oauth2.go - oauth2 - Git at Google

 // Copyright 2014 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 // Package oauth2 provides support for making
 // OAuth2 authorized and authenticated HTTP requests,
 // as specified in RFC 6749.
 // It can additionally grant authorization with Bearer JWT.
 package oauth2 // import "golang.org/x/oauth2"

 import (
 	"bytes"
 	"context"
 	"errors"
 	"net/http"
 	"net/url"
 	"strings"
 	"sync"
 	"time"

 	"golang.org/x/oauth2/internal"
 )

 // NoContext is the default context you should supply if not using
 // your own context.Context (see https://golang.org/x/net/context).
 //
 // Deprecated: Use context.Background() or context.TODO() instead.
 var NoContext = context.TODO()

 // RegisterBrokenAuthHeaderProvider previously did something. It is now a no-op.
 //
 // Deprecated: this function no longer does anything. Caller code that
 // wants to avoid potential extra HTTP requests made during
 // auto-probing of the provider's auth style should set
 // Endpoint.AuthStyle.
 func RegisterBrokenAuthHeaderProvider(tokenURL string) {}

 // Config describes a typical 3-legged OAuth2 flow, with both the
 // client application information and the server's endpoint URLs.
 // For the client credentials 2-legged OAuth2 flow, see the clientcredentials
 // package (https://golang.org/x/oauth2/clientcredentials).
 type Config struct {
 	// ClientID is the application's ID.
 	ClientID string

 	// ClientSecret is the application's secret.
 	ClientSecret string

 	// Endpoint contains the resource server's token endpoint
 	// URLs. These are constants specific to each server and are
 	// often available via site-specific packages, such as
 	// google.Endpoint or github.Endpoint.
 	Endpoint Endpoint

 	// RedirectURL is the URL to redirect users going through
 	// the OAuth flow, after the resource owner's URLs.
 	RedirectURL string

 	// Scope specifies optional requested permissions.
 	Scopes []string

 	// authStyleCache caches which auth style to use when Endpoint.AuthStyle is
 	// the zero value (AuthStyleAutoDetect).
 	authStyleCache internal.LazyAuthStyleCache
 }

 // A TokenSource is anything that can return a token.
 type TokenSource interface {
 	// Token returns a token or an error.
 	// Token must be safe for concurrent use by multiple goroutines.
 	// The returned Token must not be modified.
 	Token() (*Token, error)
 }

 // Endpoint represents an OAuth 2.0 provider's authorization and token
 // endpoint URLs.
 type Endpoint struct {
 	AuthURL       string
 	DeviceAuthURL string
 	TokenURL      string

 	// AuthStyle optionally specifies how the endpoint wants the
 	// client ID & client secret sent. The zero value means to
 	// auto-detect.
 	AuthStyle AuthStyle
 }

 // AuthStyle represents how requests for tokens are authenticated
 // to the server.
 type AuthStyle int

 const (
 	// AuthStyleAutoDetect means to auto-detect which authentication
 	// style the provider wants by trying both ways and caching
 	// the successful way for the future.
 	AuthStyleAutoDetect AuthStyle = 0

 	// AuthStyleInParams sends the "client_id" and "client_secret"
 	// in the POST body as application/x-www-form-urlencoded parameters.
 	AuthStyleInParams AuthStyle = 1

 	// AuthStyleInHeader sends the client_id and client_password
 	// using HTTP Basic Authorization. This is an optional style
 	// described in the OAuth2 RFC 6749 section 2.3.1.
 	AuthStyleInHeader AuthStyle = 2
 )

 var (
 	// AccessTypeOnline and AccessTypeOffline are options passed
 	// to the Options.AuthCodeURL method. They modify the
 	// "access_type" field that gets sent in the URL returned by
 	// AuthCodeURL.
 	//
 	// Online is the default if neither is specified. If your
 	// application needs to refresh access tokens when the user
 	// is not present at the browser, then use offline. This will
 	// result in your application obtaining a refresh token the
 	// first time your application exchanges an authorization
 	// code for a user.
 	AccessTypeOnline  AuthCodeOption = SetAuthURLParam("access_type", "online")
 	AccessTypeOffline AuthCodeOption = SetAuthURLParam("access_type", "offline")

