Incorrect requests to websockets raise validation error exception

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Example Code

from fastapi import FastAPI, WebSocket
from fastapi.websockets import WebSocketDisconnect


app = FastAPI()


@app.websocket('/test')
async def test(websocket: WebSocket, prefix: str):
	await websocket.accept()
	try:
		while True:
			message = await websocket.receive_text()
			await websocket.send_text(prefix+message)
	except WebSocketDisconnect:
		pass

Description

Hello, When i connect to websocket with prefix query parameter all works fine. But when i connect without prefix parameter, a few bad things happen.

Console 1

D:\temp>uvicorn --no-use-colors ws_test:app
INFO:     Started server process [20404]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)

Console 2

C:\Users\Danstiv>python -m websockets ws://localhost:8000/test
Failed to connect to ws://localhost:8000/test: server rejected WebSocket connection: HTTP 403.

Console 1

INFO:     ('127.0.0.1', 65175) - "WebSocket /test" 403
ERROR:    Exception in ASGI application
Traceback (most recent call last):
  File "c:\python39\lib\site-packages\uvicorn\protocols\websockets\websockets_impl.py", line 171, in run_asgi
    result = await self.app(self.scope, self.asgi_receive, self.asgi_send)
  File "c:\python39\lib\site-packages\uvicorn\middleware\proxy_headers.py", line 59, in __call__
    return await self.app(scope, receive, send)
  File "c:\python39\lib\site-packages\fastapi\applications.py", line 208, in __call__
    await super().__call__(scope, receive, send)
  File "c:\python39\lib\site-packages\starlette\applications.py", line 112, in __call__
    await self.middleware_stack(scope, receive, send)
  File "c:\python39\lib\site-packages\starlette\middleware\errors.py", line 146, in __call__
    await self.app(scope, receive, send)
  File "c:\python39\lib\site-packages\starlette\exceptions.py", line 58, in __call__
    await self.app(scope, receive, send)
  File "c:\python39\lib\site-packages\starlette\routing.py", line 580, in __call__
    await route.handle(scope, receive, send)
  File "c:\python39\lib\site-packages\starlette\routing.py", line 297, in handle
    await self.app(scope, receive, send)
  File "c:\python39\lib\site-packages\starlette\routing.py", line 68, in app
    await func(session)
  File "c:\python39\lib\site-packages\fastapi\routing.py", line 264, in app
    raise WebSocketRequestValidationError(errors)
fastapi.exceptions.WebSocketRequestValidationError: 1 validation error for WebSocket
query -> prefix
  field required (type=value_error.missing)

Intercepted traffic

GET /test HTTP/1.1
Host: localhost:8000
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Key: LKV8aWdjooqq25dxMBhDNQ==
Sec-WebSocket-Version: 13
Sec-WebSocket-Extensions: permessage-deflate; client_max_window_bits
User-Agent: Python/3.9 websockets/9.1


HTTP/1.1 403 Forbidden
Date: Sat, 14 Aug 2021 11:09:36 GMT
Server: Python/3.9 websockets/9.1
Content-Length: 0
Content-Type: text/plain
Connection: close

Problems

1. I do not want invalid requests to lead to exceptions on the server side, as happens with http requests.
2. I want to get detaild error in response to an upgrade request instead of an empty 403 response. I’m sure this is technically possible, but it might be against the websocket spec.

Operating System

Windows

Operating System Details

No response

FastAPI Version

0.68.0

Python Version

Python 3.9.5

Additional Context

websockets 9.1 (pip install -U websockets)

[eboddington]

I am having the same issue and agree with @Danstiv’s conclusion.

From what I recall, the WebSocket spec says that the server should respond with 403 if the path is unexpected. Try, for example, connecting to some random path that you haven’t specified in your FastAPI code - it will respond with 403. So I think that is expected.

What isn’t expected is the server raising exceptions. FastAPI provides the ability to specify input parameters and automatically validates them. This happens outside the scope of a path operation function, so from my eyes if it is raising an exception there then that is a bug as there’s no way for us to catch it. It seems like this should either be handled and the 403 response given for us, or maybe this functionality should be unavailable for WebSocket endpoints.

