perf(website): speed up 'yarn build' on multi-core machines

Process multiple files in parallel. This causes esbuild loading+building
(in demo pages) to overlap with quick-lint-js loading (in error docs
pages) on multi-core machines, reducing wall times for 'yarn build'.

On my MacBookPro (M1) machine:

Before:                     24.176 seconds
After but no promise-limit: 14.508 seconds
After:                      11.903 seconds

Author: strager

website/package.json

@@ -15,6 +15,7 @@
     "jsdom": "19.0.0",
     "markdown-it": "13.0.1",
     "mime": "3.0.0",
+    "promise-limit": "^2.7.0",
     "quick-lint-js-node-test-runner": "../tools/quick-lint-js-node-test-runner/"
   },
   "devDependencies": {

website/run-build.mjs

@@ -3,6 +3,7 @@
 
 import fs from "fs";
 import path from "path";
+import promiseLimit from "promise-limit";
 import { websiteConfig } from "./src/config.mjs";
 import {
   IndexConflictVFSError,
@@ -12,6 +13,9 @@ import {
   VFSDirectory,
 } from "./src/vfs.mjs";
 
+let maxConcurrentJobs = 8;
+let semaphore = promiseLimit(maxConcurrentJobs);
+
 async function mainAsync() {
   let { targetDirectory } = parseArguments(process.argv.slice(2));
 
@@ -25,22 +29,26 @@ async function mainAsync() {
 async function buildDirAsync(vfs, uri, outputDirectory) {
   let listing = await vfs.listDirectoryAsync(uri);
   await fs.promises.mkdir(outputDirectory, { recursive: true });
-  for (let childName of listing.names()) {
-    let child = listing.get(childName);
-    let childPath = path.join(
-      outputDirectory,
-      childName === "" ? "index.html" : childName
-    );
-    if (child instanceof VFSDirectory) {
-      await buildDirAsync(vfs, `${uri}${childName}/`, childPath);
-    } else if (child instanceof IndexConflictVFSError) {
-      throw child;
-    } else {
-      console.log(`generating ${childPath} ...`);
-      let data = await child.getContentsAsync();
-      await fs.promises.writeFile(childPath, data);
-    }
-  }
+  await Promise.all(
+    listing.names().map(async (childName) => {
+      let child = listing.get(childName);
+      let childPath = path.join(
+        outputDirectory,
+        childName === "" ? "index.html" : childName
+      );
+      if (child instanceof VFSDirectory) {
+        await buildDirAsync(vfs, `${uri}${childName}/`, childPath);
+      } else if (child instanceof IndexConflictVFSError) {
+        throw child;
+      } else {
+        await semaphore(async () => {
+          console.log(`generating ${childPath} ...`);
+          let data = await child.getContentsAsync();
+          await fs.promises.writeFile(childPath, data);
+        });
+      }
+    })
+  );
 }
 
 function parseArguments(args) {

website/yarn.lock

@@ -525,6 +525,11 @@ [email protected]:
   resolved "https://registry.yarnpkg.com/prettier/-/prettier-2.6.2.tgz#e26d71a18a74c3d0f0597f55f01fb6c06c206032"
   integrity sha512-PkUpF+qoXTqhOeWL9fu7As8LXsIUZ1WYaJiY/a7McAQzxjk82OF0tibkFXVCDImZtWxbvojFjerkiLb0/q8mew==
 
+promise-limit@^2.7.0:
+  version "2.7.0"
+  resolved "https://registry.yarnpkg.com/promise-limit/-/promise-limit-2.7.0.tgz#eb5737c33342a030eaeaecea9b3d3a93cb592b26"
+  integrity sha512-7nJ6v5lnJsXwGprnGXga4wx6d1POjvi5Qmf1ivTRxTjH4Z/9Czja/UCMLVmB9N93GeWOU93XaFaEt6jbuoagNw==
+
 psl@^1.1.33:
   version "1.8.0"
   resolved "https://registry.yarnpkg.com/psl/-/psl-1.8.0.tgz#9326f8bcfb013adcc005fdff056acce020e51c24"