Fixed #31056 -- Allowed disabling async-unsafe check with an environment variable.

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`).

docs/spelling_wordlist

@@ -309,6 +309,7 @@ ize
 JavaScript
 Jinja
 jQuery
+Jupyter
 jython
 Kaplan
 Kessler

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/