syncer: simply return the changes

this is easier to test. the initial code wanted to make the changes a stream.
but this never happenned since the need never arose

src/syncer.js

@@ -1,18 +1,17 @@
 'use strict';
 
 const assert = require('assert'),
-    async = require('async'),
     BoxError = require('./boxerror.js'),
     DataLayout = require('./datalayout.js'),
     debug = require('debug')('box:syncer'),
     fs = require('fs'),
     path = require('path'),
     paths = require('./paths.js'),
-    safe = require('safetydance'),
-    util = require('util');
+    safe = require('safetydance');
 
 exports = module.exports = {
-    sync: util.promisify(sync)
+    sync,
+    finalize
 };
 
 function readCache(cacheFile) {
@@ -64,19 +63,16 @@ function ISFILE(x) {
     return (x & fs.constants.S_IFREG) === fs.constants.S_IFREG;
 }
 
-function sync(dataLayout, taskProcessor, concurrency, callback) {
+async function sync(dataLayout) {
     assert(dataLayout instanceof DataLayout, 'Expecting dataLayout to be a DataLayout');
-    assert.strictEqual(typeof taskProcessor, 'function');
-    assert.strictEqual(typeof concurrency, 'number');
-    assert.strictEqual(typeof callback, 'function');
 
-    const addQueue = [], delQueue = [];
+    const addQueue = [], delQueue = []; // separate queues. we have to process the del first and then the add
     let curCacheIndex = 0;
 
     const cacheFile = path.join(paths.BACKUP_INFO_DIR, dataLayout.getBasename() + '.sync.cache'),
         newCacheFile = path.join(paths.BACKUP_INFO_DIR, dataLayout.getBasename() + '.sync.cache.new');
 
-    let cache = [ ];
+    let cache = [];
 
     // if cache is missing or if we crashed/errored in previous run, start out empty. TODO: do a remote listDir and rebuild
     if (!safe.fs.existsSync(cacheFile)) {
@@ -88,7 +84,7 @@ function sync(dataLayout, taskProcessor, concurrency, callback) {
     }
 
     const newCacheFd = safe.fs.openSync(newCacheFile, 'w'); // truncates any existing file
-    if (newCacheFd === -1) return callback(new BoxError(BoxError.FS_ERROR, 'Error opening new cache file: ' + safe.error.message));
+    if (newCacheFd === -1) throw new BoxError(BoxError.FS_ERROR, 'Error opening new cache file: ' + safe.error.message);
 
     function advanceCache(entryPath) {
         let lastRemovedDir = null;
@@ -157,21 +153,18 @@ function sync(dataLayout, taskProcessor, concurrency, callback) {
 
     safe.fs.closeSync(newCacheFd);
 
-    debug(`sync: processing ${delQueue.length} deletes and ${addQueue.length} additions`);
-
-    async.eachLimit(delQueue, concurrency, taskProcessor, function (error) {
-        debug('sync: done processing deletes. error: %o', error);
-
-        async.eachLimit(addQueue, concurrency, taskProcessor, function (error) {
-            debug('sync: done processing adds. error: %o', error);
-
-            if (error) return callback(error);
-
-            safe.fs.unlinkSync(cacheFile);
-
-            if (!safe.fs.renameSync(newCacheFile, cacheFile)) debug('Unable to save new cache file');
-
-            callback();
-        });
-    });
+    return {
+        delQueue,
+        addQueue,
+        cacheFile,
+        newCacheFile
+    };
+}
+
+async function finalize(changes) {
+    assert.strictEqual(typeof changes, 'object');
+
+    safe.fs.unlinkSync(changes.cacheFile);
+
+    if (!safe.fs.renameSync(changes.newCacheFile, changes.cacheFile)) debug('Unable to save new cache file');
 }