perf: run HTML checks concurrently

Runs all Content Document checks concurrently, earch within a new
page of a shared Chrome browser instance.

The results are aggregated in document order.

The concurrency level is currently hard-coded to 4, but it should
be made configurable when implementing issue #77.

Based on very naïve –and probably not very significant– benchmarking,
the execution time is reduced by a factor of at least 2 on large
publications.

package.json

@@ -59,6 +59,7 @@
     "marko": "^4.4.26",
     "meow": "^3.7.0",
     "multer": "^1.3.0",
+    "p-map": "^1.2.0",
     "path": "^0.12.7",
     "puppeteer": "^0.12.0",
     "shortid": "^2.2.8",

src/checker/checker-chromium.js

@@ -2,6 +2,7 @@
 
 const fs = require('fs');
 const path = require('path');
+const pMap = require('p-map');
 const puppeteer = require('puppeteer');
 const axe2ace = require('../report/axe2ace.js');
 const winston = require('winston');
@@ -43,7 +44,7 @@ if (!fs.existsSync(PATH_TO_ACE_EXTRACTION)) {
 }
 
 async function checkSingle(spineItem, epub, browser) {
-  winston.info(`- ${spineItem.relpath}`);
+  winston.verbose(`- Processing ${spineItem.relpath}`);
   try {
     const page = await browser.newPage();
     await page.goto(spineItem.url);
@@ -92,10 +93,10 @@ async function checkSingle(spineItem, epub, browser) {
     // Post-process results
     results.assertions = (results.axe != null) ? axe2ace.axe2ace(spineItem, results.axe) : [];
     delete results.axe;
-    winston.info(`- ${
-      (results.assertions == null)
-        ? 'No'
-        : results.assertions.assertions.length} issues found`);
+    winston.info(`- ${spineItem.relpath}: ${
+      (results.assertions && results.assertions.lentgh > 0)
+        ? results.assertions.assertions.length
+        : 'No'} issues found`);
     // Resolve path and locators for extracted data
     if (results.data != null) {
       Object.getOwnPropertyNames(results.data).forEach((key) => {
@@ -133,13 +134,7 @@ async function checkSingle(spineItem, epub, browser) {
 module.exports.check = async (epub) => {
   const browser = await puppeteer.launch();
   winston.info('Checking documents...');
-  return epub.contentDocs.reduce((sequence, spineItem) =>
-    sequence.then(results =>
-      checkSingle(spineItem, epub, browser)
-      .then((result) => {
-        results.push(result);
-        return results;
-      })), Promise.resolve([]))
+  return pMap(epub.contentDocs, doc => checkSingle(doc, epub, browser), { concurrency: 4 })
   .then(async (results) => {
     await browser.close();
     return results;

yarn.lock

@@ -3846,6 +3846,10 @@ p-locate@^2.0.0:
   dependencies:
     p-limit "^1.1.0"
 
+p-map@^1.2.0:
+  version "1.2.0"
+  resolved "https://registry.yarnpkg.com/p-map/-/p-map-1.2.0.tgz#e4e94f311eabbc8633a1e79908165fca26241b6b"
+
 package-json@^1.0.0:
   version "1.2.0"
   resolved "https://registry.yarnpkg.com/package-json/-/package-json-1.2.0.tgz#c8ecac094227cdf76a316874ed05e27cc939a0e0"