feat: introduce optional stateful user management

[JulienMalka]

In non-nixos settings, it is frequent that some users/groups are created by default by the distribution, or impurely by the user. Because userborn is stateless, those users/groups are disabled when they are not included in the userborn configuration. This change introduce a stateful mode, activated by the USERBORN_STATEFUL environement variable, where userborn maintain a list of users/groups it owns and only act on entities of that list.

[nikstur]

Thank you for raising this issue and working on this implementation!

However, I feel that this is very complicated. In fact too complicated for my liking. It touches much of the core code of Userborn which makes it impossible for me to say in good conscience that this change is correct.

I believe this can be implemented in a much simpler and less intrusive way. See #38

[nikstur]

This whole thing could be `

Suggested change

	let json = serde_json::to_string_pretty(&managed)
	.with_context(|| "Failed to serialize managed entities")?;
	let mut file = OpenOptions::new()
	.write(true)
	.create(true)
	.truncate(true)
	.open(&self.state_file_path)
	.with_context(|| format!("Failed to create/open state file: {:?}", self.state_file_path))?;
	file.write_all(json.as_bytes())
	.with_context(|| "Failed to write state file")?;
	fs::write(&self.state_file_path, serde_json::to_vec_pretty(&managed).context("")?).context("")?

[nikstur]

Suggested change

	log::debug!("Saved managed entities to state file: {:?}", self.state_file_path);
	log::info!("Saved managed entities to state file: {:?}", self.state_file_path);

[nikstur]

Suggested change

	let mut file = File::open(&self.state_file_path)
	.with_context(|| format!("Failed to open state file: {:?}", self.state_file_path))?;
	let mut contents = String::new();
	file.read_to_string(&mut contents)
	.with_context(|| "Failed to read state file")?;
	let contents = fs::read(&self.state_file_path).context("")?;

[nikstur]

Suggested change

	let managed: ManagedEntities = serde_json::from_str(&contents)
	let managed: ManagedEntities = serde_json::from_slice(&contents)

[nikstur]

serde is not used if I see that correctly.

Suggested change

	#[derive(Deserialize, Debug, Clone, PartialEq, serde::Serialize)]
	#[derive(Deserialize, Debug, Clone, PartialEq)]

[nikstur]

This would currently write this file right to /etc. This should be the StateDirectory of the Userborn service.

[nikstur]

This would log this info on every single invocation. That's too much noise for an info log.

[JulienMalka]

Thanks for the review! I do agree that your version is more clean and minimalist, so closing this!