refactor: 计费退款不变量收敛为 Billing.scope()（download.py 25 处手工退款 → 0）

为什么：「任何失败路径必须退款」此前靠路由里每条早退/except 手工调
refund_open 维持（download.py 单文件 25 处），上一轮拆 register/login
时已经体会过这个模式的代价——改任何一条返回路径都要核对还原时序，
漏一处不会有测试报警，只剩重启对账兜底在跑。

改动：
- Billing.scope()（asynccontextmanager）：作用域退出时自动退还仍 open
  的账本行。正确性完全依赖 refund_open 的既有幂等口径（只退仍 open
  的行）——settle()/rename_to() 消费过的行天然豁免，成功路径零动作，
  无需手动「取消自动退」；未捕获异常先退款再原样穿透。
- download.py 四个端点全部改用 async with billing.scope()，删除全部
  手工退款与 _fail 收尾闭包；_sync_download_json 的内部退款一并移除
  （docstring 标明必须在 scope 内运行）。
- 语义逐路径等价：错误早退→自动退（同原手工）；register 成功后
  rename→自动退 no-op（同原手工退的 no-op）；start 失败行挂 task_id
  留给对账（同原）；CancelledError 语义不变（退款随取消中断，对账兜底）。

先写测试后动代码：tests/test_billing_scope.py 5 例（TDD 红→绿），
对真实沙箱账本断言——open 行自动退、settle 后 no-op 且余额恰 -1、
rename 后请求级行消失且任务级行保持 open、异常穿透、free 路径零动作。
既有 30 例计费/下载端到端测试作迁移回归网。
门禁：ruff 全过 · pyright 0/0 · pytest 387 passed（382→387）。

