proposal: app engine support

App Engine unfortunately can't make outbound requests without using
the appengine urlfetch library and a request-specific context.

I don't see a way around this other than to change the signature of
the Provider and Session interfaces.

This really has a huge amount of downsides obviously, but before I
go and fix stuff for providers besides Facebook, G+, and Twitter
(Twitter support requires mrjones/oauth#60
to get merged), I figured I'd open a pull request and discuss the
tradeoffs.

Unfortunately I think we need to do something like this to support
App Engine. Perhaps this is an opportunity to version goth using
gopkg.in or something?

client.go

@@ -0,0 +1,15 @@
+// +build !appengine
+
+package goth
+
+import (
+	"net/http"
+
+	"golang.org/x/net/context"
+)
+
+// Provider implementations should use this method for making outbound HTTP
+// requests.
+var HTTPClient = func(ctx context.Context) (*http.Client, error) {
+	return http.DefaultClient, nil
+}

client_appengine.go

@@ -0,0 +1,16 @@
+// +build appengine
+
+package goth
+
+import (
+	"net/http"
+
+	"golang.org/x/net/context"
+	"google.golang.org/appengine/urlfetch"
+)
+
+// Provider implementations should use this method for making outbound HTTP
+// requests.
+var HTTPClient = func(ctx context.Context) (*http.Client, error) {
+	return urlfetch.Client(ctx), nil
+}

provider.go

@@ -1,18 +1,34 @@
 package goth
 
-import "fmt"
-import "golang.org/x/oauth2"
+import (
+	"fmt"
+
+	"golang.org/x/net/context"
+	"golang.org/x/oauth2"
+)
 
 // Provider needs to be implemented for each 3rd party authentication provider
 // e.g. Facebook, Twitter, etc...
 type Provider interface {
+	// When implementing a provider, these methods should not make outbound
+	// requests.
 	Name() string
-	BeginAuth(state string) (Session, error)
 	UnmarshalSession(string) (Session, error)
-	FetchUser(Session) (User, error)
 	Debug(bool)
-	RefreshToken(refreshToken string) (*oauth2.Token, error) //Get new access token based on the refresh token
-	RefreshTokenAvailable() bool                             //Refresh token is provided by auth provider or not
+	// Refresh token is provided by auth provider or not
+	RefreshTokenAvailable() bool
+
+	// These three methods are deprecated. See the appropriate *Ctx replacement.
+	BeginAuth(state string) (Session, error)
+	FetchUser(Session) (User, error)
+	RefreshToken(refreshToken string) (*oauth2.Token, error)
+
+	// These methods are now preferred.
+	BeginAuthCtx(ctx context.Context, state string) (Session, error)
+	FetchUserCtx(context.Context, Session) (User, error)
+	// Get new access token based on the refresh token.
+	// Only works if RefreshTokenAvailable() is true
+	RefreshTokenCtx(ctx context.Context, refreshToken string) (*oauth2.Token, error)
 }
 
 // Providers is list of known/available providers.

providers/facebook/facebook.go

@@ -8,10 +8,10 @@ import (
 	"errors"
 	"io"
 	"io/ioutil"
-	"net/http"
 	"net/url"
 
 	"github.com/markbates/goth"
+	"golang.org/x/net/context"
 	"golang.org/x/oauth2"
 )
 
@@ -50,25 +50,39 @@ func (p *Provider) Name() string {
 // Debug is a no-op for the facebook package.
 func (p *Provider) Debug(debug bool) {}
 
-// BeginAuth asks Facebook for an authentication end-point.
 func (p *Provider) BeginAuth(state string) (goth.Session, error) {
+	return p.BeginAuthCtx(context.TODO(), state)
+}
+
+// BeginAuthCtx asks Facebook for an authentication end-point.
+func (p *Provider) BeginAuthCtx(ctx context.Context, state string) (goth.Session, error) {
 	url := p.config.AuthCodeURL(state)
 	session := &Session{
 		AuthURL: url,
 	}
 	return session, nil
 }
 
-// FetchUser will go to Facebook and access basic information about the user.
 func (p *Provider) FetchUser(session goth.Session) (goth.User, error) {
+	return p.FetchUserCtx(context.TODO(), session)
+}
+
+// FetchUserCtx will go to Facebook and access basic information about the user.
+func (p *Provider) FetchUserCtx(ctx context.Context, session goth.Session) (
+	goth.User, error) {
 	sess := session.(*Session)
 	user := goth.User{
 		AccessToken: sess.AccessToken,
 		Provider:    p.Name(),
 		ExpiresAt:   sess.ExpiresAt,
 	}
 
