Support for Google Service Accounts

Fixes #879.

Allows using Google Service Accounts.

To do so, initialize it with GSA credentials in JSON form:
```shell
$ drive init -service-account-file ~/Desktop/gsaFile.json
```

Please make sure that you've enabled the Drive API
and if you've enabled the API recently, wait a couple of minutes
for the action to propagate through Google's systems, then retry.

README.md

@@ -173,11 +173,23 @@ A single hypen `-` can be used to specify options. However two hypens `--` can b
 
 Before you can use `drive`, you'll need to mount your Google Drive directory on your local file system:
 
+#### OAuth2.0 credentials
 ```shell
 drive init ~/gdrive
 cd ~/gdrive
 ```
 
+#### Google Service Account credentials
+```shell
+drive init --service-account-file <gsa_json_file_path> ~/gdrive
+cd ~/gdrive
+```
+
+where <gsa_json_file_path> must the GSA credentials in JSON form.
+This feature was implemented as requested by:
++ https://github.com/odeke-em/drive/issues/879
+
+
 ### De Initializing
 
 The opposite of `drive init`, it will remove your credentials locally as well as configuration associated files.

cmd/drive/main.go

@@ -164,14 +164,23 @@ func (cmd *versionCmd) Run(args []string, definedFlags map[string]*flag.Flag) {
 	exitWithError(nil)
 }
 
-type initCmd struct{}
+type initCmd struct {
+	ServiceAccountJSONFile *string `json:"-"`
+}
 
 func (cmd *initCmd) Flags(fs *flag.FlagSet) *flag.FlagSet {
+	cmd.ServiceAccountJSONFile = fs.String(drive.ServiceAccountJSONFileKey, "", "points the Google Service Account JSON file")
 	return fs
 }
 
 func (cmd *initCmd) Run(args []string, definedFlags map[string]*flag.Flag) {
-	exitWithError(drive.New(initContext(args), nil).Init())
+	comm := drive.New(initContext(args), nil)
+	gcsJSONFile := *cmd.ServiceAccountJSONFile
+	if gcsJSONFile == "" {
+		exitWithError(comm.Init())
+	} else {
+		exitWithError(comm.InitWithServiceAccount(gcsJSONFile))
+	}
 }
 
 type deInitCmd struct {

config/config.go

@@ -24,6 +24,8 @@ import (
 	"path/filepath"
 	"strings"
 
+	"golang.org/x/oauth2/jwt"
+
 	"github.com/boltdb/bolt"
 )
 
@@ -49,6 +51,8 @@ const (
 )
 
 type Context struct {
+	GSAJWTConfig *jwt.Config `json:"gsa_jwt_config,omitempty"`
+
 	ClientId     string `json:"client_id"`
 	ClientSecret string `json:"client_secret"`
 	RefreshToken string `json:"refresh_token"`

src/commands.go

@@ -16,6 +16,7 @@ package drive
 
 import (
 	"errors"
+	"fmt"
 	"io"
 	"os"
 	"path"
@@ -193,9 +194,17 @@ func (opts *Options) rcPath() (string, error) {
 }
 
 func New(context *config.Context, opts *Options) *Commands {
-	var r *Remote
-	if context != nil {
-		r = NewRemoteContext(context)
+	var rem *Remote
+	var err error
+
+	if context.GSAJWTConfig != nil {
+		rem, err = NewRemoteContextFromServiceAccount(context.GSAJWTConfig)
+	} else {
+		rem, err = NewRemoteContext(context)
+	}
+
+	if err != nil {
+		panic(fmt.Errorf("failed to initialize remoteContext: %v", err))
 	}
 
 	stdin, stdout, stderr := os.Stdin, os.Stdout, os.Stderr
@@ -236,7 +245,7 @@ func New(context *config.Context, opts *Options) *Commands {
 
 	return &Commands{
 		context:       context,
-		rem:           r,
+		rem:           rem,
 		opts:          opts,
 		log:           logger,
 		mkdirAllCache: expirableCache.New(),

src/errors.go

@@ -41,6 +41,7 @@ const (
 	StatusNamedPipeReadAttempt        ErrorStatus = 22
 	StatusContentTooLarge             ErrorStatus = 23
 	StatusClashesFixed                ErrorStatus = 24
+	StatusSecurityException           ErrorStatus = 25
 )
 
 type Error struct {

src/help.go

@@ -25,47 +25,48 @@ import (
 )
 
