Add route to download app backups

src/apps.js

@@ -56,6 +56,7 @@ exports = module.exports = {
     backup,
     listBackups,
     updateBackup,
+    getBackupDownloadStream,
 
     getTask,
     getLogPaths,
@@ -175,9 +176,11 @@ const appstore = require('./appstore.js'),
     shell = require('./shell.js'),
     spawn = require('child_process').spawn,
     split = require('split'),
+    storage = require('./storage.js'),
     superagent = require('superagent'),
     system = require('./system.js'),
     tasks = require('./tasks.js'),
+    tgz = require('./backupformat/tgz.js'),
     TransformStream = require('stream').Transform,
     users = require('./users.js'),
     util = require('util'),
@@ -2617,6 +2620,25 @@ async function updateBackup(app, backupId, data) {
     await backups.update(backupId, data);
 }
 
+async function getBackupDownloadStream(app, backupId) {
+    assert.strictEqual(typeof app, 'object');
+    assert.strictEqual(typeof backupId, 'string');
+
+    const backup = await backups.get(backupId);
+    if (!backup) throw new BoxError(BoxError.NOT_FOUND, 'Backup not found');
+    if (backup.identifier !== app.id) throw new BoxError(BoxError.NOT_FOUND, 'Backup not found'); // some other app's backup
+    if (backup.format !== 'tgz') throw new BoxError(BoxError.BAD_STATE, 'only tgz backups can be downloaded');
+
+    const backupConfig = await settings.getBackupConfig();
+
+    return new Promise((resolve, reject) => {
+        storage.api(backupConfig.provider).download(backupConfig, tgz.getBackupFilePath(backupConfig, backup.remotePath), function (error, sourceStream) {
+            if (error) return reject(error);
+            resolve(sourceStream);
+        });
+    });
+}
+
 async function restoreInstalledApps(options, auditSource) {
     assert.strictEqual(typeof options, 'object');
     assert.strictEqual(typeof auditSource, 'object');