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/filesystem.js

@@ -2,6 +2,7 @@
 
 exports = module.exports = {
     getBackupPath: getBackupPath,
+    checkPreconditions: checkPreconditions,
 
     upload: upload,
     download: download,
@@ -20,11 +21,14 @@ exports = module.exports = {
 
 var assert = require('assert'),
     BoxError = require('../boxerror.js'),
+    DataLayout = require('../datalayout.js'),
     debug = require('debug')('box:storage/filesystem'),
+    df = require('@sindresorhus/df'),
     EventEmitter = require('events'),
     fs = require('fs'),
     mkdirp = require('mkdirp'),
     path = require('path'),
+    prettyBytes = require('pretty-bytes'),
     readdirp = require('readdirp'),
     safe = require('safetydance'),
     shell = require('../shell.js');
@@ -36,6 +40,32 @@ function getBackupPath(apiConfig) {
     return apiConfig.backupFolder;
 }
 
+// the du call in the function below requires root
+function checkPreconditions(apiConfig, dataLayout, callback) {
+    assert.strictEqual(typeof apiConfig, 'object');
+    assert(dataLayout instanceof DataLayout, 'dataLayout must be a DataLayout');
+    assert.strictEqual(typeof callback, 'function');
+
+    let used = 0;
+    for (let localPath of dataLayout.localPaths()) {
+        debug(`checkPreconditions: getting disk usage of ${localPath}`);
+        let result = safe.child_process.execSync(`du -Dsb ${localPath}`, { encoding: 'utf8' });
+        if (!result) return callback(new BoxError(BoxError.FS_ERROR, safe.error));
+        used += parseInt(result, 10);
+    }
+
+    debug(`checkPreconditions: ${used} bytes`);
+
+    df.file(apiConfig.backupFolder).then(function (diskUsage) {
+        const needed = used + (1024 * 1024 * 1024); // check if there is atleast 1GB left afterwards
+        if (diskUsage.available <= needed) return callback(new BoxError(BoxError.FS_ERROR, `Not enough disk space for backup. Needed: ${prettyBytes(needed)} Available: ${prettyBytes(diskUsage.available)}`));
+
+        callback(null);
+    }).catch(function (error) {
+        callback(new BoxError(BoxError.FS_ERROR, error));
+    });
+}
+
 function upload(apiConfig, backupFilePath, sourceStream, callback) {
     assert.strictEqual(typeof apiConfig, 'object');
     assert.strictEqual(typeof backupFilePath, 'string');