feature/featuretags: make QR codes modular (#18358)

QR codes are used by `tailscale up --qr` to provide an easy way to
open a web-page without transcribing a difficult URI. However, there’s
no need for this feature if the client will never be called
interactively. So this PR adds the `ts_omit_qrcodes` build tag.

Updates #18182

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

util/qrcodes/format.go

@@ -0,0 +1,22 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package qrcodes
+
+// 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"
+)

util/qrcodes/qrcodes.go

@@ -1,6 +1,8 @@
 // Copyright (c) Tailscale Inc & AUTHORS
 // SPDX-License-Identifier: BSD-3-Clause
 
+//go:build !ts_omit_qrcodes
+
 // Package qrcodes provides functions to render or format QR codes.
 package qrcodes
 
@@ -12,24 +14,6 @@ import (
 	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.

util/qrcodes/qrcodes_disabled.go

@@ -0,0 +1,16 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build ts_omit_qrcodes
+
+package qrcodes
+
+import "io"
+
+func Fprintln(w io.Writer, format Format, s string) (n int, err error) {
+	panic("omitted")
+}
+
+func EncodePNG(s string, size int) ([]byte, error) {
+	panic("omitted")
+}

util/qrcodes/qrcodes_linux.go

@@ -1,7 +1,7 @@
 // Copyright (c) Tailscale Inc & AUTHORS
 // SPDX-License-Identifier: BSD-3-Clause
 
-//go:build linux
+//go:build linux && !ts_omit_qrcodes
 
 package qrcodes
 

util/qrcodes/qrcodes_notlinux.go

@@ -1,7 +1,7 @@
 // Copyright (c) Tailscale Inc & AUTHORS
 // SPDX-License-Identifier: BSD-3-Clause
 
-//go:build !linux
+//go:build !linux && !ts_omit_qrcodes
 
 package qrcodes