Provide a way to stream output of services

[mboes]

Say I run a service called tickle that immediately prints an error but does not terminate. If I do so using nix .#start-services start, then all I see is:

$ nix run '.#start-services' start
21:03:16 system   | tickle.1 started (pid=415992)

I don't see the error message until the service stops. If it doesn't stop unless I SIGTERM on the terminal, then the output is lost.

[mboes]

I'm not sure what I did different, but I now see the error log. Closing for now. Will reopen if I can reproduce.

[mboes]

Reopening. It's possible to see the output of a service, but only of the service exits. So long as that hasn't happened, then the output is not available.

[thufschmitt]

I can't reproduce. I've tried with the project file below, but the output comes properly streamed (both for stdin and stderr):

let inputs = import "./nickel.lock.ncl" in
let organist = inputs.organist in

{
  shells = organist.shells.Bash,

  services.tickle =
    let start_script =
        {
          name = "start-tickle",
          content.text = nix-s%"
            echo "Starting tickle"
            echo "Starting tickle -- stderr" >&2

            cat
        "%
        } | organist.nix.builders.ShellApplication
      in
      nix-s%"%{start_script}/bin/start-tickle"%,

}
  | organist.OrganistExpression
  & organist.services.Schema

Can you share the service that you were running?

[mboes]

Simple python code that retries connecting to a (non-existent) server:

import requests
import time

def send_request(url):
    url = f"{url}/tickle"
    try:
        response = requests.post(url)
        print("Response Status Code:", response.status_code)
    except requests.exceptions.ConnectionError as e:
        print(e)

def main():
    while True:
        send_request("https://localhost:5000")
        time.sleep(60)

if __name__ == "__main__":
    session = requests.Session()
    session.verify = False
    main()

When run on the command line using poetry run python src/tickle.py, the exception is printed on the terminal every minute. When run through nix run .#start-services start, I see a message saying the service started (as in the description of this issue) and that's it.

[jmou]

Hi I just saw this while driving by. Do you think the issue might be output buffering? I think Python line buffers when attached to a terminal but otherwise aggressively buffers stdout and stderr. Could be worth trying python -u or setting PYTHONUNBUFFERED.