-	response, err := http.Get(endpointProfile + "&access_token=" + url.QueryEscape(sess.AccessToken))
+	client, err := goth.HTTPClient(ctx)
+	if err != nil {
+		return user, err
+	}
+
+	response, err := client.Get(endpointProfile + "&access_token=" + url.QueryEscape(sess.AccessToken))
 	if err != nil {
 		return user, err
 	}
@@ -152,12 +166,16 @@ func newConfig(provider *Provider, scopes []string) *oauth2.Config {
 	return c
 }
 
-//RefreshToken refresh token is not provided by facebook
 func (p *Provider) RefreshToken(refreshToken string) (*oauth2.Token, error) {
+	return p.RefreshTokenCtx(context.TODO(), refreshToken)
+}
+
+// RefreshTokenCtx refresh token is not provided by facebook
+func (p *Provider) RefreshTokenCtx(ctx context.Context, refreshToken string) (*oauth2.Token, error) {
 	return nil, errors.New("Refresh token is not provided by facebook")
 }
 
-//RefreshTokenAvailable refresh token is not provided by facebook
+// RefreshTokenAvailable refresh token is not provided by facebook
 func (p *Provider) RefreshTokenAvailable() bool {
 	return false
 }

providers/facebook/session.go

@@ -3,10 +3,11 @@ package facebook
 import (
 	"encoding/json"
 	"errors"
-	"github.com/markbates/goth"
-	"golang.org/x/oauth2"
 	"strings"
 	"time"
+
+	"github.com/markbates/goth"
+	"golang.org/x/net/context"
 )
 
 // Session stores data during the auth process with Facebook.
@@ -24,10 +25,14 @@ func (s Session) GetAuthURL() (string, error) {
 	return s.AuthURL, nil
 }
 
-// Authorize the session with Facebook and return the access token to be stored for future use.
 func (s *Session) Authorize(provider goth.Provider, params goth.Params) (string, error) {
+	return s.AuthorizeCtx(context.TODO(), provider, params)
+}
+
+// AuthorizeCtx the session with Facebook and return the access token to be stored for future use.
+func (s *Session) AuthorizeCtx(ctx context.Context, provider goth.Provider, params goth.Params) (string, error) {
 	p := provider.(*Provider)
-	token, err := p.config.Exchange(oauth2.NoContext, params.Get("code"))
+	token, err := p.config.Exchange(ctx, params.Get("code"))
 	if err != nil {
 		return "", err
 	}

providers/gplus/gplus.go

@@ -7,11 +7,11 @@ import (
 	"encoding/json"
 	"io"
 	"io/ioutil"
-	"net/http"
 	"net/url"
 	"strings"
 
 	"github.com/markbates/goth"
+	"golang.org/x/net/context"
 	"golang.org/x/oauth2"
 )
 
@@ -51,8 +51,12 @@ func (p *Provider) Name() string {
 // Debug is a no-op for the gplus package.
 func (p *Provider) Debug(debug bool) {}
 
-// BeginAuth asks Google+ for an authentication end-point.
 func (p *Provider) BeginAuth(state string) (goth.Session, error) {
+	return p.BeginAuthCtx(context.TODO(), state)
+}
+
+// BeginAuthCtx asks Google+ for an authentication end-point.
+func (p *Provider) BeginAuthCtx(ctx context.Context, state string) (goth.Session, error) {
 	var opts []oauth2.AuthCodeOption
 	if p.prompt != nil {
 		opts = append(opts, p.prompt)
@@ -64,8 +68,14 @@ func (p *Provider) BeginAuth(state string) (goth.Session, error) {
 	return session, nil
 }
 
-// FetchUser will go to Google+ and access basic information about the user.
-func (p *Provider) FetchUser(session goth.Session) (goth.User, error) {
+func (p *Provider) FetchUser(session goth.Session) (
+	goth.User, error) {
+	return p.FetchUserCtx(context.TODO(), session)
+}
+
+// FetchUserCtx will go to Google+ and access basic information about the user.
+func (p *Provider) FetchUserCtx(ctx context.Context, session goth.Session) (
+	goth.User, error) {
 	sess := session.(*Session)
 	user := goth.User{
 		AccessToken:  sess.AccessToken,
@@ -74,7 +84,12 @@ func (p *Provider) FetchUser(session goth.Session) (goth.User, error) {
 		ExpiresAt:    sess.ExpiresAt,
 	}
 
