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58 lines
2.2 KiB
Python
58 lines
2.2 KiB
Python
import asyncio
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import functools
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from fastapi import Request
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async def listen_for_disconnect(request: Request) -> None:
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"""Returns if a disconnect message is received"""
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while True:
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message = await request.receive()
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if message["type"] == "http.disconnect":
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break
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def with_cancellation(handler_func):
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"""Decorator that allows a route handler to be cancelled by client
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disconnections.
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This does _not_ use request.is_disconnected, which does not work with
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middleware. Instead this follows the pattern from
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starlette.StreamingResponse, which simultaneously awaits on two tasks- one
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to wait for an http disconnect message, and the other to do the work that we
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want done. When the first task finishes, the other is cancelled.
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A core assumption of this method is that the body of the request has already
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been read. This is a safe assumption to make for fastapi handlers that have
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already parsed the body of the request into a pydantic model for us.
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This decorator is unsafe to use elsewhere, as it will consume and throw away
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all incoming messages for the request while it looks for a disconnect
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message.
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In the case where a `StreamingResponse` is returned by the handler, this
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wrapper will stop listening for disconnects and instead the response object
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will start listening for disconnects.
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"""
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# Functools.wraps is required for this wrapper to appear to fastapi as a
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# normal route handler, with the correct request type hinting.
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@functools.wraps(handler_func)
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async def wrapper(*args, **kwargs):
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# The request is either the second positional arg or `raw_request`
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request = args[1] if len(args) > 1 else kwargs["raw_request"]
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handler_task = asyncio.create_task(handler_func(*args, **kwargs))
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cancellation_task = asyncio.create_task(listen_for_disconnect(request))
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done, pending = await asyncio.wait([handler_task, cancellation_task],
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return_when=asyncio.FIRST_COMPLETED)
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for task in pending:
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task.cancel()
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if handler_task in done:
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return handler_task.result()
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return None
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return wrapper
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