Fixed #31224 -- Added support for asynchronous views and middleware.

This implements support for asynchronous views, asynchronous tests,
asynchronous middleware, and an asynchronous test client.

docs/topics/http/middleware.txt

@@ -71,6 +71,10 @@ method from the handler which takes care of applying :ref:`view middleware
 applying :ref:`template-response <template-response-middleware>` and
 :ref:`exception <exception-middleware>` middleware.
 
+Middleware can either support only synchronous Python (the default), only
+asynchronous Python, or both. See :ref:`async-middleware` for details of how to
+advertise what you support, and know what kind of request you are getting.
+
 Middleware can live anywhere on your Python path.
 
 ``__init__(get_response)``
@@ -282,6 +286,81 @@ if the very next middleware in the chain raises an
 that exception; instead it will get an :class:`~django.http.HttpResponse`
 object with a :attr:`~django.http.HttpResponse.status_code` of 404.
 
+.. _async-middleware:
+
+Asynchronous support
+====================
+
+.. versionadded:: 3.1
+
+Middleware can support any combination of synchronous and asynchronous
+requests. Django will adapt requests to fit the middleware's requirements if it
+cannot support both, but at a performance penalty.
+
+By default, Django assumes that your middleware is capable of handling only
+synchronous requests. To change these assumptions, set the following attributes
+on your middleware factory function or class:
+
+* ``sync_capable`` is a boolean indicating if the middleware can handle
+  synchronous requests. Defaults to ``True``.
+
+* ``async_capable`` is a boolean indicating if the middleware can handle
+  asynchronous requests. Defaults to ``False``.
+
+If your middleware has both ``sync_capable = True`` and
+``async_capable = True``, then Django will pass it the request in whatever form
+it is currently in. You can work out what type of request you have by seeing
+if the ``get_response`` object you are passed is a coroutine function or not
+(using :py:func:`asyncio.iscoroutinefunction`).
+
+The ``django.utils.decorators`` module contains
+:func:`~django.utils.decorators.sync_only_middleware`,
+:func:`~django.utils.decorators.async_only_middleware`, and
+:func:`~django.utils.decorators.sync_and_async_middleware` decorators that
+allow you to apply these flags to middleware factory functions.
+
+The returned callable must match the sync or async nature of the
+``get_response`` method. If you have an asynchronous ``get_response``, you must
+return a coroutine function (``async def``).
+
+``process_view``, ``process_template_response`` and ``process_exception``
+methods, if they are provided, should also be adapted to match the sync/async
+mode. However, Django will individually adapt them as required if you do not,
+at an additional performance penalty.
+
+Here's an example of how to detect and adapt your middleware if it supports
+both::
+
+    import asyncio
+    from django.utils.decorators import sync_and_async_middleware
+
+    @sync_and_async_middleware
+    def simple_middleware(get_response):
+        # One-time configuration and initialization goes here.
+        if asyncio.iscoroutinefunction(get_response):
+            async def middleware(request):
+                # Do something here!
+                response = await get_response(request)
+                return response
+
+        else:
+            def middleware(request):
+                # Do something here!
+                response = get_response(request)
+                return response
+
+        return middleware
+
+.. note::
+
+    If you declare a hybrid middleware that supports both synchronous and
+    asynchronous calls, the kind of call you get may not match the underlying
+    view. Django will optimize the middleware call stack to have as few
+    sync/async transitions as possible.
+
+    Thus, even if you are wrapping an async view, you may be called in sync
+    mode if there is other, synchronous middleware between you and the view.
+
 .. _upgrading-middleware:
 
 Upgrading pre-Django 1.10-style middleware
@@ -292,8 +371,8 @@ Upgrading pre-Django 1.10-style middleware
 
 Django provides ``django.utils.deprecation.MiddlewareMixin`` to ease creating
 middleware classes that are compatible with both :setting:`MIDDLEWARE` and the
-old ``MIDDLEWARE_CLASSES``. All middleware classes included with Django
-are compatible with both settings.
+old ``MIDDLEWARE_CLASSES``, and support synchronous and asynchronous requests.
+All middleware classes included with Django are compatible with both settings.
 
 The mixin provides an ``__init__()`` method that requires a ``get_response``
 argument and stores it in ``self.get_response``.
@@ -345,3 +424,7 @@ These are the behavioral differences between using :setting:`MIDDLEWARE` and
    HTTP response, and then the next middleware in line will see that
    response. Middleware are never skipped due to a middleware raising an
    exception.
+
+.. versionchanged:: 3.1
+
+    Support for asynchronous requests was added to the ``MiddlewareMixin``.

docs/topics/http/views.txt

@@ -202,3 +202,28 @@ in a test view. For example::
             response = self.client.get('/403/')
             # Make assertions on the response here. For example:
             self.assertContains(response, 'Error handler content', status_code=403)
+
+.. _async-views:
+
+Asynchronous views
+==================
+
+.. versionadded:: 3.1
+
+As well as being synchronous functions, views can also be asynchronous
+functions (``async def``). Django will automatically detect these and run them
+in an asynchronous context. You will need to be using an asynchronous (ASGI)
+server to get the full power of them, however.
+
+Here's an example of an asynchronous view::
+
+    from django.http import HttpResponse
+    import datetime
+
+    async def current_datetime(request):
+        now = datetime.datetime.now()
+        html = '<html><body>It is now %s.</body></html>' % now
+        return HttpResponse(html)
+
+You can read more about Django's asynchronous support, and how to best use
+asynchronous views, in :doc:`/topics/async`.