oauth2: exchange and app for auth implemented

Fixes #21.

* Implements OAuth2.0 authorization, as well as an
app that can be used to retrieve OAuth2 tokens for each
client by an app.

* Implements an oauth2 package for Uber with the respective
scopes and token exchange utilities.

* Started a CLI so that Uber can be usable on the command-line.
The CLI only currently runs the authorization e.g
```shell
$ go get github.com/orijtech/uber/cmd/uber
$ uber --init
Please visit this URL for auth dialog: <authorization_URL>
```

* Note to use the cli to authorize with your application, you'll
need to set and export, in your environment, these keys:
```
UBER_APP_OAUTH2_CLIENT_ID
UBER_APP_OAUTH2_CLIENT_SECRET
```

cmd/uber/main.go

@@ -0,0 +1,84 @@
+// Copyright 2017 orijtech. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package main
+
+import (
+	"encoding/json"
+	"flag"
+	"log"
+	"os"
+	"path/filepath"
+
+	"github.com/orijtech/uber/oauth2"
+)
+
+func main() {
+	var init bool
+	flag.BoolVar(&init, "init", false, "allow a user to authorize this app to make requests on their behalf")
+	flag.Parse()
+
+	// Make log not print out time info in its prefix.
+	log.SetFlags(0)
+
+	switch {
+	case init:
+		authorize()
+	}
+}
+
+func authorize() {
+	uberCredsDirPath, err := ensureUberCredsDirExists()
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	scopes := []string{
+		oauth2.ScopeProfile, oauth2.ScopeRequest,
+		oauth2.ScopeHistory, oauth2.ScopePlaces,
+		oauth2.ScopeRequestReceipt,
+	}
+
+	token, err := oauth2.AuthorizeByEnvApp(scopes...)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	blob, err := json.Marshal(token)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	credsPath := filepath.Join(uberCredsDirPath, "credentials.json")
+	f, err := os.Create(credsPath)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	f.Write(blob)
+	log.Printf("Successfully saved your OAuth2.0 token to %q", credsPath)
+}
+
+func ensureUberCredsDirExists() (string, error) {
+	wdir, err := os.Getwd()
+	if err != nil {
+		return "", err
+	}
+
+	curDirPath := filepath.Join(wdir, ".uber")
+	if err := os.MkdirAll(curDirPath, 0777); err != nil {
+		return "", err
+	}
+	return curDirPath, nil
+}

