[BUG] async BackgroundTasks run in the foreground

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).

[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.

[gary-x0pa]

I realised if you replaced asyncio.sleep with time.sleep it blocks the request anyone has a reason why async background tasks does not run in separate thread and how to potentially solve this problem