display QR codes for device flow authentication

use package rsc.io/qr to generate QR codes and display them in the
terminal using ANSI escape codes.

Author: ccmtaylor

go.mod

@@ -5,3 +5,5 @@ go 1.23.0
 toolchain go1.24.1
 
 require golang.org/x/oauth2 v0.30.0
+
+require rsc.io/qr v0.2.0

go.sum

@@ -1,2 +1,4 @@
 golang.org/x/oauth2 v0.30.0 h1:dnDm7JmhM45NNpd8FDDeLhK6FwqbOf4MLCM9zb1BOHI=
 golang.org/x/oauth2 v0.30.0/go.mod h1:B++QgG3ZKulg6sRPGD/mqlHQs5rB3Ml9erfeDY7xKlU=
+rsc.io/qr v0.2.0 h1:6vBLea5/NRMVTz8V66gipeLycZMl/+UlFmk8DvqQ6WY=
+rsc.io/qr v0.2.0/go.mod h1:IF+uZjkb9fqyeF/4tlBoynqmQxUoPfWEKh921coOuXs=

main.go

@@ -14,6 +14,7 @@
 package main
 
 import (
+	"bytes"
 	"context"
 	"flag"
 	"fmt"
@@ -31,6 +32,7 @@ import (
 
 	"golang.org/x/oauth2"
 	"golang.org/x/oauth2/endpoints"
+	"rsc.io/qr"
 )
 
 // configByHost lists default config for several public hosts.
@@ -499,10 +501,47 @@ func getDeviceToken(ctx context.Context, c oauth2.Config) (*oauth2.Token, error)
 	if verbose {
 		fmt.Fprintf(os.Stderr, "%+v\n", deviceAuth)
 	}
-	fmt.Fprintf(os.Stderr, "Please enter code %s at %s\n", deviceAuth.UserCode, deviceAuth.VerificationURI)
+	if deviceAuth.VerificationURIComplete != "" {
+		fmt.Fprintf(os.Stderr, "Please scan the QR code or enter code %s at %s\n", deviceAuth.UserCode, deviceAuth.VerificationURI)
+		writeQRCode(os.Stderr, deviceAuth.VerificationURIComplete)
+	} else {
+		fmt.Fprintf(os.Stderr, "Please enter code %s at %s\n", deviceAuth.UserCode, deviceAuth.VerificationURI)
+	}
 	return c.DeviceAccessToken(ctx, deviceAuth)
 }
 
+func writeQRCode(w io.Writer, data string) error {
+	// use low redundancy to generate small QR codes for terminal output.
+	// we assume the user is sitting in front of their screen so image quality shouldn't be an issue
+	code, err := qr.Encode(data, qr.L)
+	if err != nil {
+		return err
+	}
+
+	var (
+		black     = "\033[40m  \033[0m"
+		white     = "\033[107m  \033[0m"
+		whiteLine = strings.Repeat(white, code.Size+2) + "\n"
+		buf       bytes.Buffer
+	)
+	buf.WriteString(whiteLine)
+	for y := range code.Size {
+		for x := range code.Size {
+			if code.Black(x, y) {
+				buf.WriteString(black)
+			} else {
+				buf.WriteString(white)
+			}
+		}
+		buf.WriteString(white)
+		buf.WriteRune('\n')
+	}
+	buf.WriteString(whiteLine)
+
+	_, err = buf.WriteTo(w)
+	return err
+}
+
 func replaceHost(e oauth2.Endpoint, host string) oauth2.Endpoint {
 	e.AuthURL = replaceHostInURL(e.AuthURL, host)
 	e.TokenURL = replaceHostInURL(e.TokenURL, host)

main_test.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"os"
 	"strings"
 	"testing"
 )
@@ -22,6 +23,16 @@ func TestConfig(t *testing.T) {
 	}
 }
 
+func TestQR(t *testing.T) {
+	msg := os.Getenv("QR_MSG")
+	if msg == "" {
+		t.Skip("no QR_MSG set, skipping")
+	}
+	if err := writeQRCode(os.Stdout, msg); err != nil {
+		t.Fatal(err)
+	}
+}
+
 func FuzzParse(f *testing.F) {
 	f.Add("key=value")
 	f.Add("key=")