content size limit

This is a middleware for ASGI which intercepts the receive() method to raise an exception when the read bytes exceed the given limit.

Example

import uvicorn

from starlette.applications import Starlette
from starlette.responses import PlainTextResponse

from content_size_limit_asgi import ContentSizeLimitMiddleware


app = Starlette()

@app.route("/", methods=["POST"])
async def index(request):
    body = await request.body()
    return PlainTextResponse(f"body: {body.decode('utf-8')}")


app.add_middleware(ContentSizeLimitMiddleware, max_content_size=512)


if __name__ == "__main__":
    uvicorn.run(app, host="127.0.0.1", port=6001, log_level='debug')

To test the app:

$ curl --limit-rate 5k -q -v http://localhost:6001/ -d `printf 'A%.0s' {1..99999}`
*   Trying 127.0.0.1...
* TCP_NODELAY set
* Connected to localhost (127.0.0.1) port 6001 (#0)
> POST / HTTP/1.1
> Host: localhost:6001
> User-Agent: curl/7.54.0
> Accept: */*
> Content-Length: 99999
> Content-Type: application/x-www-form-urlencoded
> Expect: 100-continue
>
< HTTP/1.1 100 Continue
< HTTP/1.1 500 Internal Server Error
< date: Wed, 12 Jun 2019 14:41:28 GMT
< server: uvicorn
< content-length: 21
< content-type: text/plain; charset=utf-8
* HTTP error before end of send, stop sending
<
* Closing connection 0
Internal Server Error

The app console log should read:

$ PYTHONPATH=. python tapp.py
INFO: Started server process [48160]
INFO: Waiting for application startup.
DEBUG: None - ASGI [1] Started
WARNING 2019-06-12 14:42:18,003 content_size_limit.middleware: ASGI scope of type lifespan is not supported yet
WARNING: ASGI scope of type lifespan is not supported yet
DEBUG: None - ASGI [1] Sent {'type': 'lifespan.startup'}
DEBUG: None - ASGI [1] Received {'type': 'lifespan.startup.complete'}
INFO: Uvicorn running on http://127.0.0.1:6001 (Press CTRL+C to quit)
DEBUG: ('127.0.0.1', 52103) - Connected
DEBUG: ('127.0.0.1', 52103) - ASGI [2] Started
DEBUG: ('127.0.0.1', 52103) - ASGI [2] Sent {'type': 'http.request', 'body': '<16384 bytes>', 'more_body': True}
DEBUG: ('127.0.0.1', 52103) - ASGI [2] Received {'type': 'http.response.start', 'status': 500, 'headers': '<...>'}
INFO: ('127.0.0.1', 52103) - "POST / HTTP/1.1" 500
DEBUG: ('127.0.0.1', 52103) - ASGI [2] Received {'type': 'http.response.body', 'body': '<21 bytes>'}
DEBUG: ('127.0.0.1', 52103) - ASGI [2] Raised exception
ERROR: Exception in ASGI application
Traceback (most recent call last):
  File "/Users/ses/.pyenv/versions/3.7.3/lib/python3.7/site-packages/uvicorn/protocols/http/httptools_impl.py", line 368, in run_asgi
    result = await app(self.scope, self.receive, self.send)
  File "/Users/ses/.pyenv/versions/3.7.3/lib/python3.7/site-packages/uvicorn/middleware/message_logger.py", line 58, in __call__
    raise exc from None
  File "/Users/ses/.pyenv/versions/3.7.3/lib/python3.7/site-packages/uvicorn/middleware/message_logger.py", line 54, in __call__
    await self.app(scope, inner_receive, inner_send)
  File "/Users/ses/.pyenv/versions/3.7.3/lib/python3.7/site-packages/starlette/applications.py", line 133, in __call__
    await self.error_middleware(scope, receive, send)
  File "/Users/ses/.pyenv/versions/3.7.3/lib/python3.7/site-packages/starlette/middleware/errors.py", line 122, in __call__
    raise exc from None
  File "/Users/ses/.pyenv/versions/3.7.3/lib/python3.7/site-packages/starlette/middleware/errors.py", line 100, in __call__
    await self.app(scope, receive, _send)
  File "/Users/ses/w/content-size-limit-asgi/content_size_limit/middleware.py", line 48, in __call__
    await self.app(scope, wrapper, send)
  File "/Users/ses/.pyenv/versions/3.7.3/lib/python3.7/site-packages/starlette/exceptions.py", line 73, in __call__
    raise exc from None
  File "/Users/ses/.pyenv/versions/3.7.3/lib/python3.7/site-packages/starlette/exceptions.py", line 62, in __call__
    await self.app(scope, receive, sender)
  File "/Users/ses/.pyenv/versions/3.7.3/lib/python3.7/site-packages/starlette/routing.py", line 585, in __call__
    await route(scope, receive, send)
  File "/Users/ses/.pyenv/versions/3.7.3/lib/python3.7/site-packages/starlette/routing.py", line 207, in __call__
    await self.app(scope, receive, send)
  File "/Users/ses/.pyenv/versions/3.7.3/lib/python3.7/site-packages/starlette/routing.py", line 40, in app
    response = await func(request)
  File "tapp.py", line 13, in index
    body = await request.body()
  File "/Users/ses/.pyenv/versions/3.7.3/lib/python3.7/site-packages/starlette/requests.py", line 167, in body
    async for chunk in self.stream():
  File "/Users/ses/.pyenv/versions/3.7.3/lib/python3.7/site-packages/starlette/requests.py", line 152, in stream
    message = await self._receive()
  File "/Users/ses/w/content-size-limit-asgi/content_size_limit/middleware.py", line 36, in inner
    f"Maximum content size limit ({self.max_content_size}) exceeded ({received} bytes read)"
content_size_limit.errors.ContentSizeExceeded: Maximum content size limit (512) exceeded (16384 bytes read)
DEBUG: ('127.0.0.1', 52103) - Disconnected

Why not just raise in the route / view functon itself?

Depending on the ASGI server/framework used, you might not have access to the raw stream to stop reading immediately once the maximum content size has been exceeded.

Take this Starlette view for example:

@app.route("/documents/upload", methods=["POST"])
def upload_document(request):
    data = await request.body()
    if len(data) > Config.MAX_FILE_SIZE:
        return api_400(
            f"This file exceeds the maximum file size we support at this time ({Config.MAX_FILE_SIZE})",
            code=MAX_FILE_SIZE_EXCEEDED,
        )
    ...

If the maximum file size is 5MB, and the uploaded file was 50MB, then this implementation reads the entire 50MB into memory before rejecting the request.