将烧瓶请求/应用程序上下文复制到另一个进程

Posted 2023-02-16

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【中文标题】将烧瓶请求/应用程序上下文复制到另一个进程【英文标题】：Copy flask request/app context to another process 【发布时间】：2019-10-02 05:58:57 【问题描述】：

tl;博士

如何序列化 Flask 应用程序或请求上下文，或该上下文的子集（即任何可以成功序列化），以便我可以从另一个进程，而不是一个线程？

加长版

我有一些需要访问 Flask 请求上下文或 App 上下文的函数，我想在后台运行这些函数。

Flask 有一个内置的 @copy_current_request_context 装饰器，用于将函数包装在请求上下文的副本中，因此您可以在不同的线程中运行它：

from threading import Thread
from flask import Flask, request, copy_current_request_context

app = Flask(__name__)

@app.route('/')
def index():
    request.foo = 'bar'
    @copy_current_request_context
    def baz():
        print(request.foo)
    thr = Thread(target=baz)
    thr.start()
    return 'ok'

虽然 Flask 不提供用于复制应用上下文的内置装饰器，但它提供了执行此操作的机制 - Access flask.g inside greenlet 中描述了一种解决方案

我尝试过的

首先，我希望用 Pickle 来解决。这是concurrent.futures.ProcessPoolExecutor 使用的。不幸的是，由于应用上下文中存在线程锁对象，Pickle 失败了：

>>> with app.test_request_context('/'):
...     pickle.dumps(appctx)
...
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
TypeError: can't pickle _thread.lock objects

它也不能复制修饰/包装的函数：

>>> with app.test_request_context('/'):
...     bar = copy_current_request_context(pow)
...     pickle.dumps(bar)
...
Traceback (most recent call last):
  File "<stdin>", line 3, in <module>
_pickle.PicklingError: Can't pickle <function pow at 0x110bee8c8>: it's not the same object as builtins.pow

接下来我尝试了dill。它通过了上述两个测试，但它不能序列化许多其他可能最终出现在上下文中的东西。应用程序上下文特别容易将扩展链接到其中：SQLAlchemy 就是一个很好的例子。当您使用 SQLAlchemy 并尝试序列化您的应用上下文时，会发生以下情况：

