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. The prompt now sets the terminal as
ReadPassword does and reads the line from standard input, which
reports the end of input. 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/cli/prompt_test.go

@@ -0,0 +1,83 @@
+package cli_test
+
+import (
+	"bufio"
+	"io"
+	"os"
+	"os/exec"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/creack/pty"
+	"github.com/stretchr/testify/require"
+	"golang.org/x/term"
+	"sneak.berlin/go/keyfunc/internal/mnemonic"
+)
+
+// TestEndOfInputAtThePromptEndsTheTool runs "age pub" with no mnemonic
+// given and a terminal on all three streams, waits for the prompt, and
+// types Ctrl-D. The tool has to end with status 1, say that the
+// mnemonic is empty, and leave the terminal set as it found it, so with
+// echo on again.
+//
+// pty.Open returns the two ends of a new terminal: tty is the end the
+// tool uses as its terminal, and ptmx the end the test reads what the
+// terminal shows from and types into. Reading ptmx stops at the
+// deadline, so a prompt that never comes fails the test instead of
+// hanging it.
+func TestEndOfInputAtThePromptEndsTheTool(t *testing.T) {
+	t.Setenv(mnemonic.Variable, "")
+	t.Setenv(mnemonic.CommandVariable, "")
+
+	ptmx, tty, err := pty.Open()
+	require.NoError(t, err)
+
+	defer func() { _ = ptmx.Close() }()
+	defer func() { _ = tty.Close() }()
+
+	require.NoError(t, ptmx.SetReadDeadline(time.Now().Add(10*time.Second)))
+
+	before, err := term.GetState(int(tty.Fd()))
+	require.NoError(t, err)
+
+	//nolint:gosec // this test's own binary as the tool
+	command := exec.CommandContext(t.Context(), os.Args[0], "age", "pub")
+
+	command.Env = append(os.Environ(), runAsTool+"=1")
+	command.Stdin = tty
+	command.Stdout = tty
+	command.Stderr = tty
+
+	require.NoError(t, command.Start())
+
+	var (
+		shown string
+		char  byte
+	)
+
+	terminal := bufio.NewReader(ptmx)
+	for !strings.HasSuffix(shown, "mnemonic: ") {
+		char, err = terminal.ReadByte()
+		require.NoError(t, err, "the tool did not prompt: %q", shown)
+
+		shown += string(char)
+	}
+
+	_, err = ptmx.WriteString("\x04") // Ctrl-D
+	require.NoError(t, err)
+
+	waitForTool(t, "Ctrl-D at the prompt", command)
+	require.Equal(t, failedStatus, command.ProcessState.ExitCode())
+
+	after, err := term.GetState(int(tty.Fd()))
+	require.NoError(t, err)
+	require.Equal(t, before, after, "the terminal settings were not restored")
+
+	// With the tool ended and this end closed too, ptmx gives what is
+	// left on the terminal and then an error.
+	require.NoError(t, tty.Close())
+
+	said, _ := io.ReadAll(terminal)
+	require.Contains(t, string(said), mnemonic.ErrEmpty.Error())
+}