As it stands, I’m not sure how to proceed as there seems to be no way to prevent clients being able to trigger exceptions on the server (in my case, the parameter is an int so passing an invalid integer can trigger the exception).

[Danstiv]

Code

from fastapi import FastAPI, WebSocket, WebSocketDisconnect, Query
from typing import Optional


app = FastAPI()


@app.get('/test1')
async def test1(value: Optional[int] = Query(1)):
    return 'msg'*value


@app.websocket('/test2')
async def test2(websocket: WebSocket, value: Optional[int] = Query(1)):
    await websocket.accept()
    try:
        while True:
            message = await websocket.receive_text()
            message = message*value
            await websocket.send_text(message)
    except WebSocketDisconnect:
        pass

Console 1

D:\temp>uvicorn --no-use-colors ws_test:app
INFO:     Started server process [52612]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)

Example 1

console 2

C:\Users\Danstiv>curl http://localhost:8000/test1?value=3
"msgmsgmsg"

Console 1

INFO:     127.0.0.1:49950 - "GET /test1?value=3 HTTP/1.1" 200 OK

Console 2

C:\Users\Danstiv>curl http://localhost:8000/test1?value=abc
{"detail":[{"loc":["query","value"],"msg":"value is not a valid integer","type":"type_error.integer"}]}

Console 1

INFO:     127.0.0.1:49952 - "GET /test1?value=abc HTTP/1.1" 422 Unprocessable Entity

Example 2

console 2

C:\Users\Danstiv>python -m websockets ws://localhost:8000/test2?value=3
Connected to ws://localhost:8000/test2?value=3.
> test
< testtesttest

ctrl+c

Connection closed: code = 1000 (OK), no reason.

Console 1

INFO:     ('127.0.0.1', 49960) - "WebSocket /test2" [accepted]

Console 2

C:\Users\Danstiv>python -m websockets ws://localhost:8000/test2?value=abc
Failed to connect to ws://localhost:8000/test2?value=abc: server rejected WebSocket connection: HTTP 403.

Console 1

INFO:     ('127.0.0.1', 49987) - "WebSocket /test2" 403
ERROR:    Exception in ASGI application
Traceback (most recent call last):
  File "c:\python39\lib\site-packages\uvicorn\protocols\websockets\websockets_impl.py", line 171, in run_asgi
    result = await self.app(self.scope, self.asgi_receive, self.asgi_send)
  File "c:\python39\lib\site-packages\uvicorn\middleware\proxy_headers.py", line 59, in __call__
    return await self.app(scope, receive, send)
  File "c:\python39\lib\site-packages\fastapi\applications.py", line 208, in __call__
    await super().__call__(scope, receive, send)
  File "c:\python39\lib\site-packages\starlette\applications.py", line 112, in __call__
    await self.middleware_stack(scope, receive, send)
  File "c:\python39\lib\site-packages\starlette\middleware\errors.py", line 146, in __call__
    await self.app(scope, receive, send)
  File "c:\python39\lib\site-packages\starlette\exceptions.py", line 58, in __call__
    await self.app(scope, receive, send)
  File "c:\python39\lib\site-packages\starlette\routing.py", line 580, in __call__
    await route.handle(scope, receive, send)
  File "c:\python39\lib\site-packages\starlette\routing.py", line 297, in handle
    await self.app(scope, receive, send)
  File "c:\python39\lib\site-packages\starlette\routing.py", line 68, in app
    await func(session)
  File "c:\python39\lib\site-packages\fastapi\routing.py", line 264, in app
    raise WebSocketRequestValidationError(errors)
fastapi.exceptions.WebSocketRequestValidationError: 1 validation error for WebSocket
query -> value
  value is not a valid integer (type=type_error.integer)

Explanation

In example 1, server never raise exceptions, and if validation fails, server sends validation error to the client. In example2 server raise exception, if validation fails, and client receives empty 403 response, that do not contain useful information about validation error. I don’t want a request from the client to be able to raise exceptions on the server, is it possible to avoid raising exceptions? Also, ideally, I would like to return a response to invalid requests containing information about what exactly is wrong.