Fixed #30451 -- Added ASGI handler and coroutine-safety.

This adds an ASGI handler, asgi.py file for the default project layout,
a few async utilities and adds async-safety to many parts of Django.

tests/asgi/tests.py

@@ -0,0 +1,84 @@
+import sys
+
+from asgiref.sync import async_to_sync
+from asgiref.testing import ApplicationCommunicator
+
+from django.core.asgi import get_asgi_application
+from django.core.signals import request_started
+from django.db import close_old_connections
+from django.test import SimpleTestCase, override_settings
+
+from .urls import test_filename
+
+
+@override_settings(ROOT_URLCONF='asgi.urls')
+class ASGITest(SimpleTestCase):
+
+    def setUp(self):
+        request_started.disconnect(close_old_connections)
+
+    def _get_scope(self, **kwargs):
+        return {
+            'type': 'http',
+            'asgi': {'version': '3.0', 'spec_version': '2.1'},
+            'http_version': '1.1',
+            'method': 'GET',
+            'query_string': b'',
+            'server': ('testserver', 80),
+            **kwargs,
+        }
+
+    def tearDown(self):
+        request_started.connect(close_old_connections)
+
+    @async_to_sync
+    async def test_get_asgi_application(self):
+        """
+        get_asgi_application() returns a functioning ASGI callable.
+        """
+        application = get_asgi_application()
+        # Construct HTTP request.
+        communicator = ApplicationCommunicator(application, self._get_scope(path='/'))
+        await communicator.send_input({'type': 'http.request'})
+        # Read the response.
+        response_start = await communicator.receive_output()
+        self.assertEqual(response_start['type'], 'http.response.start')
+        self.assertEqual(response_start['status'], 200)
+        self.assertEqual(
+            set(response_start['headers']),
+            {
+                (b'Content-Length', b'12'),
+                (b'Content-Type', b'text/html; charset=utf-8'),
+            },
+        )
+        response_body = await communicator.receive_output()
+        self.assertEqual(response_body['type'], 'http.response.body')
+        self.assertEqual(response_body['body'], b'Hello World!')
+
+    @async_to_sync
+    async def test_file_response(self):
+        """
+        Makes sure that FileResponse works over ASGI.
+        """
+        application = get_asgi_application()
+        # Construct HTTP request.
+        communicator = ApplicationCommunicator(application, self._get_scope(path='/file/'))
+        await communicator.send_input({'type': 'http.request'})
+        # Get the file content.
+        with open(test_filename, 'rb') as test_file:
+            test_file_contents = test_file.read()
+        # Read the response.
+        response_start = await communicator.receive_output()
+        self.assertEqual(response_start['type'], 'http.response.start')
+        self.assertEqual(response_start['status'], 200)
+        self.assertEqual(
+            set(response_start['headers']),
+            {
+                (b'Content-Length', str(len(test_file_contents)).encode('ascii')),
+                (b'Content-Type', b'text/plain' if sys.platform.startswith('win') else b'text/x-python'),
+                (b'Content-Disposition', b'inline; filename="urls.py"'),
+            },
+        )
+        response_body = await communicator.receive_output()
+        self.assertEqual(response_body['type'], 'http.response.body')
+        self.assertEqual(response_body['body'], test_file_contents)