id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
32798	StreamingHttpResponse raises SynchronousOnlyOperation in ASGI server	Ralph Broenink	Michael Brown	"When using a ASGI-compliant server, such as Daphne, StreamingHttpResponse's iterator will be executed in an asynchronous context, preventing database queries from being performed (raising SynchronousOnlyOperation). This is not expected behaviour, and appears not to be documented.

*Steps to reproduce*
1. Create a simple project and app. Set-up for use with Channels. We are not going to use this component and the bug does seem to originate from Django itself, but Channels ensures that runserver is ASGI-compliant.
2. Create the following view:

{{{
from django.contrib.contenttypes.models import ContentType
from django.http import StreamingHttpResponse

def test_view(request):
    def generate():
        yield ""hello\n""
        list(ContentType.objects.all())
        yield ""bye\n""
    return StreamingHttpResponse(generate(), content_type=""text/plain"")
}}}

3. Open the page served by test_view
4. Observe the following trace:

{{{
Exception inside application: You cannot call this from an async context - use a thread or sync_to_async.
Traceback (most recent call last):
  File ""venv/lib/python3.8/site-packages/channels/staticfiles.py"", line 44, in __call__
    return await self.application(scope, receive, send)
  File ""venv/lib/python3.8/site-packages/channels/routing.py"", line 71, in __call__
    return await application(scope, receive, send)
  File ""venv/lib/python3.8/site-packages/django/core/handlers/asgi.py"", line 168, in __call__
    await self.send_response(response, send)
  File ""venv/lib/python3.8/site-packages/django/core/handlers/asgi.py"", line 242, in send_response
    for part in response:
  File ""channelstest/testapp/views.py"", line 9, in generate
    list(ContentType.objects.all())
  File ""venv/lib/python3.8/site-packages/django/db/models/query.py"", line 287, in __iter__
    self._fetch_all()
  File ""venv/lib/python3.8/site-packages/django/db/models/query.py"", line 1303, in _fetch_all
    self._result_cache = list(self._iterable_class(self))
  File ""venv/lib/python3.8/site-packages/django/db/models/query.py"", line 53, in __iter__
    results = compiler.execute_sql(chunked_fetch=self.chunked_fetch, chunk_size=self.chunk_size)
  File ""venv/lib/python3.8/site-packages/django/db/models/sql/compiler.py"", line 1152, in execute_sql
    cursor = self.connection.cursor()
  File ""venv/lib/python3.8/site-packages/django/utils/asyncio.py"", line 24, in inner
    raise SynchronousOnlyOperation(message)
django.core.exceptions.SynchronousOnlyOperation: You cannot call this from an async context - use a thread or sync_to_async.
}}}

This error is probably caused by the fact that Django 3 now actively prevents this kind of error (it would have gone undetected in Django 2) and the fact that the iterator is called in an asynchronous context in handlers/asgi.py.

As mentioned above, this issue should at the very least be documented in the documentation, but preferably should be resolved altogether."	Bug	closed	HTTP handling	3.2	Normal	duplicate	ASGI, async	Andrew Godwin Jon Janzen Carles Pina Estany	Accepted	1	0	1	0	0	0