tests/test_billing_scope.py

@@ -0,0 +1,123 @@
+# -*- coding: utf-8 -*-
+"""Billing.scope() 契约：作用域退出自动退还仍 open 的账本行。
+
+设计前提（已由既有实现保证）：refund_open 只退「仍 open」的行——
+settle/rename_to 消费过的行天然不在退款范围，因此自动退与成功路径
+互不干扰，调用方无需手动取消。测试直接对真实账本（沙箱库）断言。
+"""
+import pytest
+
+import auth_system as auth
+from web.billing import Billing
+from web.deps import SiteUser
+
+
+@pytest.fixture()
+def ledger_env(tmp_path, monkeypatch):
+    monkeypatch.setattr(auth.db, 'DATA_DIR', tmp_path)
+    monkeypatch.setattr(auth.db, 'DB_PATH', tmp_path / 'site.db')
+    monkeypatch.setattr(auth.db, 'BACKUP_DIR', tmp_path / 'backups')
+    monkeypatch.setattr(auth.db, 'SECRET_KEY_PATH', tmp_path / 'secret.key')
+    auth.init_db()
+    row = auth.create_user('payer', 'Passw0rd12', email='p@example.com', credits=5)
+    assert row is not None, '建号应成功'
+    yield {'uid': int(row['id'])}
+    # 无需清理：tmp_path 由 pytest 回收
+
+
+def _row(key: str):
+    """取账本行，不存在则测试失败（供「行必须存在」的断言用）。"""
+    row = auth.ledger_get(key)
+    assert row is not None, f'账本行 {key} 应存在'
+    return row
+
+
+def _user_credits() -> int:
+    u = auth.get_user_by_name('payer')
+    assert u is not None, '用户 payer 应存在'
+    return int(u['duration_seconds'] or 0)
+
+
+def _make_billing(ledger_env, key: str | None) -> Billing:
+    user = SiteUser(id=ledger_env['uid'], username='payer', role='user', credits=5)
+    return Billing(user=user, ledger_key=key, need=1, free=False)
+
+
+@pytest.mark.anyio
+async def test_scope_auto_refunds_open_row(ledger_env):
+    """正常退出作用域：仍 open 的行被退（rejected），余额恢复。"""
+    key = 'req-aaa'
+    reserved = auth.reserve_and_open(ledger_env['uid'], 1, key)
+    assert reserved is not None
+    assert _row(key)['status'] == 'open'
+
+    billing = _make_billing(ledger_env, key)
+    async with billing.scope():
+        pass  # 业务体正常走完但未结算（等价于路由里的错误早退）
+
+    assert _row(key)['status'] == 'rejected'
+    assert _user_credits() == 5
+
+
+@pytest.mark.anyio
+async def test_scope_noop_after_settle(ledger_env):
+    """作用域内已结算：退出自动退为 no-op，交付行不受影响。"""
+    key = 'req-bbb'
+    auth.reserve_and_open(ledger_env['uid'], 1, key)
+    billing = _make_billing(ledger_env, key)
+
+    async with billing.scope():
+        await billing.settle(1, 'delivered')
+
+    assert _row(key)['status'] == 'delivered'
+    assert _user_credits() == 4  # 恰好扣 1，未被二次退
+
+
+@pytest.mark.anyio
+async def test_scope_noop_after_rename(ledger_env):
+    """作用域内账本已改挂 task_id：请求级行不存在，自动退为 no-op；
+    任务级行保持 open 交给 worker 结算——与迁移前手工语义逐路径一致。"""
+    key = 'req-ccc'
+    auth.reserve_and_open(ledger_env['uid'], 1, key)
+    billing = _make_billing(ledger_env, key)
+
+    async with billing.scope():
+        ok = await billing.rename_to('task-xyz')
+        assert ok is True
+
+    assert auth.ledger_get(key) is None
+    moved = _row('task-xyz')
+    assert moved['status'] == 'open'
+
+
+@pytest.mark.anyio
+async def test_scope_refunds_on_exception_and_reraises(ledger_env):
+    """业务体抛异常：先退款，异常原样穿透（路由自行转错误响应）。"""
+    key = 'req-ddd'
+    auth.reserve_and_open(ledger_env['uid'], 1, key)
+    billing = _make_billing(ledger_env, key)
+
+    with pytest.raises(ValueError, match='boom'):
+        async with billing.scope():
+            raise ValueError('boom')
+
+    assert _row(key)['status'] == 'rejected'
+
+
+@pytest.mark.anyio
+async def test_scope_free_billing_noop(ledger_env):
+    """free/unlimited（ledger_key=None）：作用域进出零动作。"""
+    billing = _make_billing(ledger_env, None)
+    async with billing.scope():
+        pass
+    # 无账本行可动，不抛异常即为通过
+    assert _open_rows() == []
+
+
+def _open_rows():
+    conn = auth.get_conn()
+    try:
+        rows = conn.execute("SELECT task_id FROM billing_ledger WHERE status='open'").fetchall()
+        return [dict(r) for r in rows]
+    finally:
+        conn.close()

web/billing.py

@@ -12,7 +12,8 @@
 import asyncio
 import logging
 import secrets
-from collections.abc import Callable
+from collections.abc import AsyncGenerator, Callable
+from contextlib import asynccontextmanager
 from dataclasses import dataclass
 
 import auth_system
@@ -63,6 +64,25 @@ class Billing:
             return True
         return await asyncio.to_thread(auth_system.ledger_rename, self.ledger_key, task_id)
 
+    @asynccontextmanager
+    async def scope(self) -> AsyncGenerator['Billing', None]:
+        """计费作用域：退出时自动退还仍 open 的账本行。
+
+        把「任何失败路径必须退款」这条不变量从路由的每条早退/except 里
+        收敛到这里。正确性依赖 refund_open 的既有幂等口径（只退仍 open
+        的行），因此：
+        - 错误早退 / 异常转错误响应 → 正常退出作用域 → 自动退；
+        - settle()/rename_to() 已消费的行不在退款范围，成功路径自然豁免，
+          无需手动「取消自动退」；
+        - 未捕获异常先退款再原样穿透，由路由既有 except 转错误响应；
+        - 客户端断连（CancelledError）与迁移前行为一致：退款可能被一并
+          取消，open 行留给重启对账兜底，不额外加 shield 复杂化。
+        """
+        try:
+            yield self
+        finally:
+            await self.refund_open('rejected')
+
 
 async def require_billing(
     user: SiteUser | None,

web/routers/download.py

@@ -7,8 +7,9 @@
 - POST      /api/v1/podcast/download        ↔ Flask /podcast/download（计费 1 次）
 - POST      /api/v1/podcast/download/batch_zip ↔ Flask /podcast/download/batch_zip
 