-	response, err := http.Get(endpointProfile + "?access_token=" + url.QueryEscape(sess.AccessToken))
+	client, err := goth.HTTPClient(ctx)
+	if err != nil {
+		return user, err
+	}
+
+	response, err := client.Get(endpointProfile + "?access_token=" + url.QueryEscape(sess.AccessToken))
 	if err != nil {
 		return user, err
 	}
@@ -145,15 +160,21 @@ func newConfig(provider *Provider, scopes []string) *oauth2.Config {
 	return c
 }
 
-//RefreshTokenAvailable refresh token is provided by auth provider or not
+// RefreshTokenAvailable refresh token is provided by auth provider or not
 func (p *Provider) RefreshTokenAvailable() bool {
 	return true
 }
 
-//RefreshToken get new access token based on the refresh token
-func (p *Provider) RefreshToken(refreshToken string) (*oauth2.Token, error) {
+func (p *Provider) RefreshToken(refreshToken string) (
+	*oauth2.Token, error) {
+	return p.RefreshTokenCtx(context.TODO(), refreshToken)
+}
+
+// RefreshTokenCtx get new access token based on the refresh token
+func (p *Provider) RefreshTokenCtx(ctx context.Context, refreshToken string) (
+	*oauth2.Token, error) {
 	token := &oauth2.Token{RefreshToken: refreshToken}
-	ts := p.config.TokenSource(oauth2.NoContext, token)
+	ts := p.config.TokenSource(ctx, token)
 	newToken, err := ts.Token()
 	if err != nil {
 		return nil, err

providers/gplus/session.go

@@ -3,10 +3,11 @@ package gplus
 import (
 	"encoding/json"
 	"errors"
-	"github.com/markbates/goth"
-	"golang.org/x/oauth2"
 	"strings"
 	"time"
+
+	"github.com/markbates/goth"
+	"golang.org/x/net/context"
 )
 
 // Session stores data during the auth process with Facebook.
@@ -25,10 +26,14 @@ func (s Session) GetAuthURL() (string, error) {
 	return s.AuthURL, nil
 }
 
-// Authorize the session with Google+ and return the access token to be stored for future use.
 func (s *Session) Authorize(provider goth.Provider, params goth.Params) (string, error) {
+	return s.AuthorizeCtx(context.TODO(), provider, params)
+}
+
+// AuthorizeCtx the session with Google+ and return the access token to be stored for future use.
+func (s *Session) AuthorizeCtx(ctx context.Context, provider goth.Provider, params goth.Params) (string, error) {
 	p := provider.(*Provider)
-	token, err := p.config.Exchange(oauth2.NoContext, params.Get("code"))
+	token, err := p.config.Exchange(ctx, params.Get("code"))
 	if err != nil {
 		return "", err
 	}

providers/twitter/session.go

@@ -3,9 +3,11 @@ package twitter
 import (
 	"encoding/json"
 	"errors"
+	"strings"
+
 	"github.com/markbates/goth"
 	"github.com/mrjones/oauth"
-	"strings"
+	"golang.org/x/net/context"
 )
 
 // Session stores data during the auth process with Twitter.
@@ -23,10 +25,15 @@ func (s Session) GetAuthURL() (string, error) {
 	return s.AuthURL, nil
 }
 
-// Authorize the session with Twitter and return the access token to be stored for future use.
 func (s *Session) Authorize(provider goth.Provider, params goth.Params) (string, error) {
+	return s.AuthorizeCtx(context.TODO(), provider, params)
+}
+
+// AuthorizeCtx the session with Twitter and return the access token to be stored for future use.
+func (s *Session) AuthorizeCtx(ctx context.Context, provider goth.Provider, params goth.Params) (string, error) {
 	p := provider.(*Provider)
-	accessToken, err := p.consumer.AuthorizeToken(s.RequestToken, params.Get("oauth_verifier"))
+	accessToken, err := p.consumer.AuthorizeTokenWithParamsCtx(
+		ctx, s.RequestToken, params.Get("oauth_verifier"), p.consumer.AdditionalParams)
 	if err != nil {
 		return "", err
 	}