oauth2: support device flow

Device Authorization Grant following RFC 8628 https://datatracker.ietf.org/doc/html/rfc8628

Tested with GitHub

Fixes #418

Fixes golang/go#58126

Co-authored-by: cmP <centimitr@gmail.com>

Change-Id: Id588867110c6a5289bf1026da5d7ead88f9c7d14
GitHub-Last-Rev: 9a126d7b534532c7d18fb8d6796ad673b95fc09f
GitHub-Pull-Request: golang/oauth2#609
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/450155
Commit-Queue: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Matt Hickford <matt.hickford@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>

deviceauth_test.go

@@ -0,0 +1,97 @@
+package oauth2
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/google/go-cmp/cmp"
+	"github.com/google/go-cmp/cmp/cmpopts"
+)
+
+func TestDeviceAuthResponseMarshalJson(t *testing.T) {
+	tests := []struct {
+		name     string
+		response DeviceAuthResponse
+		want     string
+	}{
+		{
+			name:     "empty",
+			response: DeviceAuthResponse{},
+			want:     `{"device_code":"","user_code":"","verification_uri":""}`,
+		},
+		{
+			name: "soon",
+			response: DeviceAuthResponse{
+				Expiry: time.Now().Add(100*time.Second + 999*time.Millisecond),
+			},
+			want: `{"expires_in":100,"device_code":"","user_code":"","verification_uri":""}`,
+		},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			begin := time.Now()
+			gotBytes, err := json.Marshal(tc.response)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if strings.Contains(tc.want, "expires_in") && time.Since(begin) > 999*time.Millisecond {
+				t.Skip("test ran too slowly to compare `expires_in`")
+			}
+			got := string(gotBytes)
+			if got != tc.want {
+				t.Errorf("want=%s, got=%s", tc.want, got)
+			}
+		})
+	}
+}
+
+func TestDeviceAuthResponseUnmarshalJson(t *testing.T) {
+	tests := []struct {
+		name string
+		data string
+		want DeviceAuthResponse
+	}{
+		{
+			name: "empty",
+			data: `{}`,
+			want: DeviceAuthResponse{},
+		},
+		{
+			name: "soon",
+			data: `{"expires_in":100}`,
+			want: DeviceAuthResponse{Expiry: time.Now().UTC().Add(100 * time.Second)},
+		},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			begin := time.Now()
+			got := DeviceAuthResponse{}
+			err := json.Unmarshal([]byte(tc.data), &got)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if !cmp.Equal(got, tc.want, cmpopts.IgnoreUnexported(DeviceAuthResponse{}), cmpopts.EquateApproxTime(time.Second+time.Since(begin))) {
+				t.Errorf("want=%#v, got=%#v", tc.want, got)
+			}
+		})
+	}
+}
+
+func ExampleConfig_DeviceAuth() {
+	var config Config
+	ctx := context.Background()
+	response, err := config.DeviceAuth(ctx)
+	if err != nil {
+		panic(err)
+	}
+	fmt.Printf("please enter code %s at %s\n", response.UserCode, response.VerificationURI)
+	token, err := config.DeviceAccessToken(ctx, response)
+	if err != nil {
+		panic(err)
+	}
+	fmt.Println(token)
+}