-计费：经 web.billing.require_billing（预扣+开账同事务 / 失败只退 open 行），
-任务登记与启动由 web.tasks_registry 承担；
+计费：经 web.billing.require_billing（预扣+开账同事务）+ billing.scope()
+（「任何失败路径必须退款」不变量收敛在作用域退出：自动退仍 open 的行，
+settle/rename 消费过的行天然豁免）；任务登记与启动由 web.tasks_registry 承担；
 worker 复用 services 层既有实现，账本与交付语义不变。
 """
 import asyncio
@@ -67,14 +68,14 @@ async def _sync_download_json(downloader, music_id: str, quality: str, cookies:
                               billing, ip: str):
     """format=json 的同步下载分支：取信息 → 下载 → 组响应 → 结算。
 
-    语义：任一步失败退回预扣（账本守恒），只有成功才 settle(1, 'delivered')。
+    必须运行在 billing.scope() 内：失败退款由作用域退出统一完成，本函数
+    只负责在成功路径 settle(1, 'delivered')（结算后自动退自然豁免）。
     """
     try:
         info = await asyncio.to_thread(
             downloader.get_music_info, int(music_id), quality, cookies)
     except Exception as e:
         logger.error('获取音乐信息失败: %s', e)
-        await billing.refund_open('rejected')
         return error('获取失败：歌曲不存在或暂无版权', 404)
     from services._shared import _build_download_base
     base = await asyncio.to_thread(
@@ -88,11 +89,9 @@ async def _sync_download_json(downloader, music_id: str, quality: str, cookies:
             cookies=cookies, music_info=info)
         if not dl.success:
             logger.error('下载失败: %s', dl.error_message)
-            await billing.refund_open('rejected')
             return error('下载失败，请稍后重试或更换音质', 500)
     except Exception as e:
         logger.error('下载异常: %s', e)
-        await billing.refund_open('rejected')
         return error('服务暂时不可用，请稍后再试', 500)
     try:
         _record_log('下载', ip, info.name)
@@ -143,8 +142,8 @@ async def _resolve_personal(data: dict[str, Any], user: SiteUser | None, request
 async def download(user: Annotated[SiteUser | None, Depends(current_user)], request: Request):
     """下载单曲（file=后台任务 / json=同步返回）。
 
-    顺序：限流 → 预扣计费 → 四道参数闸（失败即退款）→ 账号 Cookie →
-    可选参数 → json 同步分支 / file 后台任务分支。
+    顺序：限流 → 预扣计费 → billing.scope()（失败自动退款）→ 四道参数闸 →
+    账号 Cookie → 可选参数 → json 同步分支 / file 后台任务分支。
     """
     await ratelimit.guard(request, bucket='dl', limit=20, window=60)
     data = await _request_data(request)
@@ -155,28 +154,24 @@ async def download(user: Annotated[SiteUser | None, Depends(current_user)], requ
 
     billing = await require_billing(user, action='音乐下载', ip=ip)
 
-    async def _fail(msg: str, code: int = 400):
-        """参数/校验失败：退回本次预扣后返回错误（账本守恒）。"""
-        await billing.refund_open('rejected')
-        return error(msg, code)
-
+    async with billing.scope():
         try:
             if not music_id:
-            return await _fail("参数 'music_id' 不能为空", 400)
+                return error("参数 'music_id' 不能为空", 400)
             music_id = str(music_id).strip()
             if not music_id.isdigit():
-            return await _fail('无效的歌曲 ID')
+                return error('无效的歌曲 ID')
             if quality not in VALID_QUALITY_LEVELS:
-            return await _fail(f"无效的音质参数，支持: {', '.join(VALID_QUALITY_LEVELS)}")
+                return error(f"无效的音质参数，支持: {', '.join(VALID_QUALITY_LEVELS)}")
             if return_format not in ('file', 'json'):
-            return await _fail("返回格式只支持 'file' 或 'json'")
+                return error("返回格式只支持 'file' 或 'json'")
             music_id = _extract_id(music_id)
 
