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

fixes #772

src/storage/gcs.js

@@ -5,6 +5,7 @@ exports = module.exports = {
     checkPreconditions,
 
     upload,
+    exists,
     download,
     copy,
 
@@ -100,6 +101,36 @@ function upload(apiConfig, backupFilePath, sourceStream, callback) {
     sourceStream.pipe(uploadStream);
 }
 
+function exists(apiConfig, backupFilePath, callback) {
+    assert.strictEqual(typeof apiConfig, 'object');
+    assert.strictEqual(typeof backupFilePath, 'string');
+    assert.strictEqual(typeof callback, 'function');
+
+    const bucket = getBucket(apiConfig);
+
+    if (!backupFilePath.endsWith('/')) {
+        const file = bucket.file(backupFilePath);
+        file.getMetadata(function (error) {
+            if (error && error.code === 404) return callback(null, false);
+            if (error) return callback(new BoxError(BoxError.EXTERNAL_ERROR, error.message));
+
+            callback(null, true);
+        });
+    } else {
+        const query = {
+            prefix: backupFilePath,
+            maxResults: 1,
+            autoPaginate: true
+        };
+
+        bucket.getFiles(query, function (error, files) {
+            if (error) return callback(error);
+
+            callback(null, files.length !== 0);
+        });
+    }
+}
+
 function download(apiConfig, backupFilePath, callback) {
     assert.strictEqual(typeof apiConfig, 'object');
     assert.strictEqual(typeof backupFilePath, 'string');