Using Prefect 2 with FastAPI, RuntimeError on event loop

[soleilchang]

I'm trying to use FastAPI + Prefect 2(2.8.3), the idea is to use RESTful API defined in FastAPI to trigger workflow defined in Prefect.
Code as following:
main.py

from fastapi import FastAPI
from test_workflow import test

app = FastAPI(
    title="Data Services",
    description="REST API for data services",
    version="0.1.0",
)

@app.get("/")
async def root():
    return {"message": "Data Services"}

@app.get("/test")
def trigger():
    test()
    return "completed"

test_workflow.py

from prefect import flow, task
import time

@task
def sleep1():
    print("sleep1")
    time.sleep(5)  // to monitor calculation that needs time to finish
    return 1
    
@flow()
def test():
    task_1 = sleep1.submit()
    x = task_1.result()
    print(x)

Need to run the following commands to enable the services：

$ prefect server start

$ uvicorn main:app --reload

When I try GET /test, the following error encountered:
RuntimeError: <asyncio.locks.Event object at 0x7f0de4d19bd0 [unset]> is bound to a different event loop

If I define as asynchronous function in FastAPI async def trigger(): it will finish without error. But the problem is that, in this way FastAPI can only handle one /test request at a time (because the workflow is supposed to be blocking that needs time to finish).

If it is defined without async, normally FastAPI should run it in its ThreadPool to avoid blocking, but got the error as above.

Any idea on this?