fastapi background tasks do not work on azure function apps

[warreee]

Please provide us with the following information:

This issue is for a: (mark with an x)

- [x] bug report -> please search issues before submitting
- [ ] feature request
- [ ] documentation issue or request
- [ ] regression (a behavior that used to work and stopped in a new release)

Minimal steps to reproduce

I added background tasks as described in https://fastapi.tiangolo.com/tutorial/background-tasks/.
You can see this in my fork of this repo: https://github.com/warreee/fastapi-on-azure-functions-background-tasks.
When running locally or in Azure will see that a response is not returned immediately but only after 5 seconds (the delay set in the background task).

Any log messages given by the failure

Expected/desired behavior

I expect the function app to give an immediate response while still finishing the background task afterwards.

OS and Version?

MacOs Sonoma

Versions

Function runtime: 4.33.2.2
Python: 3.11

Mention any other details that might be useful

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[eddison-shyi]

Me too, I've add "await asyncio.sleep(20)" in my background task function to test, then response back after 20 secs, and also block another request at same time.