 	// ApprovalForce forces the users to view the consent dialog
 	// and confirm the permissions request at the URL returned
 	// from AuthCodeURL, even if they've already done so.
 	ApprovalForce AuthCodeOption = SetAuthURLParam("prompt", "consent")
 )

 // An AuthCodeOption is passed to Config.AuthCodeURL.
 type AuthCodeOption interface {
 	setValue(url.Values)
 }

 type setParam struct{ k, v string }

 func (p setParam) setValue(m url.Values) { m.Set(p.k, p.v) }

 // SetAuthURLParam builds an AuthCodeOption which passes key/value parameters
 // to a provider's authorization endpoint.
 func SetAuthURLParam(key, value string) AuthCodeOption {
 	return setParam{key, value}
 }

 // AuthCodeURL returns a URL to OAuth 2.0 provider's consent page
 // that asks for permissions for the required scopes explicitly.
 //
 // State is an opaque value used by the client to maintain state between the
 // request and callback. The authorization server includes this value when
 // redirecting the user agent back to the client.
 //
 // Opts may include AccessTypeOnline or AccessTypeOffline, as well
 // as ApprovalForce.
 //
 // To protect against CSRF attacks, opts should include a PKCE challenge
 // (S256ChallengeOption). Not all servers support PKCE. An alternative is to
 // generate a random state parameter and verify it after exchange.
 // See https://datatracker.ietf.org/doc/html/rfc6749#section-10.12 (predating
 // PKCE), https://www.oauth.com/oauth2-servers/pkce/ and
 // https://www.ietf.org/archive/id/draft-ietf-oauth-v2-1-09.html#name-cross-site-request-forgery (describing both approaches)
 func (c *Config) AuthCodeURL(state string, opts ...AuthCodeOption) string {
 	var buf bytes.Buffer
 	buf.WriteString(c.Endpoint.AuthURL)
 	v := url.Values{
 		"response_type": {"code"},
 		"client_id":     {c.ClientID},
 	}
 	if c.RedirectURL != "" {
 		v.Set("redirect_uri", c.RedirectURL)
 	}
 	if len(c.Scopes) > 0 {
 		v.Set("scope", strings.Join(c.Scopes, " "))
 	}
 	if state != "" {
 		v.Set("state", state)
 	}
 	for _, opt := range opts {
 		opt.setValue(v)
 	}
 	if strings.Contains(c.Endpoint.AuthURL, "?") {
 		buf.WriteByte('&')
 	} else {
 		buf.WriteByte('?')
 	}
 	buf.WriteString(v.Encode())
 	return buf.String()
 }

 // PasswordCredentialsToken converts a resource owner username and password
 // pair into a token.
 //
 // Per the RFC, this grant type should only be used "when there is a high
 // degree of trust between the resource owner and the client (e.g., the client
 // is part of the device operating system or a highly privileged application),
 // and when other authorization grant types are not available."
 // See https://tools.ietf.org/html/rfc6749#section-4.3 for more info.
 //
 // The provided context optionally controls which HTTP client is used. See the HTTPClient variable.
 func (c *Config) PasswordCredentialsToken(ctx context.Context, username, password string) (*Token, error) {
 	v := url.Values{
 		"grant_type": {"password"},
 		"username":   {username},
 		"password":   {password},
 	}
 	if len(c.Scopes) > 0 {
 		v.Set("scope", strings.Join(c.Scopes, " "))
 	}
 	return retrieveToken(ctx, c, v)
 }

 // Exchange converts an authorization code into a token.
 //
 // It is used after a resource provider redirects the user back
 // to the Redirect URI (the URL obtained from AuthCodeURL).
 //
 // The provided context optionally controls which HTTP client is used. See the HTTPClient variable.
 //
 // The code will be in the *http.Request.FormValue("code"). Before
 // calling Exchange, be sure to validate FormValue("state") if you are
 // using it to protect against CSRF attacks.
 //
 // If using PKCE to protect against CSRF attacks, opts should include a
 // VerifierOption.
 func (c *Config) Exchange(ctx context.Context, code string, opts ...AuthCodeOption) (*Token, error) {
 	v := url.Values{
 		"grant_type": {"authorization_code"},
 		"code":       {code},
 	}
 	if c.RedirectURL != "" {
 		v.Set("redirect_uri", c.RedirectURL)
 	}
 	for _, opt := range opts {
 		opt.setValue(v)
 	}
 	return retrieveToken(ctx, c, v)
 }

