Put the terminal back when a signal ends the mnemonic prompt (closes #91)

A SIGINT, SIGTERM or SIGHUP at the mnemonic prompt killed the tool
with echo still off. The prompt now catches them with signals.Notify
from before echo goes off, and reads the line in a goroutine. When one
arrives it puts the terminal settings back, stops catching, and sends
the tool the same signal again, so the tool is still ended by it (130
for SIGINT in a shell). A signal the tool was started with set to be
ignored stays ignored. A test sends SIGINT at the prompt on a
pseudo-terminal.

Unchanged: the Windows prompt, which uses term.ReadPassword.

Model: opus-5-5

internal/mnemonic/prompt_unix.go

@@ -8,8 +8,10 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"os/signal"
 
 	"golang.org/x/sys/unix"
+	"sneak.berlin/go/keyfunc/internal/cli/signals"
 )
 
 // readWithoutEcho reads one line from the terminal on standard input
@@ -17,6 +19,10 @@ import (
 // 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.
+//
+// Meanwhile SIGINT, SIGTERM and SIGHUP are caught, as signals.Notify
+// catches them. One that arrives puts the terminal back as it was and
+// then ends the tool, as it would have ended it if it were not caught.
 func readWithoutEcho() (string, error) {
 	fd := int(os.Stdin.Fd())
 
@@ -25,6 +31,13 @@ func readWithoutEcho() (string, error) {
 		return "", fmt.Errorf("reading the terminal settings: %w", err)
 	}
 
+	// Caught from before echo is turned off, so that no signal can end
+	// the tool with echo off.
+	received := make(chan os.Signal, 1)
+	signals.Notify(received)
+
+	defer signal.Stop(received)
+
 	hidden := *settings
 	hidden.Lflag &^= unix.ECHO
 	hidden.Lflag |= unix.ICANON | unix.ISIG
@@ -35,14 +48,54 @@ func readWithoutEcho() (string, error) {
 		return "", fmt.Errorf("turning echo off: %w", err)
 	}
 
-	defer func() { _ = unix.IoctlSetTermios(fd, ioctlWriteTermios, settings) }()
+	restore := func() {
+		_ = unix.IoctlSetTermios(fd, ioctlWriteTermios, settings)
+	}
+	defer restore()
 
-	// 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) {
+	var (
+		typed  string
+		failed error
+	)
+
+	read := make(chan struct{})
+
+	go func() {
+		// With ICANON set, a read from the terminal returns at most one
+		// line, so the buffer takes nothing past the newline.
+		typed, failed = bufio.NewReader(os.Stdin).ReadString('\n')
+
+		close(read)
+	}()
+
+	select {
+	case <-read:
+	case ending := <-received:
+		restore()
+		signal.Stop(received)
+
+		return "", endBy(ending)
+	}
+
+	if errors.Is(failed, io.EOF) {
 		return typed, nil
 	}
 
-	return typed, err
+	return typed, failed
+}
+
+// endBy sends the tool the signal again, now that nothing catches it,
+// so that it ends the tool, and waits for that. It returns only if the
+// signal could not be sent.
+func endBy(ending os.Signal) error {
+	tool, err := os.FindProcess(os.Getpid())
+	if err == nil {
+		err = tool.Signal(ending)
+	}
+
+	if err != nil {
+		return fmt.Errorf("ending the tool by %s: %w", ending, err)
+	}
+
+	select {}
 }