add storage api to make preflight checks

Currently there is only disk space checking but sshfs and cifs need
mount point checking as well

src/storage/s3.js

@@ -2,6 +2,7 @@
 
 exports = module.exports = {
     getBackupPath: getBackupPath,
+    checkPreconditions: checkPreconditions,
 
     upload: upload,
     download: download,
@@ -27,6 +28,7 @@ var assert = require('assert'),
     BoxError = require('../boxerror.js'),
     chunk = require('lodash.chunk'),
     constants = require('../constants.js'),
+    DataLayout = require('../datalayout.js'),
     debug = require('debug')('box:storage/s3'),
     EventEmitter = require('events'),
     https = require('https'),
@@ -91,6 +93,14 @@ function getBackupPath(apiConfig) {
     return apiConfig.prefix;
 }
 
+function checkPreconditions(apiConfig, dataLayout, callback) {
+    assert.strictEqual(typeof apiConfig, 'object');
+    assert(dataLayout instanceof DataLayout, 'dataLayout must be a DataLayout');
+    assert.strictEqual(typeof callback, 'function');
+
+    callback(null);
+}
+
 function upload(apiConfig, backupFilePath, sourceStream, callback) {
     assert.strictEqual(typeof apiConfig, 'object');
     assert.strictEqual(typeof backupFilePath, 'string');