oauth2/oauth2.go

@@ -0,0 +1,200 @@
+// Copyright 2017 orijtech. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package oauth2
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"log"
+	"math/rand"
+	"net/http"
+	"net/url"
+	"os"
+	"strings"
+	"sync"
+	"time"
+
+	"golang.org/x/oauth2"
+)
+
+type OAuth2AppConfig struct {
+	ClientID     string `json:"client_id"`
+	ClientSecret string `json:"client_secret"`
+}
+
+var (
+	envOAuth2ClientIDKey     = "UBER_APP_OAUTH2_CLIENT_ID"
+	envOAuth2ClientSecretKey = "UBER_APP_OAUTH2_CLIENT_SECRET"
+)
+
+func Transport(token *oauth2.Token) *oauth2.Transport {
+	// Once we have the token we can now make the TokenSource
+	ts := &tokenSourcer{token: token}
+	return &oauth2.Transport{Source: ts}
+}
+
+type tokenSourcer struct {
+	sync.RWMutex
+	token *oauth2.Token
+}
+
+var _ oauth2.TokenSource = (*tokenSourcer)(nil)
+
+func (ts *tokenSourcer) Token() (*oauth2.Token, error) {
+	ts.RLock()
+	defer ts.RUnlock()
+
+	return ts.token, nil
+}
+
+const (
+	OAuth2AuthURL  = "https://login.uber.com/oauth/v2/authorize"
+	OAuth2TokenURL = "https://login.uber.com/oauth/v2/token"
+)
+
+// OAuth2ConfigFromEnv retrieves your app's client id and client
+// secret from your environment, with the purpose of later being
+// able to perform application functions on behalf of users.
+func OAuth2ConfigFromEnv() (*OAuth2AppConfig, error) {
+	var errsList []string
+	oauth2ClientID := strings.TrimSpace(os.Getenv(envOAuth2ClientIDKey))
+	if oauth2ClientID == "" {
+		errsList = append(errsList, fmt.Sprintf("%q was not set", envOAuth2ClientIDKey))
+	}
+	oauth2ClientSecret := strings.TrimSpace(os.Getenv(envOAuth2ClientSecretKey))
+	if oauth2ClientSecret == "" {
+		errsList = append(errsList, fmt.Sprintf("%q was not set", envOAuth2ClientSecretKey))
+	}
+
+	if len(errsList) > 0 {
+		return nil, errors.New(strings.Join(errsList, "\n"))
+	}
+
+	config := &OAuth2AppConfig{
+		ClientID:     oauth2ClientID,
+		ClientSecret: oauth2ClientSecret,
+	}
+
+	return config, nil
+}
+
+const (
+	// Access the user's basic profile information
+	// on a user's Uber account including their
+	// firstname, email address and profile picture.
+	ScopeProfile = "profile"
+
+	// Pull trip data including times, product
+	// type andd city information of a user's
+	// historical pickups and drop-offs.
+	ScopeHistory = "history"
+
+	// ScopeHistoryLite is the same as
+	// ScopeHistory but without city information.
+	ScopeHistoryLite = "history_lite"
+
+	// Access to get and update your saved places.
+	// This includes your home and work addresses if
+	// you have saved them with Uber.
+	ScopePlaces = "places"
+
+	// Allows developers to provide a complete
+	// Uber ride experience inside their app
+	// using the widget. Enables users to access
+	// trip information for rides requested through
+	// the app and the current ride, available promos,
+	// and payment methods (last two digits only)
+	// using the widget. Uber's charges, terms
+	// and policies will apply.
+	ScopeRideWidgets = "ride_widgets"
+
+	// ScopeRequest is a privileged scope that
+	// allows your application to make requests
+	// for Uber products on behalf of users.
+	ScopeRequest = "request"
+
+	// ScopeRequestReceipt is a privileged scope that
+	// allows your application to get receipt details
+	// for requests made by the application.
+	// Restrictions: This scope is only granted to apps
+	//  that request Uber rides directly and receipts
+	//  as part of the trip lifecycle. We do not allow
+	// apps to aggregate receipt information. The receipt
+	// endpoint will only provide receipts for ride requests
+	// origination from your application. It is not
+	// currently possible to receive receipt
+	// data for all trips, as of: (Fri 12 May 2017 18:18:42 MDT).
+	ScopeRequestReceipt = "request_receipt"
+
+	// ScopeAllTrips is a privileged scope that allows
+	// access to trip details about all future Uber trips,
+	// including pickup, destination and real-time
+	// location for all of your future rides.
+	ScopeAllTrips = "all_trips"
+)
+
+func AuthorizeByEnvApp(scopes ...string) (*oauth2.Token, error) {
+	oconfig, err := OAuth2ConfigFromEnv()
+	if err != nil {
+		return nil, err
+	}
+	return Authorize(oconfig, scopes...)
+}
+
+func Authorize(oconfig *OAuth2AppConfig, scopes ...string) (*oauth2.Token, error) {
+	config := &oauth2.Config{
+		ClientID:     oconfig.ClientID,
+		ClientSecret: oconfig.ClientSecret,
+		Scopes:       scopes,
+		Endpoint: oauth2.Endpoint{
+			AuthURL:  OAuth2AuthURL,
+			TokenURL: OAuth2TokenURL,
+		},
+	}
+
+	state := fmt.Sprintf("%v%s", time.Now().Unix(), rand.Float32())
+	urlToVisit := config.AuthCodeURL(state, oauth2.AccessTypeOffline)
+	fmt.Printf("Please visit this URL for the auth dialog: %v\n", urlToVisit)
+
+	callbackURLChan := make(chan url.Values)
+	go func() {
+		http.HandleFunc("/", func(rw http.ResponseWriter, req *http.Request) {
+			query := req.URL.Query()
+			callbackURLChan <- query
+			fmt.Fprintf(rw, "Received the token successfully. Please return to your terminal")
+		})
+
+		defer close(callbackURLChan)
+		addr := ":8889"
+		if err := http.ListenAndServe(addr, nil); err != nil {
+			log.Fatal(err)
+		}
+	}()
+
+	urlValues := <-callbackURLChan
+	gotState, wantState := urlValues.Get("state"), state
+	if gotState != wantState {
+		return nil, fmt.Errorf("states do not match: got: %q want: %q", gotState, wantState)
+	}
+	code := urlValues.Get("code")
+
+	ctx := context.Background()
+	token, err := config.Exchange(ctx, code)
+	if err != nil {
+		return nil, err
+	}
+	return token, nil
+}