// Package sysuser runs commands as the current process or as another user,
// and answers questions about local user accounts.
package sysuser

import (
	"fmt"
	"os"
	"os/exec"
	"os/user"
	"path/filepath"
)

// IsRoot reports whether the process runs with euid 0.
func IsRoot() bool { return os.Geteuid() == 0 }

// Current returns the username of the effective user.
func Current() string {
	u, err := user.LookupId(fmt.Sprint(os.Geteuid()))
	if err != nil {
		return ""
	}
	return u.Username
}

// IsSelf reports whether name is the effective user (or empty).
func IsSelf(name string) bool {
	return name == "" || name == Current()
}

// HomeDir returns the home directory of the given user.
func HomeDir(name string) (string, error) {
	u, err := user.Lookup(name)
	if err != nil {
		return "", fmt.Errorf("looking up user %s: %w", name, err)
	}
	return u.HomeDir, nil
}

// UID returns the numeric uid of the given user as a string.
func UID(name string) (string, error) {
	u, err := user.Lookup(name)
	if err != nil {
		return "", fmt.Errorf("looking up user %s: %w", name, err)
	}
	return u.Uid, nil
}

// Command builds a command that runs bin as the given user.
// An empty user, or the current user, runs it directly. Otherwise runuser is
// used (requires root) with XDG_RUNTIME_DIR pointed at the user's runtime dir,
// which rootless podman and systemctl --user need.
func Command(name string, bin string, args ...string) (*exec.Cmd, error) {
	if IsSelf(name) {
		return exec.Command(bin, args...), nil
	}
	if !IsRoot() {
		return nil, fmt.Errorf("running as user %s requires root", name)
	}
	uid, err := UID(name)
	if err != nil {
		return nil, err
	}
	full := append([]string{"-u", name, "--", "env",
		"XDG_RUNTIME_DIR=/run/user/" + uid,
		"DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/" + uid + "/bus",
		bin}, args...)
	return exec.Command("runuser", full...), nil
}

// LookPath finds bin in PATH, then in the user's ~/.local/bin.
// Returns "" if not found.
func LookPath(name string, bin string) string {
	if p, err := exec.LookPath(bin); err == nil {
		return p
	}
	if name == "" {
		name = Current()
	}
	home, err := HomeDir(name)
	if err != nil {
		return ""
	}
	p := filepath.Join(home, ".local", "bin", bin)
	if fi, err := os.Stat(p); err == nil && !fi.IsDir() {
		return p
	}
	return ""
}

// LingerEnabled reports whether systemd lingering is enabled for the user,
// so user units start at boot without a login session.
func LingerEnabled(name string) bool {
	_, err := os.Stat(filepath.Join("/var/lib/systemd/linger", name))
	return err == nil
}