//go:build linux

package terminal

import (
	"fmt"
	"os"
	"syscall"

	"golang.org/x/sys/unix"
)

// openPTY allocates a pseudo-terminal and returns both of its ends.
//
// Linux hands out the slave by number: unlock the pair, ask which number it
// got, and the device is /dev/pts/<number>.
func openPTY() (master, slave *os.File, err error) {
	master, err = os.OpenFile("/dev/ptmx", os.O_RDWR|unix.O_NOCTTY, 0)
	if err != nil {
		return nil, nil, fmt.Errorf("terminal: opening /dev/ptmx: %w", err)
	}

	// A pair starts locked so that nothing can open the slave between its
	// creation and the master being ready for it.
	if err := unix.IoctlSetPointerInt(int(master.Fd()), unix.TIOCSPTLCK, 0); err != nil {
		master.Close()
		return nil, nil, fmt.Errorf("terminal: unlocking the pseudo-terminal: %w", err)
	}

	number, err := unix.IoctlGetInt(int(master.Fd()), unix.TIOCGPTN)
	if err != nil {
		master.Close()
		return nil, nil, fmt.Errorf("terminal: naming the pseudo-terminal: %w", err)
	}

	slave, err = os.OpenFile(fmt.Sprintf("/dev/pts/%d", number), os.O_RDWR|unix.O_NOCTTY, 0)
	if err != nil {
		master.Close()
		return nil, nil, fmt.Errorf("terminal: opening the slave side: %w", err)
	}
	return master, slave, nil
}

// setWinsize tells the pseudo-terminal how big it is, which the kernel passes
// on to the shell as SIGWINCH.
func setWinsize(master *os.File, width, height int) error {
	return unix.IoctlSetWinsize(int(master.Fd()), unix.TIOCSWINSZ, &unix.Winsize{
		Row: uint16(height),
		Col: uint16(width),
	})
}

// childAttributes put the shell in a session of its own with the slave as its
// controlling terminal.
//
// Without this the shell has no controlling terminal, and job control — Ctrl-C,
// Ctrl-Z, running something in the background — does not work at all.
func childAttributes() *syscall.SysProcAttr {
	return &syscall.SysProcAttr{Setsid: true, Setctty: true}
}