCount).Error; err != nil {
+		return err
+	}
+	if resultCount > 0 {
+		return fmt.Errorf("instance_name %q is already in use by a delivered instance", instanceName)
+	}
+	return nil
+}
+
 // 版本白名单与 playbook 的 mysql_package_map 保持同步：
 // 8.0 走 Ubuntu 自带源，8.4 走 MySQL 官方 APT 源；5.6/5.7 已 EOL 且无 noble 包，不支持。
 var supportedMySQLVersions = map[string]bool{"8.0": true, "8.4": true}