RuntimeError: Response content longer than Content-Length

[saddy001]

This is uvicorn==0.34.0 with FastAPI. In some rare cases I get the above exception. I have read #1442 and I'm not sure if the clients request incomplete content. All I can tell is the clients should be usual web browsers.

Since I collect the local variables beneth the traceback I had a look at what's being sent and there are some special characters being sent as bytes (see traceback with variables below):

body = b'1\n00:00:00,000 --> 00:00:01,900\n\xe9\x80\x99\xe5\x80\x8b\xe7\xbe\x8e\xe9\xba\x97\xe7\x9a\x84\xe5\xa5\xb3\xe5\xad\xa9\xe5\x8f\xab\xe5 ...

Maybe the content length is calculated incorrectly with those characters?

The traceback:

Traceback (most recent call last):
  File "/opt/python3/lib/python3.12/site-packages/uvicorn/protocols/http/httptools_impl.py", line 409, in run_asgi
    result = await app(  # type: ignore[func-returns-value]
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/python3/lib/python3.12/site-packages/uvicorn/middleware/proxy_headers.py", line 60, in __call__
    return await self.app(scope, receive, send)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/python3/lib/python3.12/site-packages/fastapi/applications.py", line 1054, in __call__
    await super().__call__(scope, receive, send)
  File "/opt/python3/lib/python3.12/site-packages/starlette/applications.py", line 112, in __call__
    await self.middleware_stack(scope, receive, send)
  File "/opt/python3/lib/python3.12/site-packages/starlette/middleware/errors.py", line 187, in __call__
    raise exc
  File "/opt/python3/lib/python3.12/site-packages/starlette/middleware/errors.py", line 165, in __call__
    await self.app(scope, receive, _send)
  File "/opt/python3/lib/python3.12/site-packages/starlette/middleware/exceptions.py", line 62, in __call__
    await wrap_app_handling_exceptions(self.app, conn)(scope, receive, send)
  File "/opt/python3/lib/python3.12/site-packages/starlette/_exception_handler.py", line 53, in wrapped_app
    raise exc
  File "/opt/python3/lib/python3.12/site-packages/starlette/_exception_handler.py", line 42, in wrapped_app
    await app(scope, receive, sender)
  File "/opt/python3/lib/python3.12/site-packages/starlette/routing.py", line 714, in __call__
    await self.middleware_stack(scope, receive, send)
  File "/opt/python3/lib/python3.12/site-packages/starlette/routing.py", line 734, in app
    await route.handle(scope, receive, send)
  File "/opt/python3/lib/python3.12/site-packages/starlette/routing.py", line 288, in handle
    await self.app(scope, receive, send)
  File "/opt/python3/lib/python3.12/site-packages/starlette/routing.py", line 76, in app
    await wrap_app_handling_exceptions(app, request)(scope, receive, send)
  File "/opt/python3/lib/python3.12/site-packages/starlette/_exception_handler.py", line 53, in wrapped_app
    raise exc
  File "/opt/python3/lib/python3.12/site-packages/starlette/_exception_handler.py", line 42, in wrapped_app
    await app(scope, receive, sender)
  File "/opt/python3/lib/python3.12/site-packages/starlette/routing.py", line 74, in app
    await response(scope, receive, send)
  File "/opt/python3/lib/python3.12/site-packages/starlette/responses.py", line 359, in __call__
    await self._handle_simple(send, send_header_only)
  File "/opt/python3/lib/python3.12/site-packages/starlette/responses.py", line 388, in _handle_simple
    await send({"type": "http.response.body", "body": chunk, "more_body": more_body})
  File "/opt/python3/lib/python3.12/site-packages/starlette/_exception_handler.py", line 39, in sender
    await send(message)
  File "/opt/python3/lib/python3.12/site-packages/starlette/_exception_handler.py", line 39, in sender
    await send(message)
  File "/opt/python3/lib/python3.12/site-packages/starlette/middleware/errors.py", line 162, in _send
    await send(message)
  File "/opt/python3/lib/python3.12/site-packages/uvicorn/protocols/http/httptools_impl.py", line 536, in send
    raise RuntimeError("Response content longer than Content-Length")
RuntimeError: Response content longer than Content-Length

