Extending systemd generator?

[Lombra]

Hi,

I want to add some extra functionality to quadlets, and I figured systemd generators combined with unit dropin files should come in handy. First I thought of defining entirely new directives, but I think that might not be doable without breaking the existing generator.

One thing I want to do is use a couple of shorthands for the rather verbose Traefik labelling system. Writing your own generators seems simple enough. Ditto for some basic parsing of quadlets, but I'm curious as to whether there are any existing utilities I can use to this end? I was unable to find the source for podman-system-generator.

[eriksjolund]

I was unable to find the source for podman-system-generator.

See the file
https://github.com/containers/podman/blob/main/cmd/quadlet/main.go

Here is how I found it

1. Run command

   /usr/lib/systemd/system-generators/podman-system-generator -help
   

   The following output is printed

   Usage of /usr/lib/systemd/system-generators/podman-system-generator:
     -dryrun
       	Run in dryrun mode printing debug information
     -no-kmsg-log
       	Don't log to kmsg
     -user
       	Run as systemd user
     -v	Print debug information
     -version
       	Print version information and exit
   

2. Clone repo

   git clone https://github.com/containers/podman.git
   

3. Change dir

   cd podman
   

4. Search for Run in dryrun mode printing debug information

    git grep "Run in dryrun mode printing debug information"
   

   The following output is printed

   cmd/quadlet/main.go:    flag.BoolVar(&dryRunFlag, "dryrun", false, "Run in dryrun mode printing debug information")
   

[ygalblum]

Yes, this is the entry point for Quadlet

[Lombra]

Ok, I guess I should have read up more on generators. They only have write access to the final unit directory, and are only intended to be used that way, which I suppose makes sense. I was hoping to write .container.d dropins, and let the existing generator take care of things. I would have to build the ExecStart directive from scratch, unless anyone have any other ideas.

[Lombra]

I've ended up with this. It's a little hacky, but it works well enough. It goes in /usr/lib/systemd/system-generators (though /etc/systemd/system-generators might be more appropriate) and parses the quadlet files (not all possible directories) and generates unit dropins to override the generated podman units. Using labels is the best way I could think of for defining arbitrary properties. Other possible features include:

• Checking the environment variable SYSTEMD_SCOPE to determine whether system or user quadlets should be processed, but user processing might require placing the generator in a different directory, such as /etc/systemd/user-generators?
• Parsing the generated ExecStart directive. You could remove your fake labels by generating a new ExecStart with labels removed. You will want to make sure the original generator runs before yours, somehow.

#!/bin/python3

import sys
import os
import glob
import re
from pathlib import Path

CONTAINER_DIR = '/etc/containers/systemd'
TRAEFIK_DIR = '/etc/traefik/dynamic/containers.d'

normal_dir = sys.argv[1]

def get_container_info(path):
        container = {
                'name': Path(path).stem,
                'labels': {},
        }

        with open(path) as file:
                for line in file:
                        line = line.strip()
                        match = re.match(r'\[(.+)\]', line)
                        if match:
                                section_name = match.group(1)
                        elif not (line.startswith('#') or line.startswith(';')):
                                match = re.match(r'(.+?)=(.+)', line)
                                if match:
                                        key = match.group(1)
                                        value = match.group(2)
                                        if key == 'Label':
                                                key, value = value.split('=', maxsplit=1)
                                                container['labels'][key] = value

        return container

for file in glob.glob(f'{CONTAINER_DIR}/*.container'):
        container = get_container_info(file)
        proxy_host = container['labels'].get('proxy.host')
        if proxy_host is not None:
                unit_file = Path(os.path.join(normal_dir, container['name'] + '.service.d', 'proxy.conf'))
                unit_file.parent.mkdir(exist_ok=True)
                with open(unit_file, 'w') as o:
                        config_path = f'{TRAEFIK_DIR}/%N.yaml'
                        proxy_args = proxy_host.replace(',', ' ')
                        proxy_port = container['labels'].get('proxy.port')
                        if proxy_port is not None:
                                proxy_args = f'-p {proxy_port} {proxy_args}'
                        o.write('[Service]\n')
                        o.write(f'ExecStartPost=/usr/local/bin/create_proxy_config %N -o {config_path} {proxy_args}\n')
                        o.write(f'ExecStopPost=rm {config_path}\n')