 // Client returns an HTTP client using the provided token.
 // The token will auto-refresh as necessary. The underlying
 // HTTP transport will be obtained using the provided context.
 // The returned client and its Transport should not be modified.
 func (c *Config) Client(ctx context.Context, t *Token) *http.Client {
 	return NewClient(ctx, c.TokenSource(ctx, t))
 }

 // TokenSource returns a TokenSource that returns t until t expires,
 // automatically refreshing it as necessary using the provided context.
 //
 // Most users will use Config.Client instead.
 func (c *Config) TokenSource(ctx context.Context, t *Token) TokenSource {
 	tkr := &tokenRefresher{
 		ctx:  ctx,
 		conf: c,
 	}
 	if t != nil {
 		tkr.refreshToken = t.RefreshToken
 	}
 	return &reuseTokenSource{
 		t:   t,
 		new: tkr,
 	}
 }

 // tokenRefresher is a TokenSource that makes "grant_type"=="refresh_token"
 // HTTP requests to renew a token using a RefreshToken.
 type tokenRefresher struct {
 	ctx          context.Context // used to get HTTP requests
 	conf         *Config
 	refreshToken string
 }

 // WARNING: Token is not safe for concurrent access, as it
 // updates the tokenRefresher's refreshToken field.
 // Within this package, it is used by reuseTokenSource which
 // synchronizes calls to this method with its own mutex.
 func (tf *tokenRefresher) Token() (*Token, error) {
 	if tf.refreshToken == "" {
 		return nil, errors.New("oauth2: token expired and refresh token is not set")
 	}

 	tk, err := retrieveToken(tf.ctx, tf.conf, url.Values{
 		"grant_type":    {"refresh_token"},
 		"refresh_token": {tf.refreshToken},
 	})

 	if err != nil {
 		return nil, err
 	}
 	if tf.refreshToken != tk.RefreshToken {
 		tf.refreshToken = tk.RefreshToken
 	}
 	return tk, err
 }

 // reuseTokenSource is a TokenSource that holds a single token in memory
 // and validates its expiry before each call to retrieve it with
 // Token. If it's expired, it will be auto-refreshed using the
 // new TokenSource.
 type reuseTokenSource struct {
 	new TokenSource // called when t is expired.

 	mu sync.Mutex // guards t
 	t  *Token

 	expiryDelta time.Duration
 }

 // Token returns the current token if it's still valid, else will
 // refresh the current token (using r.Context for HTTP client
 // information) and return the new one.
 func (s *reuseTokenSource) Token() (*Token, error) {
 	s.mu.Lock()
 	defer s.mu.Unlock()
 	if s.t.Valid() {
 		return s.t, nil
 	}
 	t, err := s.new.Token()
 	if err != nil {
 		return nil, err
 	}
 	t.expiryDelta = s.expiryDelta
 	s.t = t
 	return t, nil
 }

 // StaticTokenSource returns a TokenSource that always returns the same token.
 // Because the provided token t is never refreshed, StaticTokenSource is only
 // useful for tokens that never expire.
 func StaticTokenSource(t *Token) TokenSource {
 	return staticTokenSource{t}
 }

 // staticTokenSource is a TokenSource that always returns the same Token.
 type staticTokenSource struct {
 	t *Token
 }

 func (s staticTokenSource) Token() (*Token, error) {
 	return s.t, nil
 }

 // HTTPClient is the context key to use with golang.org/x/net/context's
 // WithValue function to associate an *http.Client value with a context.
 var HTTPClient internal.ContextKey

 // NewClient creates an *http.Client from a Context and TokenSource.
 // The returned client is not valid beyond the lifetime of the context.
 //
 // Note that if a custom *http.Client is provided via the Context it
 // is used only for token acquisition and is not used to configure the
 // *http.Client returned from NewClient.
 //
 // As a special case, if src is nil, a non-OAuth2 client is returned
 // using the provided context. This exists to support related OAuth2
 // packages.
 func NewClient(ctx context.Context, src TokenSource) *http.Client {
 	if src == nil {
 		return internal.ContextClient(ctx)
 	}
 	return &http.Client{
 		Transport: &Transport{
 			Base:   internal.ContextClient(ctx).Transport,
 			Source: ReuseTokenSource(nil, src),
 		},
 	}
 }

