Fastapi: [BUG] async BackgroundTasks run in the foreground

[ebanner]

Describe the bug
async BackgroundTasks are not running as background tasks. My response blocks until the task finishes.

To Reproduce
Steps to reproduce the behavior:

1. Create a file run.py with contents

from fastapi import FastAPI, BackgroundTasks

import time
import asyncio

app = FastAPI()

async def sleep_for_10_seconds():
    start = time.time()
    while True:
        now = time.time()
        if now - start > 10: 
            break
        asyncio.sleep(1)
    print(time.time(), 'slept for 10 seconds')


@app.get("/")
async def read_root(background_tasks: BackgroundTasks):
    print(time.time(), 'read_root')
    background_tasks.add_task(sleep_for_10_seconds)
    return {"Hello": "World"}

1. Run with uvicorn run:app --reload
2. Run curl http://localhost:8000

Expected behavior
{"Hello": "World"} returns immediately. Instead it takes 10 seconds.

Environment:

• macOS
• fastapi==0.27.0
• uvicorn==0.8.4
• Python 3.6.6 :: Anaconda, Inc.

Additional context
I originally thought this problem was confined to asyncio.ensure_future() in #403, but I'm getting the same behavior with fastapi.BackgroundTasks.

sync tasks return immediately (i.e. don't define with async).

[euri10]

try with def sleep_for_10_seconds(): instead of async def sleep_for_10_seconds():

[dmontagu]

@euri10 That may be a reasonable short term workaround, but I don’t think you should have to pay the threadpool overhead just to get background tasks to finish after returning the response.. If this is the behavior on the most recent version of FastAPI I think it merits a fix.

@ebanner I’ll look into this as soon as I can (hopefully tomorrow), but in the mean time could you confirm that it isn’t fixed by updating to the most recent version of fastapi?

[euri10]

/agree

you can also use it the Starlette way but you have to declare your JSONResponse and won't benefit from the docs goodies

import time
import asyncio

app = FastAPI()

async def sleep_for_10_seconds():
    start = time.time()
    while True:
        now = time.time()
        if now - start > 10:
            break
        asyncio.sleep(1)
    print(time.time(), 'slept for 10 seconds')


@app.get("/")
async def read_root():
    print(time.time(), 'read_root')
    background_tasks = BackgroundTasks()
    background_tasks.add_task(sleep_for_10_seconds)
    return JSONResponse({"Hello": "World"}, background=background_tasks)

[dmontagu]

@euri10 @ebanner I think I found the problem:

async def sleep_for_10_seconds():
    start = time.time()
    while True:
        now = time.time()
        if now - start > 10:
            break
        await asyncio.sleep(1)  # <-- HERE (was missing the await)
    print(time.time(), 'slept for 10 seconds')

After making this change, it seems to behave properly.

[euri10]

evil :+1:

[ebanner]

Ah, my mistake!

Thank you for pointing it out.

[tiangolo]

Thanks @dmontagu and @euri10 for the help!

Thanks @ebanner for reporting back and closing the issue.