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

fixes #772

src/storage/s3.js

@@ -5,6 +5,7 @@ exports = module.exports = {
     checkPreconditions,
 
     upload,
+    exists,
     download,
     copy,
 
@@ -56,7 +57,7 @@ function getS3Config(apiConfig, callback) {
     assert.strictEqual(typeof apiConfig, 'object');
     assert.strictEqual(typeof callback, 'function');
 
-    var credentials = {
+    let credentials = {
         signatureVersion: apiConfig.signatureVersion || 'v4',
         s3ForcePathStyle: false, // Use vhost style instead of path style - https://forums.aws.amazon.com/ann.jspa?annID=6776
         accessKeyId: apiConfig.accessKeyId,
@@ -137,6 +138,44 @@ function upload(apiConfig, backupFilePath, sourceStream, callback) {
     });
 }
 
+function exists(apiConfig, backupFilePath, callback) {
+    assert.strictEqual(typeof apiConfig, 'object');
+    assert.strictEqual(typeof backupFilePath, 'string');
+    assert.strictEqual(typeof callback, 'function');
+
+    getS3Config(apiConfig, function (error, credentials) {
+        if (error) return callback(error);
+
+        const s3 = new AWS.S3(credentials);
+
+        if (!backupFilePath.endsWith('/')) { // check for file
+            const params = {
+                Bucket: apiConfig.bucket,
+                Key: backupFilePath
+            };
+
+            s3.headObject(params, function (error) {
+                if (error && S3_NOT_FOUND(error)) return callback(null, false);
+                if (error) return callback(new BoxError(BoxError.EXTERNAL_ERROR, error.message || error.code));
+
+                callback(null, true);
+            });
+        } else { // list dir contents
+            const listParams = {
+                Bucket: apiConfig.bucket,
+                Prefix: backupFilePath,
+                MaxKeys: 1
+            };
+
+            s3.listObjects(listParams, function (error, listData) {
+                if (error) return callback(new BoxError(BoxError.EXTERNAL_ERROR, error.message || error.code));
+
+                callback(null, listData.Contents.length !== 0);
+            });
+        }
+    });
+}
+
 function download(apiConfig, backupFilePath, callback) {
     assert.strictEqual(typeof apiConfig, 'object');
     assert.strictEqual(typeof backupFilePath, 'string');