 // ReuseTokenSource returns a TokenSource which repeatedly returns the
 // same token as long as it's valid, starting with t.
 // When its cached token is invalid, a new token is obtained from src.
 //
 // ReuseTokenSource is typically used to reuse tokens from a cache
 // (such as a file on disk) between runs of a program, rather than
 // obtaining new tokens unnecessarily.
 //
 // The initial token t may be nil, in which case the TokenSource is
 // wrapped in a caching version if it isn't one already. This also
 // means it's always safe to wrap ReuseTokenSource around any other
 // TokenSource without adverse effects.
 func ReuseTokenSource(t *Token, src TokenSource) TokenSource {
 	// Don't wrap a reuseTokenSource in itself. That would work,
 	// but cause an unnecessary number of mutex operations.
 	// Just build the equivalent one.
 	if rt, ok := src.(*reuseTokenSource); ok {
 		if t == nil {
 			// Just use it directly.
 			return rt
 		}
 		src = rt.new
 	}
 	return &reuseTokenSource{
 		t:   t,
 		new: src,
 	}
 }

 // ReuseTokenSource returns a TokenSource that acts in the same manner as the
 // TokenSource returned by ReuseTokenSource, except the expiry buffer is
 // configurable. The expiration time of a token is calculated as
 // t.Expiry.Add(-earlyExpiry).
 func ReuseTokenSourceWithExpiry(t *Token, src TokenSource, earlyExpiry time.Duration) TokenSource {
 	// Don't wrap a reuseTokenSource in itself. That would work,
 	// but cause an unnecessary number of mutex operations.
 	// Just build the equivalent one.
 	if rt, ok := src.(*reuseTokenSource); ok {
 		if t == nil {
 			// Just use it directly, but set the expiryDelta to earlyExpiry,
 			// so the behavior matches what the user expects.
 			rt.expiryDelta = earlyExpiry
 			return rt
 		}
 		src = rt.new
 	}
 	if t != nil {
 		t.expiryDelta = earlyExpiry
 	}
 	return &reuseTokenSource{
 		t:           t,
 		new:         src,
 		expiryDelta: earlyExpiry,
 	}
 }
	// Copyright 2014 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	// Package oauth2 provides support for making
	// OAuth2 authorized and authenticated HTTP requests,
	// as specified in RFC 6749.
	// It can additionally grant authorization with Bearer JWT.
	package oauth2 // import "golang.org/x/oauth2"

	import (
	"bytes"
	"context"
	"errors"
	"net/http"
	"net/url"
	"strings"
	"sync"
	"time"

	"golang.org/x/oauth2/internal"
	)

	// NoContext is the default context you should supply if not using
	// your own context.Context (see https://golang.org/x/net/context).
	//
	// Deprecated: Use context.Background() or context.TODO() instead.
	var NoContext = context.TODO()

	// RegisterBrokenAuthHeaderProvider previously did something. It is now a no-op.
	//
	// Deprecated: this function no longer does anything. Caller code that
	// wants to avoid potential extra HTTP requests made during
	// auto-probing of the provider's auth style should set
	// Endpoint.AuthStyle.
	func RegisterBrokenAuthHeaderProvider(tokenURL string) {}

	// Config describes a typical 3-legged OAuth2 flow, with both the
	// client application information and the server's endpoint URLs.
	// For the client credentials 2-legged OAuth2 flow, see the clientcredentials
	// package (https://golang.org/x/oauth2/clientcredentials).
	type Config struct {
	// ClientID is the application's ID.
	ClientID string

	// ClientSecret is the application's secret.
	ClientSecret string

	// Endpoint contains the resource server's token endpoint
	// URLs. These are constants specific to each server and are
	// often available via site-specific packages, such as
	// google.Endpoint or github.Endpoint.
	Endpoint Endpoint

	// RedirectURL is the URL to redirect users going through
	// the OAuth flow, after the resource owner's URLs.
	RedirectURL string

