mgsh/input.go

package main

// input.go — the y/n prompts that guard the destructive commands.
//
// These questions are the only thing standing between `init` and a wiped
// server repository, so the terminal handling here has to be exactly right: a
// question that cannot be answered is worse than no question at all.

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

	"github.com/chzyer/readline"
)

// yesno asks a y/n question with a default and reads a single keypress. It is a
// variable because it guards the destructive operations: tests replace it to
// drive those paths without a terminal, and to assert that the question was
// asked at all.
//
// When the keypress cannot be read at all the answer is no, whatever the
// default says — a question nobody saw must never be taken as consent.
var yesno = func(prompt string, def bool) bool {
	suffix := " y/N ? "
	if def {
		suffix = " Y/n ? "
	}
	key, ok := getkey(prompt + suffix)
	if !ok {
		errorln("could not read an answer from the terminal — assuming no")
		return false
	}
	ans := strings.ToLower(strings.TrimSpace(key))
	if ans == "" {
		return def
	}
	return ans == "y"
}

// keyModeArgs put the terminal into single-key input: one keypress is delivered
// as it is typed, and it is not echoed.
//
// MIN and TIME are set explicitly, and that is not decoration. They are not
// part of the canonical/non-canonical switch — they live in their own slots of
// the control-character array and survive `stty icanon`. drainTTY leaves them
// at "min 0 time 0" ("return what is buffered, do not wait"), so without this
// the *second* question of a session read zero bytes, answered itself with its
// default, and left the keypress queued for the next prompt line.
var keyModeArgs = []string{"-icanon", "-echo", "min", "1", "time", "0"}

// getkey reads a single keypress from the terminal without echo, and reports
// whether it got one. It reads a single byte directly from stdin; readline only
// reads while it is inside Readline(), and we are inside command execution
// here, so there is no reader to desync with.
//
// Anything else already typed on the same line is discarded: answering "yes"
// to a y/n prompt must not leave "es\n" queued for the next readline call,
// where it would come back as a bogus command.
func getkey(prompt string) (string, bool) {
	fmt.Print(prompt)

	restore := func() {}
	if stdinIsTTY() {
		restore = singleKeyMode()
	}
	var buf [1]byte
	n, err := os.Stdin.Read(buf[:])
	restore()

	if err != nil || n == 0 {
		fmt.Println()
		return "", false
	}
	key := strings.Trim(string(buf[:n]), "\r\n\t")
	fmt.Println(key)
	return key, true
}

// singleKeyMode switches the terminal to single-key input and returns the
// function that puts it back. The previous settings are restored verbatim from
// `stty -g` rather than by naming the flags we changed: naming them is how the
// MIN/TIME above were left behind in the first place, and mgsh should hand the
// terminal back exactly as it found it.
func singleKeyMode() func() {
	saved, err := sttyRun("-g")
	saved = strings.TrimSpace(saved)
	sttyRun(keyModeArgs...)

	if err != nil || saved == "" {
		return func() { drainTTY(); sttyRun("icanon", "echo") } // best effort
	}
	return func() { drainTTY(); sttyRun(strings.Fields(saved)...) }
}

// drainTTY discards input already queued on the terminal. `min 0 time 0` makes
// a read return whatever is buffered without waiting, so this cannot block when
// nothing is pending. Its caller restores the terminal afterwards.
func drainTTY() {
	sttyRun("-icanon", "-echo", "min", "0", "time", "0")
	buf := make([]byte, 256)
	for {
		n, err := os.Stdin.Read(buf)
		if err != nil || n == 0 {
			return
		}
	}
}

// stdinIsTTY reports whether keypresses come from a terminal. A variable so the
// tests can exercise the terminal path without one.
var stdinIsTTY = func() bool { return readline.IsTerminal(int(os.Stdin.Fd())) }

// sttyRun runs stty on the terminal and returns its output. Errors are silent:
// every caller has a fallback, and a stray "stty: ..." line in the middle of a
// half-printed question helps nobody.
var sttyRun = func(args ...string) (string, error) {
	c := exec.Command("stty", args...)
	c.Stdin = os.Stdin
	out, err := c.Output()
	return string(out), err
}