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>
This commit is contained in:
Simon Law
GitHub
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// Copyright (c) Tailscale Inc & AUTHORS
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// SPDX-License-Identifier: BSD-3-Clause
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// Package qrcodes provides functions to render or format QR codes.
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package qrcodes
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import (
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"fmt"
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"io"
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"strings"
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qrcode "github.com/skip2/go-qrcode"
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)
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// Format selects the text representation used to print QR codes.
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type Format string
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const (
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// FormatAuto will format QR codes to best fit the capabilities of the
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// [io.Writer].
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FormatAuto Format = "auto"
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// FormatASCII will format QR codes with only ASCII characters.
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FormatASCII Format = "ascii"
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// FormatLarge will format QR codes with full block characters.
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FormatLarge Format = "large"
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// FormatSmall will format QR codes with full and half block characters.
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FormatSmall Format = "small"
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)
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// Fprintln formats s according to [Format] and writes a QR code to w, along
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// with a newline. It returns the number of bytes written and any write error
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// encountered.
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func Fprintln(w io.Writer, format Format, s string) (n int, err error) {
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const inverse = false // Modern scanners can read QR codes of any colour.
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q, err := qrcode.New(s, qrcode.Medium)
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if err != nil {
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return 0, fmt.Errorf("QR code error: %w", err)
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}
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if format == FormatAuto {
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format, err = detectFormat(w, inverse)
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if err != nil {
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return 0, fmt.Errorf("QR code error: %w", err)
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}
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}
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var out string
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switch format {
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case FormatASCII:
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out = q.ToString(inverse)
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out = strings.ReplaceAll(out, "█", "#")
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case FormatLarge:
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out = q.ToString(inverse)
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case FormatSmall:
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out = q.ToSmallString(inverse)
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default:
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return 0, fmt.Errorf("unknown QR code format: %q", format)
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}
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return fmt.Fprintln(w, out)
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}
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// EncodePNG renders a QR code for s as a PNG, with a width and height of size
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// pixels.
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func EncodePNG(s string, size int) ([]byte, error) {
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q, err := qrcode.New(s, qrcode.Medium)
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if err != nil {
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return nil, err
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}
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return q.PNG(size)
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}
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@@ -0,0 +1,160 @@
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// Copyright (c) Tailscale Inc & AUTHORS
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// SPDX-License-Identifier: BSD-3-Clause
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//go:build linux
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package qrcodes
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import (
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"errors"
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"fmt"
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"io"
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"os"
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"os/exec"
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"strconv"
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"strings"
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"syscall"
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"unsafe"
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"github.com/mattn/go-isatty"
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"golang.org/x/sys/unix"
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)
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func detectFormat(w io.Writer, inverse bool) (format Format, _ error) {
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var zero Format
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// Almost every terminal supports UTF-8, but the Linux
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// console may have partial or no support, which is
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// especially painful inside VMs. See tailscale/tailscale#12935.
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format = FormatSmall
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// Is the locale (LC_CTYPE) set to UTF-8?
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locale, err := locale()
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if err != nil {
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return FormatASCII, fmt.Errorf("QR: %w", err)
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}
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const utf8 = ".UTF-8"
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if !strings.HasSuffix(locale["LC_CTYPE"], utf8) &&
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!strings.HasSuffix(locale["LANG"], utf8) {
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return FormatASCII, nil
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}
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// Are we printing to a terminal?
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f, ok := w.(*os.File)
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if !ok {
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return format, nil
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}
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if !isatty.IsTerminal(f.Fd()) {
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return format, nil
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}
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fd := f.Fd()
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// On a Linux console, check that the current keyboard
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// is in Unicode mode. See unicode_start(1).
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const K_UNICODE = 0x03
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kbMode, err := ioctlGetKBMode(fd)
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if err != nil {
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if errors.Is(err, syscall.ENOTTY) {
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return format, nil
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}
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return zero, err
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}
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if kbMode != K_UNICODE {
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return FormatASCII, nil
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}
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// On a raw Linux console, detect whether the block
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// characters are available in the current font by
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// consulting the Unicode-to-font mapping.
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unimap, err := ioctlGetUniMap(fd)
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if err != nil {
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return zero, err
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}
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if _, ok := unimap['█']; ok {
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format = FormatLarge
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}
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if _, ok := unimap['▀']; ok && inverse {
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format = FormatSmall
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}
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if _, ok := unimap['▄']; ok && !inverse {
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format = FormatSmall
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}
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return format, nil
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}
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func locale() (map[string]string, error) {
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locale := map[string]string{
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"LANG": os.Getenv("LANG"),
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"LC_CTYPE": os.Getenv("LC_CTYPE"),
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}
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cmd := exec.Command("locale")
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out, err := cmd.Output()
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if err != nil {
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if errors.Is(err, exec.ErrNotFound) {
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return locale, nil
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}
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return nil, fmt.Errorf("locale error: %w", err)
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}
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for line := range strings.SplitSeq(string(out), "\n") {
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if line == "" {
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continue
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}
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k, v, found := strings.Cut(line, "=")
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if !found {
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continue
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}
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v, err := strconv.Unquote(v)
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if err != nil {
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continue
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}
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locale[k] = v
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}
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return locale, nil
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}
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func ioctlGetKBMode(fd uintptr) (int, error) {
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const KDGKBMODE = 0x4b44
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mode, err := unix.IoctlGetInt(int(fd), KDGKBMODE)
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if err != nil {
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return 0, fmt.Errorf("keyboard mode error: %w", err)
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}
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return mode, nil
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}
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func ioctlGetUniMap(fd uintptr) (map[rune]int, error) {
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const GIO_UNIMAP = 0x4B66 // get unicode-to-font mapping from kernel
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var ud struct {
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Count uint16
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Entries uintptr // pointer to unipair array
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}
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type unipair struct {
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Unicode uint16 // Unicode value
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FontPos uint16 // Font position in the console font
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}
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// First, get the number of entries:
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_, _, errno := unix.Syscall(unix.SYS_IOCTL, fd, GIO_UNIMAP, uintptr(unsafe.Pointer(&ud)))
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if errno != 0 && !errors.Is(errno, syscall.ENOMEM) {
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return nil, fmt.Errorf("unicode mapping error: %w", errno)
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}
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// Then allocate enough space and get the entries themselves:
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if ud.Count == 0 {
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return nil, nil
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}
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entries := make([]unipair, ud.Count)
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ud.Entries = uintptr(unsafe.Pointer(&entries[0]))
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_, _, errno = unix.Syscall(unix.SYS_IOCTL, fd, GIO_UNIMAP, uintptr(unsafe.Pointer(&ud)))
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if errno != 0 {
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return nil, fmt.Errorf("unicode mapping error: %w", errno)
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}
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unimap := make(map[rune]int)
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for _, e := range entries {
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unimap[rune(e.Unicode)] = int(e.FontPos)
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}
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return unimap, nil
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}
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@@ -0,0 +1,14 @@
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// Copyright (c) Tailscale Inc & AUTHORS
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// SPDX-License-Identifier: BSD-3-Clause
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//go:build !linux
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package qrcodes
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import "io"
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func detectFormat(w io.Writer, inverse bool) (Format, error) {
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// Assume all terminals can support the full set of UTF-8 block
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// characters: (█, ▀, ▄). See tailscale/tailscale#12935.
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return FormatSmall, nil
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}
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