Add service account support for GCS

docs/content/googlecloudstorage.md

@@ -57,6 +57,8 @@ Google Application Client Secret - leave blank normally.
 client_secret> 
 Project number optional - needed only for list/create/delete buckets - see your developer console.
 project_number> 12345678
+Service Account Credentials JSON file path - needed only if you want use SA instead of interactive login.
+service_account_file> 
 Access Control List for new objects.
 Choose a number from below, or type in your own value
  * Object owner gets OWNER access, and all Authenticated Users get READER access.
@@ -139,6 +141,41 @@ files in the bucket.
 
     rclone sync /home/local/directory remote:bucket
 
+### Service Account support ###
+
+You can set up rclone with Google Cloud Storage in an unattended mode,
+i.e. not tied to a specific end-user Google account. This is useful
+when you want to synchronise files onto machines that don't have
+actively logged-in users, for example build machines.
+
+To get credentials for Google Cloud Platform
+[IAM Service Accounts](https://cloud.google.com/iam/docs/service-accounts),
+please head to the
+[Service Account](https://console.cloud.google.com/permissions/serviceaccounts)
+section of the Google Developer Console. Service Accounts behave just
+like normal `User` permissions in
+[Google Cloud Storage ACLs](https://cloud.google.com/storage/docs/access-control),
+so you can limit their access (e.g. make them read only). After
+creating an account, a JSON file containing the Service Account's
+credentials will be downloaded onto your machines. These credentials
+are what rclone will use for authentication.
+
+To use a Service Account instead of OAuth2 token flow, replace the
+`token` section of your `.rclone.conf` with a `service_account_file`
+pointing to the JSON credentials.
+
+For example, here's an example `.rclone.conf` that sets up read only
+access using a service account:
+
+```
+[readonly-sync]
+type = google cloud storage
+project_number = 123456789
+service_account_file = $HOME/.rclone-service_account.json
+object_acl = authenticatedRead
+bucket_acl = authenticatedRead
+```
+
 ### Modified time ###
 
 Google google cloud storage stores md5sums natively and rclone stores

googlecloudstorage/googlecloudstorage.go

@@ -17,8 +17,10 @@ import (
 	"encoding/hex"
 	"fmt"
 	"io"
+	"io/ioutil"
 	"log"
 	"net/http"
+	"os"
 	"path"
 	"regexp"
 	"strings"
@@ -74,6 +76,9 @@ func init() {
 		}, {
 			Name: "project_number",
 			Help: "Project number optional - needed only for list/create/delete buckets - see your developer console.",
+		}, {
+			Name: "service_account_file",
+			Help: "Service Account Credentials JSON file path - needed only if you want use SA instead of interactive login.",
 		}, {
 			Name: "object_acl",
 			Help: "Access Control List for new objects.",
@@ -181,12 +186,36 @@ func parsePath(path string) (bucket, directory string, err error) {
 	return
 }
 
+func getServiceAccountClient(keyJsonfilePath string) (*http.Client, error) {
+	data, err := ioutil.ReadFile(os.ExpandEnv(keyJsonfilePath))
+	if err != nil {
+		return nil, fmt.Errorf("error opening credentials file: %v", err)
+	}
+	conf, err := google.JWTConfigFromJSON(data, storageConfig.Scopes...)
+	if err != nil {
+		return nil, fmt.Errorf("error processing credentials: %v", err)
+	}
+	ctxWithSpecialClient := oauthutil.Context()
+	return oauth2.NewClient(ctxWithSpecialClient, conf.TokenSource(ctxWithSpecialClient)), nil
+}
+
 // NewFs contstructs an Fs from the path, bucket:path
 func NewFs(name, root string) (fs.Fs, error) {
-	oAuthClient, err := oauthutil.NewClient(name, storageConfig)
+	var oAuthClient *http.Client
+	var err error
+
+	serviceAccountPath := fs.ConfigFile.MustValue(name, "service_account_file")
+	if serviceAccountPath != "" {
+		oAuthClient, err = getServiceAccountClient(serviceAccountPath)
+		if err != nil {
+			log.Fatalf("Failed configuring Google Cloud Storage Service Account: %v", err)
+		}
+	} else {
+		oAuthClient, err = oauthutil.NewClient(name, storageConfig)
 		if err != nil {
 			log.Fatalf("Failed to configure Google Cloud Storage: %v", err)
 		}
+	}
 
 	bucket, directory, err := parsePath(root)
 	if err != nil {