oauth2: support for Google Service Accounts

Fixes #879.

Allows using Google Service Accounts.
However, it is a little awkward to use them because
we still need an init/mount point on disk.
WIP that I'll get to perhaps when the school break
is on.

README.md

@@ -178,6 +178,12 @@ drive init ~/gdrive
 cd ~/gdrive
 ```
 
+If you have Google Service Account credentials, make sure the path to that file is set by
+environment variable `GOOGLE_APPLICATION_CREDENTIALS`, and make sure that your project has Google Drive
+enabled in order to use it. For more information, you can read 
++ https://developers.google.com/accounts/docs/application-default-credentials
++ 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.

src/commands.go

@@ -16,6 +16,7 @@ package drive
 
 import (
 	"errors"
+	"fmt"
 	"io"
 	"os"
 	"path"
@@ -193,9 +194,16 @@ func (opts *Options) rcPath() (string, error) {
 }
 
 func New(context *config.Context, opts *Options) *Commands {
-	var r *Remote
+	var rem *Remote
+	var err error
 	if context != nil {
-		r = NewRemoteContext(context)
+		rem, err = NewRemoteContext(context)
+	} else {
+		rem, err = NewRemoteContextFromServiceAccount()
+	}
+
+	if err != nil {
+		panic(fmt.Errorf("failed to initialize remoteContext: %v", err))
 	}
 
 	stdin, stdout, stderr := os.Stdin, os.Stdout, os.Stderr
@@ -236,7 +244,7 @@ func New(context *config.Context, opts *Options) *Commands {
 
 	return &Commands{
 		context:       context,
-		rem:           r,
+		rem:           rem,
 		opts:          opts,
 		log:           logger,
 		mkdirAllCache: expirableCache.New(),

src/remote.go

@@ -93,15 +93,38 @@ 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 set in your environment, key `GOOGLE_APPLICATION_CREDENTIALS`.
+func NewRemoteContextFromServiceAccount() (*Remote, error) {
+	client, err := google.DefaultClient(context.TODO(), DriveScope)
+	if err != nil {
+		return nil, err
+	}
+	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 {