package agent

import (
	"fmt"
	"os"
	"os/exec"

	"github.com/creack/pty"
)

// PTY is a pseudo-terminal running an interactive login shell.
type PTY struct {
	cmd *exec.Cmd
	f   *os.File
}

// StartPTY launches a login shell attached to a new PTY. With user empty (or
// equal to the agent's own user) it is the agent's shell — root by default,
// since the agent runs as a root systemd service. Any other user is entered via
// `su -l`, which only a root agent can do without a password.
func StartPTY(user string) (*PTY, error) {
	var cmd *exec.Cmd
	if user == "" || user == currentUser() {
		cmd = exec.Command(defaultShell(), "-l")
	} else {
		if !validUserName(user) {
			return nil, fmt.Errorf("недопустимое имя пользователя %q", user)
		}
		if os.Geteuid() != 0 {
			return nil, fmt.Errorf("агент запущен от %s и не может открыть сессию от %s", currentUser(), user)
		}
		if !userExists(user) {
			return nil, fmt.Errorf("пользователь %s не найден", user)
		}
		su, err := exec.LookPath("su")
		if err != nil {
			return nil, fmt.Errorf("su не найден: %w", err)
		}
		cmd = exec.Command(su, "-l", user)
	}
	cmd.Env = append(os.Environ(), "TERM=xterm-256color")
	f, err := pty.Start(cmd)
	if err != nil {
		return nil, err
	}
	return &PTY{cmd: cmd, f: f}, nil
}

// Read reads terminal output.
func (p *PTY) Read(b []byte) (int, error) { return p.f.Read(b) }

// Write writes keystrokes to the terminal.
func (p *PTY) Write(b []byte) (int, error) { return p.f.Write(b) }

// Resize changes the terminal window size.
func (p *PTY) Resize(cols, rows uint16) {
	if cols == 0 || rows == 0 {
		return
	}
	_ = pty.Setsize(p.f, &pty.Winsize{Rows: rows, Cols: cols})
}

// Close terminates the shell and releases the PTY.
func (p *PTY) Close() error {
	err := p.f.Close()
	if p.cmd.Process != nil {
		_ = p.cmd.Process.Kill()
	}
	_ = p.cmd.Wait()
	return err
}