Which version of dd-trace-py are you using?

ddtrace==0.55.4

Which version of pip are you using?

21.2.4

Which version of the libraries are you using?

fastapi==0.68.2
starlette==0.14.2

How can we reproduce your problem?

this would be a minimal proof of concept app.py, running through ddtrace-run uvicorn app:app

import asyncio

from ddtrace import tracer
from fastapi import FastAPI, BackgroundTasks

app = FastAPI()


async def some_background_task():
    with tracer.start_span("some_background_task", activate=True):
        tracer.context_provider.activate(None)
        await asyncio.sleep(10)


@app.get("/")
async def main(background_tasks: BackgroundTasks):
    background_tasks.add_task(some_background_task)
    return "ok"

What is the result that you get?

The duration of / is reported to be 10s, while the browser immediately receives the response.
some_background_task is also reported with a duration of 10s.

What is the result that you expected?

I would expect that the reported endpoint duration matches the time it took to get the response, and that the background task is reported separately. Please don't mind that tracer.context_provider.activate(None) might be redundant here, adding it here to show what I have tried.

FastAPI's add_task actually comes from starlette https://www.starlette.io/background/

I can understand why the endpoint duration includes the background task, this is the definition of starlette's Response.__call__:

https://github.com/encode/starlette/blob/ada99beee530e7b841ce320bc6e66f6dbd9ad781/starlette/responses.py#L159

    async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
        await send(
            {
                "type": "http.response.start",
                "status": self.status_code,
                "headers": self.raw_headers,
            }
        )
        await send({"type": "http.response.body", "body": self.body})

        if self.background is not None:
            await self.background()

The response header and body is sent, but the function itself is not finished until all background tasks have been processed.

I believe that this is not what users of ddtrace would expect: the background tasks are used to return a response early without waiting for background operations to finish ; the reported endpoint duration should correspond to when the body was sent