	// Scope specifies optional requested permissions.
	Scopes []string

	// authStyleCache caches which auth style to use when Endpoint.AuthStyle is
	// the zero value (AuthStyleAutoDetect).
	authStyleCache internal.LazyAuthStyleCache
	}

	// A TokenSource is anything that can return a token.
	type TokenSource interface {
	// Token returns a token or an error.
	// Token must be safe for concurrent use by multiple goroutines.
	// The returned Token must not be modified.
	Token() (*Token, error)
	}

	// Endpoint represents an OAuth 2.0 provider's authorization and token
	// endpoint URLs.
	type Endpoint struct {
	AuthURL string
	DeviceAuthURL string
	TokenURL string

	// AuthStyle optionally specifies how the endpoint wants the
	// client ID & client secret sent. The zero value means to
	// auto-detect.
	AuthStyle AuthStyle
	}

	// AuthStyle represents how requests for tokens are authenticated
	// to the server.
	type AuthStyle int

	const (
	// AuthStyleAutoDetect means to auto-detect which authentication
	// style the provider wants by trying both ways and caching
	// the successful way for the future.
	AuthStyleAutoDetect AuthStyle = 0

	// AuthStyleInParams sends the "client_id" and "client_secret"
	// in the POST body as application/x-www-form-urlencoded parameters.
	AuthStyleInParams AuthStyle = 1

	// AuthStyleInHeader sends the client_id and client_password
	// using HTTP Basic Authorization. This is an optional style
	// described in the OAuth2 RFC 6749 section 2.3.1.
	AuthStyleInHeader AuthStyle = 2
	)

	var (
	// AccessTypeOnline and AccessTypeOffline are options passed
	// to the Options.AuthCodeURL method. They modify the
	// "access_type" field that gets sent in the URL returned by
	// AuthCodeURL.
	//
	// Online is the default if neither is specified. If your
	// application needs to refresh access tokens when the user
	// is not present at the browser, then use offline. This will
	// result in your application obtaining a refresh token the
	// first time your application exchanges an authorization
	// code for a user.
	AccessTypeOnline AuthCodeOption = SetAuthURLParam("access_type", "online")
	AccessTypeOffline AuthCodeOption = SetAuthURLParam("access_type", "offline")

	// ApprovalForce forces the users to view the consent dialog
	// and confirm the permissions request at the URL returned
	// from AuthCodeURL, even if they've already done so.
	ApprovalForce AuthCodeOption = SetAuthURLParam("prompt", "consent")
	)

	// An AuthCodeOption is passed to Config.AuthCodeURL.
	type AuthCodeOption interface {
	setValue(url.Values)
	}

	type setParam struct{ k, v string }

	func (p setParam) setValue(m url.Values) { m.Set(p.k, p.v) }

	// SetAuthURLParam builds an AuthCodeOption which passes key/value parameters
	// to a provider's authorization endpoint.
	func SetAuthURLParam(key, value string) AuthCodeOption {
	return setParam{key, value}
	}

	// AuthCodeURL returns a URL to OAuth 2.0 provider's consent page
	// that asks for permissions for the required scopes explicitly.
	//
	// State is an opaque value used by the client to maintain state between the
	// request and callback. The authorization server includes this value when
	// redirecting the user agent back to the client.
	//
	// Opts may include AccessTypeOnline or AccessTypeOffline, as well
	// as ApprovalForce.
	//
	// To protect against CSRF attacks, opts should include a PKCE challenge
	// (S256ChallengeOption). Not all servers support PKCE. An alternative is to
	// generate a random state parameter and verify it after exchange.
	// See https://datatracker.ietf.org/doc/html/rfc6749#section-10.12 (predating
	// PKCE), https://www.oauth.com/oauth2-servers/pkce/ and
	// https://www.ietf.org/archive/id/draft-ietf-oauth-v2-1-09.html#name-cross-site-request-forgery (describing both approaches)
	func (c *Config) AuthCodeURL(state string, opts ...AuthCodeOption) string {
	var buf bytes.Buffer
	buf.WriteString(c.Endpoint.AuthURL)
	v := url.Values{
	"response_type": {"code"},
	"client_id": {c.ClientID},
	}
	if c.RedirectURL != "" {
	v.Set("redirect_uri", c.RedirectURL)
	}
	if len(c.Scopes) > 0 {
	v.Set("scope", strings.Join(c.Scopes, " "))
	}
	if state != "" {
	v.Set("state", state)
	}
	for _, opt := range opts {
	opt.setValue(v)
	}
	if strings.Contains(c.Endpoint.AuthURL, "?") {
	buf.WriteByte('&')
	} else {
	buf.WriteByte('?')
	}
	buf.WriteString(v.Encode())
	return buf.String()
	}