             account = data.get('account', 'admin')
             if account == 'user':
                 cookies = await _resolve_personal(data, user, request)
                 if not cookies:
-                return await _fail('请先扫码登录个人账号后再使用个人解析', 401)
+                    return error('请先扫码登录个人账号后再使用个人解析', 401)
             else:
                 cookies = _cookies()
 
@@ -204,7 +199,6 @@ async def download(user: Annotated[SiteUser | None, Depends(current_user)], requ
             }, device_id)
             reg_err = await register_download_task(billing, task, str(data.get('task_id') or '').strip())
             if reg_err:
-            await billing.refund_open('rejected')
                 return error(reg_err, 429)
             task_id = task['task_id']
             start_err = await start_task(task_id, services_downloads._single_download_worker, (task_id,))
@@ -212,11 +206,9 @@ async def download(user: Annotated[SiteUser | None, Depends(current_user)], requ
                 return error(start_err, 500)
             return success({'task_id': task_id, 'status': 'queued'}, '下载任务已提交')
         except APIException:
-        await billing.refund_open('rejected')
             return error('服务暂时不可用，请稍后再试', 500)
         except Exception as e:
             logger.error('下载音乐异常: %s', e)
-        await billing.refund_open('rejected')
             return error('服务暂时不可用，请稍后再试', 500)
 
 
@@ -250,23 +242,20 @@ async def download_batch_zip(user: Annotated[SiteUser | None, Depends(current_us
         return max(1, min(len(_dedup_ids(data.get('ids') or data.get('id') or [])), 500))
 
     billing = await require_billing(user, action='批量ZIP下载', cost=_cost, ip=ip)
+    async with billing.scope():
         try:
             raw_ids = data.get('ids') or data.get('id')
             ids = _dedup_ids(raw_ids)
             if not ids:
-            await billing.refund_open('rejected')
                 return error('缺少 ids 参数')
             non_digit = [x for x in ids if not x.isdigit()]
             if non_digit:
-            await billing.refund_open('rejected')
                 return error('ids 中含无效歌曲 ID')
             if len(ids) > 500:
-            await billing.refund_open('rejected')
                 return error('单个批量任务最多 500 首，请分批提交')
 
             quality = data.get('quality', 'lossless')
             if quality not in VALID_QUALITY_LEVELS:
-            await billing.refund_open('rejected')
                 return error(f"无效的音质参数，支持: {', '.join(VALID_QUALITY_LEVELS)}")
             filename_format = data.get('filename_format', 'artist_song')
             write_meta = str(data.get('write_meta', 'true')).lower() in ('1', 'true', 'yes')
@@ -301,7 +290,6 @@ async def download_batch_zip(user: Annotated[SiteUser | None, Depends(current_us
             }
             reg_err = await register_download_task(billing, task, str(data.get('task_id') or '').strip())
             if reg_err:
-            await billing.refund_open('rejected')
                 return error(reg_err, 429)
             task_id = task['task_id']
             start_err = await start_task(task_id, services_batch._batch_zip_worker, (task_id,))
@@ -311,7 +299,6 @@ async def download_batch_zip(user: Annotated[SiteUser | None, Depends(current_us
                            '批量打包任务已提交')
         except Exception as e:
             logger.error('批量打包提交异常: %s', e)
-        await billing.refund_open('rejected')
             return error('服务暂时不可用，请稍后再试', 500)
 
