performance: Use worker threads to minify JS/CSS files (#3823)

src/node/utils/MinifyWorker.js

@@ -0,0 +1,67 @@
+/**
+ * Worker thread to minify JS & CSS files out of the main NodeJS thread
+ */
+
+var CleanCSS = require('clean-css');
+var uglifyJS = require("uglify-js");
+var path = require('path');
+var Threads = require('threads')
+
+function compressJS(content)
+{
+  return uglifyJS.minify(content);
+}
+
+function compressCSS(filename, ROOT_DIR)
+{
+  return new Promise((res, rej) => {
+    try {
+      const absPath = path.join(ROOT_DIR, filename);
+
+      /*
+       * Changes done to migrate CleanCSS 3.x -> 4.x:
+       *
+       * 1. Rework the rebase logic, because the API was simplified (but we have
+       *    less control now). See:
+       *    https://github.com/jakubpawlowicz/clean-css/blob/08f3a74925524d30bbe7ac450979de0a8a9e54b2/README.md#important-40-breaking-changes
+       *
+       *    EXAMPLE:
+       *        The URLs contained in a CSS file (including all the stylesheets
+       *        imported by it) residing on disk at:
+       *            /home/muxator/etherpad/src/static/css/pad.css
+       *
+       *        Will be rewritten rebasing them to:
+       *            /home/muxator/etherpad/src/static/css
+       *
+       * 2. CleanCSS.minify() can either receive a string containing the CSS, or
+       *    an array of strings. In that case each array element is interpreted as
+       *    an absolute local path from which the CSS file is read.
+       *
+       *    In version 4.x, CleanCSS API was simplified, eliminating the
+       *    relativeTo parameter, and thus we cannot use our already loaded
+       *    "content" argument, but we have to wrap the absolute path to the CSS
+       *    in an array and ask the library to read it by itself.
+       */
+
+      const basePath = path.dirname(absPath);
+
+      new CleanCSS({
+        rebase: true,
+        rebaseTo: basePath,
+      }).minify([absPath], function (errors, minified) {
+        if (errors) return rej(errors)
+
+        return res(minified.styles)
+      });
+    } catch (error) {
+      // on error, just yield the un-minified original, but write a log message
+      console.error(`Unexpected error minifying ${filename} (${absPath}): ${error}`);
+      callback(null, content);
+    }
+  })
+}
+
+Threads.expose({
+  compressJS,
+  compressCSS
+})