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/interface.js

@@ -12,6 +12,7 @@
 //     them to tune the concurrency based on failures/rate limits accordingly
 exports = module.exports = {
     getBackupPath: getBackupPath,
+    checkPreconditions: checkPreconditions,
 
     upload: upload,
 
@@ -31,6 +32,7 @@ exports = module.exports = {
 
 var assert = require('assert'),
     BoxError = require('../boxerror.js'),
+    DataLayout = require('../datalayout.js'),
     EventEmitter = require('events');
 
 function removePrivateFields(apiConfig) {
@@ -50,6 +52,14 @@ function getBackupPath(apiConfig) {
     return '/';
 }
 
+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');