 
@@ -343,23 +330,21 @@ async def podcast_download(user: Annotated[SiteUser | None, Depends(current_user
     ip = ratelimit.client_ip(request)
 
     billing = await require_billing(user, action='播客单集下载', ip=ip)
+    async with billing.scope():
         try:
             if not raw:
-            await billing.refund_open('rejected')
                 return error("参数 'id' 不能为空", 400)
             from music_api import podcast
             quality = _podcast_quality(data)
             need_zip = str(data.get('zip', 'false')).lower() in ('1', 'true', 'yes')
             ref = podcast.extract_podcast_ref(raw)
             if ref['kind'] == 'radio':
-            await billing.refund_open('rejected')
                 return error('这是电台链接，请先解析电台后选择单集下载，或使用「打包整个播客」')
 
             account = data.get('account', 'admin')
             if account == 'user':
                 cookies = await _resolve_personal(data, user, request)
                 if not cookies:
-                await billing.refund_open('rejected')
                     return error('请先扫码登录个人账号后再使用个人解析', 401)
             else:
                 cookies = _cookies()
@@ -379,7 +364,6 @@ async def podcast_download(user: Annotated[SiteUser | None, Depends(current_user
             }
             reg_err = await register_download_task(billing, task, str(data.get('task_id') or '').strip())
             if reg_err:
-            await billing.refund_open('rejected')
                 return error(reg_err, 429)
             task_id = task['task_id']
             start_err = await start_task(task_id, services_podcast._podcast_download_worker, (task_id,))
@@ -387,11 +371,9 @@ async def podcast_download(user: Annotated[SiteUser | None, Depends(current_user
                 return error(start_err, 500)
             return success({'task_id': task_id, 'status': 'queued'}, '播客单集下载任务已提交')
         except APIException as e:
-        await billing.refund_open('rejected')
             return error(str(e), 404)
         except Exception as e:
             logger.error('播客单集下载提交异常: %s', e)
-        await billing.refund_open('rejected')
             return error('服务暂时不可用，请稍后再试', 500)
 
 
@@ -406,11 +388,11 @@ async def podcast_batch_zip(user: Annotated[SiteUser | None, Depends(current_use
         return _podcast_billing_count(data)
 
     billing = await require_billing(user, action='播客整台打包', cost=_cost, ip=ip)
+    async with billing.scope():
         try:
             raw_radio = data.get('radio_id')
             raw_ids = data.get('ids')
             if not raw_radio and not raw_ids:
-            await billing.refund_open('rejected')
                 return error('缺少 radio_id 或 ids 参数')
             limit = max(1, min(_safe_int(data.get('limit', 200) or 200, 200), 500))
             quality = _podcast_quality(data)
@@ -423,7 +405,6 @@ async def podcast_batch_zip(user: Annotated[SiteUser | None, Depends(current_use
             if raw_radio:
                 ref = podcast.extract_podcast_ref(raw_radio)
                 if ref['kind'] == 'program':
-                await billing.refund_open('rejected')
                     return error('这是单集链接，请使用「下载单集」或直接输入电台链接/ID')
                 radio_id = ref['id']
                 radio_name = ''
@@ -434,7 +415,6 @@ async def podcast_batch_zip(user: Annotated[SiteUser | None, Depends(current_use
             else:
                 id_list = _dedup_ids(raw_ids)[:limit]
                 if not id_list:
-                await billing.refund_open('rejected')
                     return error('缺少 ids 参数')
                 radio_id = None
                 radio_name = str(data.get('name') or '播客单集')
@@ -462,7 +442,6 @@ async def podcast_batch_zip(user: Annotated[SiteUser | None, Depends(current_use
             }
             reg_err = await register_download_task(billing, task, str(data.get('task_id') or '').strip())
             if reg_err:
-            await billing.refund_open('rejected')
                 return error(reg_err, 429)
             task_id = task['task_id']
             start_err = await start_task(task_id, services_podcast._podcast_zip_worker, (task_id,))
@@ -470,9 +449,7 @@ async def podcast_batch_zip(user: Annotated[SiteUser | None, Depends(current_use
                 return error(start_err, 500)
             return success({'task_id': task_id, 'status': 'queued'}, '播客打包任务已提交')
         except APIException as e:
-        await billing.refund_open('rejected')
             return error(str(e), 404)
         except Exception as e:
             logger.error('播客打包提交异常: %s', e)
-        await billing.refund_open('rejected')
             return error('服务暂时不可用，请稍后再试', 500)