cmd/tailscale/cli, util/qrcodes: format QR codes on Linux consoles (#18182)

Raw Linux consoles support UTF-8, but we cannot assume that all UTF-8
characters are available. The default Fixed and Terminus fonts don’t
contain half-block characters (`▀` and `▄`), but do contain the
full-block character (`█`).

Sometimes, Linux doesn’t have a framebuffer, so it falls back to VGA.
When this happens, the full-block character could be anywhere in
extended ASCII block, because we don’t know which code page is active.

This PR introduces `--qr-format=auto` which tries to heuristically
detect when Tailscale is printing to a raw Linux console, whether
UTF-8 is enabled, and which block characters have been mapped in the
console font.

If Unicode characters are unavailable, the new `--qr-format=ascii`
formatter uses `#` characters instead of full-block characters.

Fixes #12935

Signed-off-by: Simon Law <sfllaw@tailscale.com>

util/qrcodes/qrcodes.go

@@ -0,0 +1,75 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package qrcodes provides functions to render or format QR codes.
+package qrcodes
+
+import (
+	"fmt"
+	"io"
+	"strings"
+
+	qrcode "github.com/skip2/go-qrcode"
+)
+
+// Format selects the text representation used to print QR codes.
+type Format string
+
+const (
+	// FormatAuto will format QR codes to best fit the capabilities of the
+	// [io.Writer].
+	FormatAuto Format = "auto"
+
+	// FormatASCII will format QR codes with only ASCII characters.
+	FormatASCII Format = "ascii"
+
+	// FormatLarge will format QR codes with full block characters.
+	FormatLarge Format = "large"
+
+	// FormatSmall will format QR codes with full and half block characters.
+	FormatSmall Format = "small"
+)
+
+// Fprintln formats s according to [Format] and writes a QR code to w, along
+// with a newline. It returns the number of bytes written and any write error
+// encountered.
+func Fprintln(w io.Writer, format Format, s string) (n int, err error) {
+	const inverse = false // Modern scanners can read QR codes of any colour.
+
+	q, err := qrcode.New(s, qrcode.Medium)
+	if err != nil {
+		return 0, fmt.Errorf("QR code error: %w", err)
+	}
+
+	if format == FormatAuto {
+		format, err = detectFormat(w, inverse)
+		if err != nil {
+			return 0, fmt.Errorf("QR code error: %w", err)
+		}
+	}
+
+	var out string
+	switch format {
+	case FormatASCII:
+		out = q.ToString(inverse)
+		out = strings.ReplaceAll(out, "█", "#")
+	case FormatLarge:
+		out = q.ToString(inverse)
+	case FormatSmall:
+		out = q.ToSmallString(inverse)
+	default:
+		return 0, fmt.Errorf("unknown QR code format: %q", format)
+	}
+
+	return fmt.Fprintln(w, out)
+}
+
+// EncodePNG renders a QR code for s as a PNG, with a width and height of size
+// pixels.
+func EncodePNG(s string, size int) ([]byte, error) {
+	q, err := qrcode.New(s, qrcode.Medium)
+	if err != nil {
+		return nil, err
+	}
+	return q.PNG(size)
+}