The traceback with variables:

...
  File "/opt/python3/lib/python3.12/site-packages/uvicorn/protocols/http/httptools_impl.py", line 536, in send
    raise RuntimeError("Response content longer than Content-Length")
      self = <uvicorn.protocols.http.httptools_impl.RequestResponseCycle object at 0x7342ca3e3080>(access_log=False, access_logger=<Logger uvicorn.access (INFO)>, body=b'', chunked>
      message = {'type': 'http.response.body', 'body': b'1\n00:00:00,000 --> 00:00:01,900\n\xe9\x80\x99\xe5\x80\x8b\xe7\xbe\x8e\xe9\xba\x97\xe7\x9a\x84\xe5\xa5\xb3\xe5\xad\xa9\xe5\>
      message_type = 'http.response.body'
      body = b'1\n00:00:00,000 --> 00:00:01,900\n\xe9\x80\x99\xe5\x80\x8b\xe7\xbe\x8e\xe9\xba\x97\xe7\x9a\x84\xe5\xa5\xb3\xe5\xad\xa9\xe5\x8f\xab\xe5\x81\x9a\xe8\x89\xbe\xe6\x9b\xb>
      more_body = False
      num_bytes = 7322
builtins.RuntimeError: Response content longer than Content-Length

[janas-adam]

hi @saddy001
can u provide code snippet with it?

[saddy001]

I couldn't reproduce it locally. What I can provide is the following code:

from fastapi import FastAPI
from fastapi.responses import Response
import uvicorn

app = FastAPI()


@app.get("/subtitle")
async def read_subtitle():
    content = """1
00:00:00,000 --> 00:00:01,900
那個...

""".strip().encode("utf-8")  # this is equivalent to b'1\n...'

    # Set intentionally incorrect Content-Length
    headers = {
        "Content-Length": "10"
    }

    return Response(content=content, headers=headers, media_type="application/octet-stream")


if __name__ == '__main__':
    uvicorn.run('test:app', host='::', port=8001, log_level='info')

curl -g -6 'http://[::1]:8001/subtitle'

This triggers the error 'manually'. However, I don't know at which point the content length is being calculated incorrectly. But I don't set the content length anywhere in the real code ;-)

What I was able to dig out of the stack is that uvicorn.protocols.http.httptools_impl.RequestResponseCycle has set expected_content_length=0 while processing the response, which may be related.

[janas-adam]

@saddy001
i wouldn't say its uvicorn bug 🤔
is it possible that this content u want to send is modified pararelly by someone at the same time ?

[saddy001]

The content is routed through nginx as proxy. I don't think that it modifies the content in a relevant way. If not uvicorn, do you have an idea where to address the issue?

[janas-adam]

if your content/resource is modifiable, imo it's race condition
uvicorn received content-length, but before it was sent, data already had different length

u can try to wrap it in asyncio.Lock, to lock resource per coroutine

import asyncio

lock = asyncio.Lock()
...

@app.get("/subtitle")
async def read_subtitle():
    ...
    async with lock:
        content = <your data here>
    return Response(content=content, headers=headers, media_type="application/octet-stream")

u should use lock in every endpoint where resource is used

[saddy001]

Thanks for your suggestion. I see I should have given more information about the endpoint: It's a threaded, streaming one, more like the one below. Sorry that my other example above was a bit misleading in that point.

from threading import Event

from fastapi import FastAPI
from fastapi.responses import StreamingResponse
import uvicorn

app = FastAPI()


@app.get("/subtitle")
def read_subtitle():

    def gen():
        for i in range(10 ** 1000):
            yield str(i)
            Event().wait(0.01)

    return StreamingResponse(gen(), media_type='text/event-stream')


if __name__ == '__main__':
    uvicorn.run('test:app', host='::', port=8001, log_level='info')

I did some more testing to reproduce it, but still no luck:

run_curl() {
    curl -g -6 --no-buffer 'http://[::1]:8001/subtitle'
}

for i in {1..100}
do
    run_curl &
done

# pkill curl

I'm having trouble to understand how a lock would help me in that case. Can you extend on the above example to modify the content while being read to trigger the error?