Unblock periodically the main process to check for signal

[humrochagf]

Summary

This adds a periodic unblock of the main process to check for signals sent like SIGINT or SIGTERM.
In windows the main process does not listen to signals when blocked by the wait call.

Closes: #1972

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[Kludex]

This is a setting that should be used only for --reload.

[humrochagf]

Oh right, do you think it makes sense to add a config just for this? Or is it better to use a fixed timeout/constant value?

[Kludex]

Isn't there an alternative solution? This problem started happening after a feature we added, didn't it?

[humrochagf]

I tried to remove the signal added in #1909 to check but the issue was still there on windows. I'll try to diff from 0.21.1 and isolate the features to find the exact change that caused the issue.

Side note: I also noticed also a flaky startup issue happening on windows while debugging this that I'll dig more on it later.

[Kludex]

Yeah, tests on reload are flaky... I don't know why. You can add the skipif on them as well for the time being. I'll accept a PR for it.

[humrochagf]

Just tested with 0.21.1 and the error of ctrl+c is still there.

I'm running it on Windows 10 with powershell 7.3.6 but I also tested with cmd and got the same freeze.
Python version used for the test is 3.11.1 and the packages are those:

➜ pip freeze
annotated-types==0.5.0
anyio==3.7.1
click==8.1.6
colorama==0.4.6
fastapi==0.100.0
h11==0.14.0
idna==3.4
pydantic==2.1.1
pydantic_core==2.4.0
sniffio==1.3.0
starlette==0.27.0
typing_extensions==4.7.1
uvicorn==0.21.1

Also, the app used for the test was the fastapi base example:

from typing import Union

from fastapi import FastAPI

app = FastAPI()


@app.get("/")
def read_root():
    return {"Hello": "World"}


@app.get("/items/{item_id}")
def read_item(item_id: int, q: Union[str, None] = None):
    return {"item_id": item_id, "q": q}

And the command I'm running is:

uvicorn main:app --workers 2

[humrochagf]

Closing this one since #2183 seems to solve the issue on windows