# app/services/task_service.py

from typing import List, Optional

from sqlalchemy.ext.asyncio import AsyncSession
from sqlalchemy import select, update
from datetime import datetime
from app.core.biz_exception import NotFoundError
from app.models.task import Task
from app.schemas.task import TaskCreate, TaskUpdate


class TaskService:

    # ======================
    # 创建任务
    # ======================
    @staticmethod
    async def create(db: AsyncSession, obj_in: TaskCreate) -> Task:
        db_obj = Task(
            command=obj_in.command,
            args=obj_in.args,
            status=obj_in.status or 0,
        )
        db.add(db_obj)
        await db.commit()
        await db.refresh(db_obj)
        return db_obj
    
    async def pop_task(db: AsyncSession, queue_name: str) -> Task:
        """
        异步 POP 方法 (MySQL 8.0+ 专用)
        原子性获取并锁定一个任务
        """
        try:
            # --- 第一步：查询并抢占锁 (SELECT ... FOR UPDATE SKIP LOCKED) ---
            stmt = (
                select(Task)
                .where(Task.command == queue_name)  # 指定队列/命令类型
                .where(Task.status == 0)            # 只找待执行的
                .order_by(Task.id.asc()) # 优先级 > 时间
                .limit(1)
                # MySQL 8.0+ 必须加 skip_locked=True，否则并发高时会造成大量等待
                .with_for_update(skip_locked=True)  
            )
            
            result = await db.execute(stmt)
            task = result.scalar_one_or_none()
            
            # --- 第二步：更新状态 (UPDATE) ---
            if task:
                task.status = 1  # 标记为执行中
                task.update_at = datetime.now()
                
                # session.begin() 退出时会自动执行 commit()
                # 此时数据库中的状态已变更
                await db.commit()
                return task
            
            # 如果没抢到任务
            raise NotFoundError(message="Task not found")

        except Exception as e:
            # 记录日志
            # session.begin() 会自动 rollback，无需手动调用
            raise e

    # ======================
    # 根据 ID 获取
    # ======================
    @staticmethod
    async def get_by_id(db: AsyncSession, task_id: int) -> Task:
        stmt = select(Task).where(Task.id == task_id)
        obj = (await db.execute(stmt)).scalar_one_or_none()

        if not obj:
            raise NotFoundError("Task not exist")
        return obj

    # ======================
    # 更新任务
    # ======================
    @staticmethod
    async def update(
        db: AsyncSession,
        task_id: int,
        obj_in: TaskUpdate
    ) -> Task:
        stmt = select(Task).where(Task.id == task_id)
        db_obj = (await db.execute(stmt)).scalar_one_or_none()
        old_status = db_obj.status
        if not db_obj:
            raise NotFoundError("Task not exist")

        if obj_in.result is not None:
            db_obj.result = obj_in.result
        if obj_in.status is not None:
            db_obj.status = obj_in.status

        await db.commit()
        await db.refresh(db_obj)
        return db_obj

    # ======================
    # 获取待处理任务（分页）
    # ======================
    @staticmethod
    async def get_pending(
        db: AsyncSession,
        command: str,
        page: int,
        size: int
    ) -> List[Task]:
        offset = page * size

        stmt = (
            select(Task)
            .where(
                Task.command == command,
                Task.status == 0
            )
            .order_by(Task.create_at.asc())
            .offset(offset)
            .limit(size)
        )

        return (await db.execute(stmt)).scalars().all()