>>> from flask_sqlalchemy import SQLAlchemy
>>> db = SQLAlchemy(app)
>>> with app.test_request_context('/'):
...     from flask.globals import _app_ctx_stack
...     appctx = _app_ctx_stack.top
...     dill.dumps(appctx)
...
Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
  File "/Users/dchevell/Development/python3/sandbox/env/lib/python3.7/site-packages/dill/_dill.py", line 294, in dumps
    dump(obj, file, protocol, byref, fmode, recurse)#, strictio)
  File "/Users/dchevell/Development/python3/sandbox/env/lib/python3.7/site-packages/dill/_dill.py", line 287, in dump
    pik.dump(obj)
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/pickle.py", line 437, in dump
    self.save(obj)
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/pickle.py", line 549, in save
    self.save_reduce(obj=obj, *rv)
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/pickle.py", line 662, in save_reduce
    save(state)
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/pickle.py", line 504, in save
    f(self, obj) # Call unbound method with explicit self
  File "/Users/dchevell/Development/python3/sandbox/env/lib/python3.7/site-packages/dill/_dill.py", line 902, in save_module_dict
    StockPickler.save_dict(pickler, obj)
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/pickle.py", line 856, in save_dict
    self._batch_setitems(obj.items())
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/pickle.py", line 882, in _batch_setitems
    save(v)
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/pickle.py", line 549, in save
    self.save_reduce(obj=obj, *rv)
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/pickle.py", line 662, in save_reduce
    save(state)
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/pickle.py", line 504, in save
    f(self, obj) # Call unbound method with explicit self
  File "/Users/dchevell/Development/python3/sandbox/env/lib/python3.7/site-packages/dill/_dill.py", line 902, in save_module_dict
    StockPickler.save_dict(pickler, obj)
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/pickle.py", line 856, in save_dict
    self._batch_setitems(obj.items())
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/pickle.py", line 882, in _batch_setitems
    save(v)
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/pickle.py", line 504, in save
    f(self, obj) # Call unbound method with explicit self
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/pickle.py", line 816, in save_list
    self._batch_appends(obj)
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/pickle.py", line 843, in _batch_appends
    save(tmp[0])
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/pickle.py", line 504, in save
    f(self, obj) # Call unbound method with explicit self
  File "/Users/dchevell/Development/python3/sandbox/env/lib/python3.7/site-packages/dill/_dill.py", line 1386, in save_function
    obj.__dict__), obj=obj)
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/pickle.py", line 638, in save_reduce
    save(args)
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/pickle.py", line 504, in save
    f(self, obj) # Call unbound method with explicit self
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/pickle.py", line 786, in save_tuple
    save(element)
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/pickle.py", line 504, in save
    f(self, obj) # Call unbound method with explicit self
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/pickle.py", line 771, in save_tuple
    save(element)
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/pickle.py", line 504, in save
    f(self, obj) # Call unbound method with explicit self
  File "/Users/dchevell/Development/python3/sandbox/env/lib/python3.7/site-packages/dill/_dill.py", line 1129, in save_cell
    pickler.save_reduce(_create_cell, (f,), obj=obj)
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/pickle.py", line 638, in save_reduce
    save(args)
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/pickle.py", line 504, in save
    f(self, obj) # Call unbound method with explicit self
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/pickle.py", line 771, in save_tuple
    save(element)
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/pickle.py", line 549, in save
    self.save_reduce(obj=obj, *rv)
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/pickle.py", line 662, in save_reduce
    save(state)
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/pickle.py", line 504, in save
    f(self, obj) # Call unbound method with explicit self
  File "/Users/dchevell/Development/python3/sandbox/env/lib/python3.7/site-packages/dill/_dill.py", line 902, in save_module_dict
    StockPickler.save_dict(pickler, obj)
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/pickle.py", line 856, in save_dict
    self._batch_setitems(obj.items())
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/pickle.py", line 882, in _batch_setitems
    save(v)
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/pickle.py", line 549, in save
    self.save_reduce(obj=obj, *rv)
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/pickle.py", line 662, in save_reduce
    save(state)
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/pickle.py", line 504, in save
    f(self, obj) # Call unbound method with explicit self
  File "/Users/dchevell/Development/python3/sandbox/env/lib/python3.7/site-packages/dill/_dill.py", line 902, in save_module_dict
    StockPickler.save_dict(pickler, obj)
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/pickle.py", line 856, in save_dict
    self._batch_setitems(obj.items())
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/pickle.py", line 882, in _batch_setitems
    save(v)
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/pickle.py", line 549, in save
    self.save_reduce(obj=obj, *rv)
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/pickle.py", line 662, in save_reduce
    save(state)
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/pickle.py", line 504, in save
    f(self, obj) # Call unbound method with explicit self
  File "/Users/dchevell/Development/python3/sandbox/env/lib/python3.7/site-packages/dill/_dill.py", line 902, in save_module_dict
    StockPickler.save_dict(pickler, obj)
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/pickle.py", line 856, in save_dict
    self._batch_setitems(obj.items())
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/pickle.py", line 882, in _batch_setitems
    save(v)
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/pickle.py", line 504, in save
    f(self, obj) # Call unbound method with explicit self
  File "/Users/dchevell/Development/python3/sandbox/env/lib/python3.7/site-packages/dill/_dill.py", line 1330, in save_type
    StockPickler.save_global(pickler, obj)
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/pickle.py", line 957, in save_global
    (obj, module_name, name)) from None
_pickle.PicklingError: Can't pickle <class 'sqlalchemy.orm.session.SignallingSession'>: it's not found as sqlalchemy.orm.session.SignallingSession

那么：有什么办法可以解决这个问题吗？我的一个想法是“及时”分叉一个新进程 - 即在我想运行我的后台任务之前 - 可能意味着我根本不需要复制上下文，它应该已经有了它分叉发生在上下文设置之后。但是，我不知道如何使用ProcessPoolExecutor 进行这项工作，其中进程是一个长期存在的池。我唯一的想法是以某种方式强制工作进程在每个任务结束时关闭，但我很确定这只会导致池损坏。

【问题讨论】：

这是一个非常巧妙的问题，因此是一个巨大的 +1。你有任何进展吗？我试图在烧瓶中使用带有opencv的futures，但是futures + opencv不能很好地发挥作用，这将我推向了loky，这导致了我当前的问题。 【参考方案1】：

不确定这是否适合您，但有可能使用来自其他进程的 Flask 应用程序上下文。它要求您使用application factory 模式。

# your_project/app.py

def create_app(config_file):
    # Load the config file somehow
    # Create your app instance
    app = Flask(__name__)
    # Initialize your extensions
    db.init_app(app)
    # Mount routes, etc
    app.register_blueprint(...)
    # Return your app
    return app

# your_project/wsgi.py
from .app import create_app

app = create_app("./config/local.cfg")

您定义了一个应用程序工厂并使用它来创建您的应用程序作为wsgi.py 或其他方便位置的入口点，然后您可以从后台脚本访问此应用程序实例，该脚本将在另一个进程中运行。


# your_project/background_script.py
from .wsgi import app

with app.app_context():
    # Do some stuff

顺便说一句，这也是将 Flask 与 Celery 一起使用的一种方式，例如，如 in this question 所述。希望对您有所帮助。

【讨论】：

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