Purposefully drop connection?

[DanCardin]

I dont know if this is an insane question or not. But suppose I want to purposefully cause a ConnectionResetError or other sorts of connection exceptions on the client (for testing their error/retry handling).

Is it possible to induce starlette (through fastapi if relevant) to behave in that way for specific requests?

[adriangb]

Yes, but you'll have to do some pure ASGI stuff. We do it in some of our tests:

starlette/tests/middleware/test_base.py

Lines 746 to 783 in a8b8856

	@pytest.mark.anyio
	async def test_read_request_disconnected_client() -> None:
	"""If we receive a disconnect message when the downstream ASGI
	app calls receive() the Request instance passed into the dispatch function
	should get marked as disconnected.
	The downstream ASGI app should not get a ClientDisconnect raised,
	instead if should just receive the disconnect message.
	"""
	async def endpoint(scope: Scope, receive: Receive, send: Send) -> None:
	msg = await receive()
	assert msg["type"] == "http.disconnect"
	await Response()(scope, receive, send)
	class ConsumingMiddleware(BaseHTTPMiddleware):
	async def dispatch(self, request: Request, call_next: RequestResponseEndpoint):
	response = await call_next(request)
	disconnected = await request.is_disconnected()
	assert disconnected is True
	return response
	scope = {"type": "http", "method": "POST", "path": "/"}
	async def receive() -> AsyncGenerator[Message, None]:
	yield {"type": "http.disconnect"}
	raise AssertionError("Should not be called, would hang") # pragma: no cover
	async def send(msg: Message):
	if msg["type"] == "http.response.start":
	assert msg["status"] == 200
	app: ASGIApp = ConsumingMiddleware(endpoint)
	rcv = receive()
	await app(scope, rcv.__anext__, send)
	await rcv.aclose()

[DanCardin]

In this case you're reacting to the client disconnecting and returning the response normally. I want to simulate the server crashing, for example, otherwise failing to correctly finish responding to the request.

Correct me if these are subtly the same kind of code i'd need to write though. I would think in an example like this i would need to ultimately not return a Response in order to it to look like i failed the request, from the perspective of the client.

[adriangb]

I guess it depends on what you're trying to test: the server, the web framework, etc. I don't exactly understand what you want to test in particular nor can I really spend the time to give you an exact example, but yes you are right it is likely similar to that example test (there are others in the repo) and you should be able to adapt it to your needs. This may be helpful to learn more about ASGI in general: https://youtu.be/fcfyDvK_A6Q?si=sPxa5zOZzW_9VNmV

[DanCardin]

i'm trying to test the client's ability to react to the server misbehaving. but I think you've given me enough to go on (that is, it's either this, or it not supported).

thanks!