use cap_std::fs::{Dir, OpenOptions};
use pam::constants::{PamFlag, PamResultCode};
use pam::module::{PamHandle, PamHooks};
use std::ffi::CStr;
use std::io::{ErrorKind, Write};
use std::path::Path;
use std::process;

const CG_MOUNT: &str = "/sys/fs/cgroup";

struct PAMUserCG;
pam::pam_hooks!(PAMUserCG);

fn create_and_open_dir<P: AsRef<Path> + Copy>(
    d: &Dir,
    path: P,
) -> std::io::Result<Dir> {
    if let Err(e) = d.create_dir(path) {
        if e.kind() != ErrorKind::AlreadyExists {
            return Err(e);
        }
    }
    d.open_dir(path)
}

struct SessionError;

impl From<std::io::Error> for SessionError {
    fn from(_: std::io::Error) -> Self {
        SessionError
    }
}

fn open_session(h: &mut PamHandle) -> Result<(), SessionError> {
    let user = h.get_user(None).or(Err(SessionError))?;
    let user = users::get_user_by_name(&user).ok_or(SessionError)?;
    let aa = cap_std::ambient_authority();
    let d = Dir::open_ambient_dir(CG_MOUNT, aa)?;
    let d = create_and_open_dir(&d, "user")?;
    let d = create_and_open_dir(&d, &user.uid().to_string())?;
    let d = create_and_open_dir(&d, "leaf")?;
    let pid = process::id().to_string();
    let mut options = OpenOptions::new();
    options.write(true);
    let mut procs = d.open_with("cgroup.procs", &options)?;
    procs.write_all(pid.as_bytes())?;
    Ok(())
}

impl PamHooks for PAMUserCG {
    fn sm_open_session(
        h: &mut PamHandle,
        _args: Vec<&CStr>,
        _flags: PamFlag,
    ) -> PamResultCode {
        if open_session(h).is_ok() {
            PamResultCode::PAM_SUCCESS
        } else {
            PamResultCode::PAM_SESSION_ERR
        }
    }
}