	// PasswordCredentialsToken converts a resource owner username and password
	// pair into a token.
	//
	// Per the RFC, this grant type should only be used "when there is a high
	// degree of trust between the resource owner and the client (e.g., the client
	// is part of the device operating system or a highly privileged application),
	// and when other authorization grant types are not available."
	// See https://tools.ietf.org/html/rfc6749#section-4.3 for more info.
	//
	// The provided context optionally controls which HTTP client is used. See the HTTPClient variable.
	func (c Config) PasswordCredentialsToken(ctx context.Context, username, password string) (Token, error) {
	v := url.Values{
	"grant_type": {"password"},
	"username": {username},
	"password": {password},
	}
	if len(c.Scopes) > 0 {
	v.Set("scope", strings.Join(c.Scopes, " "))
	}
	return retrieveToken(ctx, c, v)
	}

	// Exchange converts an authorization code into a token.
	//
	// It is used after a resource provider redirects the user back
	// to the Redirect URI (the URL obtained from AuthCodeURL).
	//
	// The provided context optionally controls which HTTP client is used. See the HTTPClient variable.
	//
	// The code will be in the *http.Request.FormValue("code"). Before
	// calling Exchange, be sure to validate FormValue("state") if you are
	// using it to protect against CSRF attacks.
	//
	// If using PKCE to protect against CSRF attacks, opts should include a
	// VerifierOption.
	func (c Config) Exchange(ctx context.Context, code string, opts ...AuthCodeOption) (Token, error) {
	v := url.Values{
	"grant_type": {"authorization_code"},
	"code": {code},
	}
	if c.RedirectURL != "" {
	v.Set("redirect_uri", c.RedirectURL)
	}
	for _, opt := range opts {
	opt.setValue(v)
	}
	return retrieveToken(ctx, c, v)
	}

	// Client returns an HTTP client using the provided token.
	// The token will auto-refresh as necessary. The underlying
	// HTTP transport will be obtained using the provided context.
	// The returned client and its Transport should not be modified.
	func (c Config) Client(ctx context.Context, t Token) *http.Client {
	return NewClient(ctx, c.TokenSource(ctx, t))
	}

	// TokenSource returns a TokenSource that returns t until t expires,
	// automatically refreshing it as necessary using the provided context.
	//
	// Most users will use Config.Client instead.
	func (c Config) TokenSource(ctx context.Context, t Token) TokenSource {
	tkr := &tokenRefresher{
	ctx: ctx,
	conf: c,
	}
	if t != nil {
	tkr.refreshToken = t.RefreshToken
	}
	return &reuseTokenSource{
	t: t,
	new: tkr,
	}
	}

	// tokenRefresher is a TokenSource that makes "grant_type"=="refresh_token"
	// HTTP requests to renew a token using a RefreshToken.
	type tokenRefresher struct {
	ctx context.Context // used to get HTTP requests
	conf *Config
	refreshToken string
	}

	// WARNING: Token is not safe for concurrent access, as it
	// updates the tokenRefresher's refreshToken field.
	// Within this package, it is used by reuseTokenSource which
	// synchronizes calls to this method with its own mutex.
	func (tf tokenRefresher) Token() (Token, error) {
	if tf.refreshToken == "" {
	return nil, errors.New("oauth2: token expired and refresh token is not set")
	}

	tk, err := retrieveToken(tf.ctx, tf.conf, url.Values{
	"grant_type": {"refresh_token"},
	"refresh_token": {tf.refreshToken},
	})

	if err != nil {
	return nil, err
	}
	if tf.refreshToken != tk.RefreshToken {
	tf.refreshToken = tk.RefreshToken
	}
	return tk, err
	}

