Add Auth through Service Account in env variable

lib/authenticate.js

@@ -1,5 +1,6 @@
 var colors = require('colors');
 var OAuth2Client = require('googleapis').auth.OAuth2;
+var ServiceToken = require('googleapis').auth.JWT;
 
 var Promise = require('bluebird');
 var fs = Promise.promisifyAll(require('fs'));
@@ -15,6 +16,12 @@ module.exports = function authenticate() {
 };
 
 function getCredentials() {
+  if (process.env.GOOGLE_APPS_PRIVATE_KEY) {
+    const buffer = Buffer.from(process.env.GOOGLE_APPS_PRIVATE_KEY, 'base64').toString();
+    const json = JSON.parse(buffer);
+    json.serviceAccount = true;
+    return Promise.resolve(json);
+  }
   return fs.readFileAsync(defaults.STORAGE_FILE)
     .then(JSON.parse)
     .catch(SyntaxError, function(e) {
@@ -27,6 +34,34 @@ function getCredentials() {
 }
 
 function createAuthClient(credentials) {
+  if (credentials.serviceAccount == true) {
+    var auth = new ServiceToken(
+      credentials.client_email,
+      null,  // 'path/to/key.pem',
+
+      // Contents of private_key.pem if you want to load the pem file yourself
+      // (do not use the path parameter above if using this param)
+      credentials.private_key,
+
+      // Scopes can be specified either as an array or as a single, space-delimited string
+      // ['https://spreadsheets.google.com/feeds'], // TODO: check if this is correct
+      [
+        'https://www.googleapis.com/auth/drive',
+        'https://www.googleapis.com/auth/drive.file',
+        'https://www.googleapis.com/auth/drive.appdata',
+        'https://www.googleapis.com/auth/drive.apps.readonly',
+      ],
+
+      // User to impersonate (leave empty if no impersonation needed)
+      null
+    );
+
+    return Promise.promisify(auth.authorize.bind(auth))()
+      .then(function(res) {
+        return auth;
+      })
+  }
+
   var auth = new OAuth2Client(
     credentials.client_id,
     credentials.client_secret,