backups: remove entries from database that don't exist in storage

fixes #772

src/storage/filesystem.js

@@ -1,22 +1,23 @@
 'use strict';
 
 exports = module.exports = {
-    getBackupPath: getBackupPath,
-    checkPreconditions: checkPreconditions,
+    getBackupPath,
+    checkPreconditions,
 
-    upload: upload,
-    download: download,
+    upload,
+    download,
 
-    copy: copy,
+    copy,
 
-    listDir: listDir,
+    exists,
+    listDir,
 
-    remove: remove,
-    removeDir: removeDir,
+    remove,
+    removeDir,
 
-    testConfig: testConfig,
-    removePrivateFields: removePrivateFields,
-    injectPrivateFields: injectPrivateFields
+    testConfig,
+    removePrivateFields,
+    injectPrivateFields
 };
 
 const PROVIDER_FILESYSTEM = 'filesystem';
@@ -136,6 +137,20 @@ function download(apiConfig, sourceFilePath, callback) {
     callback(null, fileStream);
 }
 
+function exists(apiConfig, sourceFilePath, callback) {
+    assert.strictEqual(typeof apiConfig, 'object');
+    assert.strictEqual(typeof sourceFilePath, 'string');
+    assert.strictEqual(typeof callback, 'function');
+
+    // do not use existsSync because it does not return EPERM etc
+    if (!safe.fs.statSync(sourceFilePath)) {
+        if (safe.error && safe.error.code === 'ENOENT') return callback(null, false);
+        if (safe.error) return callback(new BoxError(BoxError.EXTERNAL_ERROR, `Exists ${sourceFilePath}: ${safe.error.message}`));
+    }
+
+    callback(null, true);
+}
+
 function listDir(apiConfig, dir, batchSize, iteratorCallback, callback) {
     assert.strictEqual(typeof apiConfig, 'object');
     assert.strictEqual(typeof dir, 'string');