	// reuseTokenSource is a TokenSource that holds a single token in memory
	// and validates its expiry before each call to retrieve it with
	// Token. If it's expired, it will be auto-refreshed using the
	// new TokenSource.
	type reuseTokenSource struct {
	new TokenSource // called when t is expired.

	mu sync.Mutex // guards t
	t *Token

	expiryDelta time.Duration
	}

	// Token returns the current token if it's still valid, else will
	// refresh the current token (using r.Context for HTTP client
	// information) and return the new one.
	func (s reuseTokenSource) Token() (Token, error) {
	s.mu.Lock()
	defer s.mu.Unlock()
	if s.t.Valid() {
	return s.t, nil
	}
	t, err := s.new.Token()
	if err != nil {
	return nil, err
	}
	t.expiryDelta = s.expiryDelta
	s.t = t
	return t, nil
	}

	// StaticTokenSource returns a TokenSource that always returns the same token.
	// Because the provided token t is never refreshed, StaticTokenSource is only
	// useful for tokens that never expire.
	func StaticTokenSource(t *Token) TokenSource {
	return staticTokenSource{t}
	}

	// staticTokenSource is a TokenSource that always returns the same Token.
	type staticTokenSource struct {
	t *Token
	}

	func (s staticTokenSource) Token() (*Token, error) {
	return s.t, nil
	}

	// HTTPClient is the context key to use with golang.org/x/net/context's
	// WithValue function to associate an *http.Client value with a context.
	var HTTPClient internal.ContextKey

	// NewClient creates an *http.Client from a Context and TokenSource.
	// The returned client is not valid beyond the lifetime of the context.
	//
	// Note that if a custom *http.Client is provided via the Context it
	// is used only for token acquisition and is not used to configure the
	// *http.Client returned from NewClient.
	//
	// As a special case, if src is nil, a non-OAuth2 client is returned
	// using the provided context. This exists to support related OAuth2
	// packages.
	func NewClient(ctx context.Context, src TokenSource) *http.Client {
	if src == nil {
	return internal.ContextClient(ctx)
	}
	return &http.Client{
	Transport: &Transport{
	Base: internal.ContextClient(ctx).Transport,
	Source: ReuseTokenSource(nil, src),
	},
	}
	}

	// ReuseTokenSource returns a TokenSource which repeatedly returns the
	// same token as long as it's valid, starting with t.
	// When its cached token is invalid, a new token is obtained from src.
	//
	// ReuseTokenSource is typically used to reuse tokens from a cache
	// (such as a file on disk) between runs of a program, rather than
	// obtaining new tokens unnecessarily.
	//
	// The initial token t may be nil, in which case the TokenSource is
	// wrapped in a caching version if it isn't one already. This also
	// means it's always safe to wrap ReuseTokenSource around any other
	// TokenSource without adverse effects.
	func ReuseTokenSource(t *Token, src TokenSource) TokenSource {
	// Don't wrap a reuseTokenSource in itself. That would work,
	// but cause an unnecessary number of mutex operations.
	// Just build the equivalent one.
	if rt, ok := src.(*reuseTokenSource); ok {
	if t == nil {
	// Just use it directly.
	return rt
	}
	src = rt.new
	}
	return &reuseTokenSource{
	t: t,
	new: src,
	}
	}

	// ReuseTokenSource returns a TokenSource that acts in the same manner as the
	// TokenSource returned by ReuseTokenSource, except the expiry buffer is
	// configurable. The expiration time of a token is calculated as
	// t.Expiry.Add(-earlyExpiry).
	func ReuseTokenSourceWithExpiry(t *Token, src TokenSource, earlyExpiry time.Duration) TokenSource {
	// Don't wrap a reuseTokenSource in itself. That would work,
	// but cause an unnecessary number of mutex operations.
	// Just build the equivalent one.
	if rt, ok := src.(*reuseTokenSource); ok {
	if t == nil {
	// Just use it directly, but set the expiryDelta to earlyExpiry,
	// so the behavior matches what the user expects.
	rt.expiryDelta = earlyExpiry
	return rt
	}
	src = rt.new
	}
	if t != nil {
	t.expiryDelta = earlyExpiry
	}
	return &reuseTokenSource{
	t: t,
	new: src,
	expiryDelta: earlyExpiry,
	}
	}