 const (
-	AboutKey                = "about"
-	AllKey                  = "all"
-	CopyKey                 = "copy"
-	DeleteKey               = "delete"
-	DeInitKey               = "deinit"
-	EditDescriptionKey      = "edit-description"
-	EditDescriptionShortKey = "edit-desc"
-	DiffKey                 = "diff"
-	AddressKey              = "address"
-	EmptyTrashKey           = "emptytrash"
-	FeaturesKey             = "features"
-	HelpKey                 = "help"
-	InitKey                 = "init"
-	LinkKey                 = "Link"
-	ListKey                 = "list"
-	DuKey                   = "du"
-	Md5sumKey               = "md5sum"
-	MoveKey                 = "move"
-	OcrKey                  = "ocr"
-	ConvertKey              = "convert"
-	OSLinuxKey              = "linux"
-	PullKey                 = "pull"
-	PipedKey                = "piped"
-	PushKey                 = "push"
-	PubKey                  = "pub"
-	QRLinkKey               = "qr"
-	RenameKey               = "rename"
-	QuotaKey                = "quota"
-	ShareKey                = "share"
-	StatKey                 = "stat"
-	TouchKey                = "touch"
-	TrashKey                = "trash"
-	UnshareKey              = "unshare"
-	UntrashKey              = "untrash"
-	UnpubKey                = "unpub"
-	VersionKey              = "version"
-	NewKey                  = "new"
-	IndexKey                = "index"
-	PruneKey                = "prune"
-	StarKey                 = "star"
-	UnStarKey               = "unstar"
+	AboutKey                  = "about"
+	AllKey                    = "all"
+	CopyKey                   = "copy"
+	DeleteKey                 = "delete"
+	DeInitKey                 = "deinit"
+	EditDescriptionKey        = "edit-description"
+	EditDescriptionShortKey   = "edit-desc"
+	ServiceAccountJSONFileKey = "service-account-file"
+	DiffKey                   = "diff"
+	AddressKey                = "address"
+	EmptyTrashKey             = "emptytrash"
+	FeaturesKey               = "features"
+	HelpKey                   = "help"
+	InitKey                   = "init"
+	LinkKey                   = "Link"
+	ListKey                   = "list"
+	DuKey                     = "du"
+	Md5sumKey                 = "md5sum"
+	MoveKey                   = "move"
+	OcrKey                    = "ocr"
+	ConvertKey                = "convert"
+	OSLinuxKey                = "linux"
+	PullKey                   = "pull"
+	PipedKey                  = "piped"
+	PushKey                   = "push"
+	PubKey                    = "pub"
+	QRLinkKey                 = "qr"
+	RenameKey                 = "rename"
+	QuotaKey                  = "quota"
+	ShareKey                  = "share"
+	StatKey                   = "stat"
+	TouchKey                  = "touch"
+	TrashKey                  = "trash"
+	UnshareKey                = "unshare"
+	UntrashKey                = "untrash"
+	UnpubKey                  = "unpub"
+	VersionKey                = "version"
+	NewKey                    = "new"
+	IndexKey                  = "index"
+	PruneKey                  = "prune"
+	StarKey                   = "star"
+	UnStarKey                 = "unstar"
 
 	CoercedMimeKeyKey        = "coerced-mime"
 	ExportsKey               = "export"

src/init.go

@@ -15,9 +15,11 @@
 package drive
 
 import (
+	"io/ioutil"
 	"os"
 
 	"golang.org/x/net/context"
+	"golang.org/x/oauth2/google"
 )
 
 func (g *Commands) Init() error {
@@ -38,6 +40,37 @@ func (g *Commands) Init() error {
 	return g.context.Write()
 }
 
+// We don't need to perform an OAuth2.0 exchange
+func (g *Commands) InitWithServiceAccount(gsaFilepath string) error {
+	if gsaFilepath == "" {
+		// We should be cautious enough not to blindly read in
+		// by default a Google Service account, because this
+		// could be a security breach to phish for accounts.
+		// The user has to explicitly pass in the path
+		return &Error{
+			code:   StatusSecurityException,
+			status: "a path has to be explicitly set for service accounts",
+		}
+	}
+
+	blob, err := ioutil.ReadFile(gsaFilepath)
+	if err != nil {
+		return err
+	}
+
+	jwtConfig, err := google.JWTConfigFromJSON(blob, DriveScope)
+	if err != nil {
+		return err
+	}
+
+	// Next we'll just transfer the attributes directly
+	// by means of JSON marshaling the already vetted JWTConfig
+	g.context.GSAJWTConfig = jwtConfig
+
+	// Since it validates alright, let's now write it to disk
+	return g.context.Write()
+}
+
 func (g *Commands) DeInit() error {
 	prompt := func(args ...interface{}) bool {
 		if !g.opts.canPrompt() {

src/remote.go

@@ -27,6 +27,7 @@ import (
 	"golang.org/x/net/context"
 	"golang.org/x/oauth2"
 	"golang.org/x/oauth2/google"
+	"golang.org/x/oauth2/jwt"
 
 	"github.com/odeke-em/drive/config"
 	"github.com/odeke-em/statos"
@@ -93,15 +94,36 @@ type Remote struct {
 	progressChan chan int
 }
 
-func NewRemoteContext(context *config.Context) *Remote {
+// NewRemoteContextFromServiceAccount returns a remote initialized
+// with credentials from a Google Service Account.
+// For more information about these accounts, see:
+// https://developers.google.com/identity/protocols/OAuth2ServiceAccount
+// https://developers.google.com/accounts/docs/application-default-credentials
+//
+// You'll also need to configure access to Google Drive.
+func NewRemoteContextFromServiceAccount(jwtConfig *jwt.Config) (*Remote, error) {
+	client := jwtConfig.Client(context.Background())
+	return remoteFromClient(client)
+}
+
+func NewRemoteContext(context *config.Context) (*Remote, error) {
 	client := newOAuthClient(context)
-	service, _ := drive.New(client)
+	return remoteFromClient(client)
+}
+
+func remoteFromClient(client *http.Client) (*Remote, error) {
+	service, err := drive.New(client)
+	if err != nil {
+		return nil, err
+	}
+
 	progressChan := make(chan int)
-	return &Remote{
+	rem := &Remote{
 		progressChan: progressChan,
 		service:      service,
 		client:       client,
 	}
+	return rem, nil
 }
 
 func hasExportLinks(f *File) bool {