End the mnemonic prompt at end of input (closes #78)

term.ReadPassword reads the terminal with a raw read that returns 0
bytes at the end of input and takes that as nothing yet, so Ctrl-D at
the prompt did nothing. On the Unix systems the prompt now sets the
terminal as ReadPassword does and reads the line from standard input,
which reports the end of input; elsewhere, such as Windows, it still
uses ReadPassword. Ctrl-D with nothing typed gives an empty mnemonic,
refused with status 1, and the terminal settings are put back.

A test runs the tool on a pseudo-terminal (github.com/creack/pty, at a
commit past v1.1.24, whose Open on Linux can return another pty's
terminal) and types Ctrl-D at the prompt.

Model: opus-5-5

internal/mnemonic/mnemonic.go

@@ -136,7 +136,7 @@ func ask() (string, error) {
 
 	fmt.Fprint(os.Stderr, "mnemonic: ")
 
-	typed, err := term.ReadPassword(fd)
+	typed, err := readWithoutEcho()
 
 	fmt.Fprintln(os.Stderr)
 
@@ -144,7 +144,7 @@ func ask() (string, error) {
 		return "", fmt.Errorf("reading the mnemonic: %w", err)
 	}
 
-	return checked(string(typed))
+	return checked(typed)
 }
 
 // checked joins the words with single spaces, whatever whitespace

internal/mnemonic/prompt_notunix.go

@@ -0,0 +1,18 @@
+//go:build !aix && !darwin && !dragonfly && !freebsd && !linux && !netbsd && !openbsd && !solaris && !zos
+
+package mnemonic
+
+import (
+	"os"
+
+	"golang.org/x/term"
+)
+
+// readWithoutEcho reads one line from the terminal on standard input
+// with echo turned off, through term.ReadPassword, on the systems
+// prompt_unix.go is not built for, such as Windows.
+func readWithoutEcho() (string, error) {
+	typed, err := term.ReadPassword(int(os.Stdin.Fd()))
+
+	return string(typed), err
+}

internal/mnemonic/prompt_unix.go

@@ -0,0 +1,48 @@
+//go:build aix || darwin || dragonfly || freebsd || linux || netbsd || openbsd || solaris || zos
+
+package mnemonic
+
+import (
+	"bufio"
+	"errors"
+	"fmt"
+	"io"
+	"os"
+
+	"golang.org/x/sys/unix"
+)
+
+// readWithoutEcho reads one line from the terminal on standard input
+// with echo turned off. The line ends at a newline or at the end of
+// input (Ctrl-D). The terminal is set as term.ReadPassword sets it;
+// ReadPassword itself is not used here because it never sees the end
+// of input and goes on waiting.
+func readWithoutEcho() (string, error) {
+	fd := int(os.Stdin.Fd())
+
+	settings, err := unix.IoctlGetTermios(fd, ioctlReadTermios)
+	if err != nil {
+		return "", fmt.Errorf("reading the terminal settings: %w", err)
+	}
+
+	hidden := *settings
+	hidden.Lflag &^= unix.ECHO
+	hidden.Lflag |= unix.ICANON | unix.ISIG
+	hidden.Iflag |= unix.ICRNL
+
+	err = unix.IoctlSetTermios(fd, ioctlWriteTermios, &hidden)
+	if err != nil {
+		return "", fmt.Errorf("turning echo off: %w", err)
+	}
+
+	defer func() { _ = unix.IoctlSetTermios(fd, ioctlWriteTermios, settings) }()
+
+	// With ICANON set, a read from the terminal returns at most one
+	// line, so the buffer takes nothing past the newline.
+	typed, err := bufio.NewReader(os.Stdin).ReadString('\n')
+	if errors.Is(err, io.EOF) {
+		return typed, nil
+	}
+
+	return typed, err
+}

internal/mnemonic/termios_bsd.go

@@ -0,0 +1,12 @@
+//go:build darwin || dragonfly || freebsd || netbsd || openbsd
+
+package mnemonic
+
+import "golang.org/x/sys/unix"
+
+// The requests that read and set the terminal settings, as
+// golang.org/x/term names them.
+const (
+	ioctlReadTermios  = unix.TIOCGETA
+	ioctlWriteTermios = unix.TIOCSETA
+)

internal/mnemonic/termios_other.go

@@ -0,0 +1,12 @@
+//go:build aix || linux || solaris || zos
+
+package mnemonic
+
+import "golang.org/x/sys/unix"
+
+// The requests that read and set the terminal settings, as
+// golang.org/x/term names them.
+const (
+	ioctlReadTermios  = unix.TCGETS
+	ioctlWriteTermios = unix.TCSETS
+)