From 9243435b5e7e327d0e1e1271b6dfd4c74ca7120b Mon Sep 17 00:00:00 2001
From: Andrew Godwin <andrew@aeracode.org>
Date: Mon, 2 Dec 2019 13:02:21 -0700
Subject: [PATCH] [3.0.x] Fixed #31056 -- Allowed disabling async-unsafe check
 with an environment variable.

Backport of c90ab30fa1305481024b9c3c50b5a6ed6cd9a2f5 from master
---
 django/utils/asyncio.py | 18 ++++++++++--------
 docs/releases/3.0.1.txt |  5 ++++-
 docs/spelling_wordlist  |  1 +
 docs/topics/async.txt   | 20 ++++++++++++++++++++
 tests/async/tests.py    | 13 ++++++++++++-
 5 files changed, 47 insertions(+), 10 deletions(-)

diff --git a/django/utils/asyncio.py b/django/utils/asyncio.py
index c4de04ba127ca..2405e3413e119 100644
--- a/django/utils/asyncio.py
+++ b/django/utils/asyncio.py
@@ -1,5 +1,6 @@
 import asyncio
 import functools
+import os
 
 from django.core.exceptions import SynchronousOnlyOperation
 
@@ -12,14 +13,15 @@ def async_unsafe(message):
     def decorator(func):
         @functools.wraps(func)
         def inner(*args, **kwargs):
-            # Detect a running event loop in this thread.
-            try:
-                event_loop = asyncio.get_event_loop()
-            except RuntimeError:
-                pass
-            else:
-                if event_loop.is_running():
-                    raise SynchronousOnlyOperation(message)
+            if not os.environ.get('DJANGO_ALLOW_ASYNC_UNSAFE'):
+                # Detect a running event loop in this thread.
+                try:
+                    event_loop = asyncio.get_event_loop()
+                except RuntimeError:
+                    pass
+                else:
+                    if event_loop.is_running():
+                        raise SynchronousOnlyOperation(message)
             # Pass onwards.
             return func(*args, **kwargs)
         return inner
diff --git a/docs/releases/3.0.1.txt b/docs/releases/3.0.1.txt
index 18e2ea44c4a77..589ef40499dcc 100644
--- a/docs/releases/3.0.1.txt
+++ b/docs/releases/3.0.1.txt
@@ -9,4 +9,7 @@ Django 3.0.1 fixes several bugs in 3.0.
 Bugfixes
 ========
 
-* ...
+* Fixed a regression in Django 3.0 by restoring the ability to use Django
+  inside Jupyter and other environments that force an async context, by adding
+  and option to disable :ref:`async-safety` mechanism with
+  ``DJANGO_ALLOW_ASYNC_UNSAFE`` environment variable (:ticket:`31056`).
diff --git a/docs/spelling_wordlist b/docs/spelling_wordlist
index 9c56339cc0fde..6a36a117073f9 100644
--- a/docs/spelling_wordlist
+++ b/docs/spelling_wordlist
@@ -309,6 +309,7 @@ ize
 JavaScript
 Jinja
 jQuery
+Jupyter
 jython
 Kaplan
 Kessler
diff --git a/docs/topics/async.txt b/docs/topics/async.txt
index 6a6f0030c9be0..b341084b1abb3 100644
--- a/docs/topics/async.txt
+++ b/docs/topics/async.txt
@@ -12,6 +12,8 @@ There is limited support for other parts of the async ecosystem; namely, Django
 can natively talk :doc:`ASGI </howto/deployment/asgi/index>`, and some async
 safety support.
 
+.. _async-safety:
+
 Async-safety
 ------------
 
@@ -34,3 +36,21 @@ code from an async context; instead, write your code that talks to async-unsafe
 in its own, synchronous function, and call that using
 ``asgiref.sync.async_to_sync``, or any other preferred way of running
 synchronous code in its own thread.
+
+If you are *absolutely* in dire need to run this code from an asynchronous
+context - for example, it is being forced on you by an external environment,
+and you are sure there is no chance of it being run concurrently (e.g. you are
+in a Jupyter_ notebook), then you can disable the warning with the
+``DJANGO_ALLOW_ASYNC_UNSAFE`` environment variable.
+
+.. warning::
+
+    If you enable this option and there is concurrent access to the
+    async-unsafe parts of Django, you may suffer data loss or corruption. Be
+    very careful and do not use this in production environments.
+
+If you need to do this from within Python, do that with ``os.environ``::
+
+    os.environ["DJANGO_ALLOW_ASYNC_UNSAFE"] = "true"
+
+.. _Jupyter: https://jupyter.org/
diff --git a/tests/async/tests.py b/tests/async/tests.py
index 450a38018da81..f42e549075593 100644
--- a/tests/async/tests.py
+++ b/tests/async/tests.py
@@ -1,5 +1,6 @@
+import os
 import sys
-from unittest import skipIf
+from unittest import mock, skipIf
 
 from asgiref.sync import async_to_sync
 
@@ -39,3 +40,13 @@ async def test_async_unsafe(self):
         )
         with self.assertRaisesMessage(SynchronousOnlyOperation, msg):
             self.dangerous_method()
+
+    @mock.patch.dict(os.environ, {'DJANGO_ALLOW_ASYNC_UNSAFE': 'true'})
+    @async_to_sync
+    async def test_async_unsafe_suppressed(self):
+        # Decorator doesn't trigger check when the environment variable to
+        # suppress it is set.
+        try:
+            self.dangerous_method()
+        except SynchronousOnlyOperation:
+            self.fail('SynchronousOnlyOperation should not be raised.')