test: fix test server on Node 15 (#4668)

This patch fixes a bug in our test server that manifests itself in
Node 15.

Context: Node 14 does not support Apple Silicon (and probably will not),
so we currently have to run tests on Node 15 on new macs.

utils/testserver/index.js

@@ -19,11 +19,16 @@ const https = require('https');
 const url = require('url');
 const fs = require('fs');
 const path = require('path');
+const zlib = require('zlib');
+const util = require('util');
 const WebSocketServer = require('ws').Server;
 
 const fulfillSymbol = Symbol('fullfil callback');
 const rejectSymbol = Symbol('reject callback');
 
+const readFileAsync = util.promisify(fs.readFile.bind(fs));
+const gzipAsync = util.promisify(zlib.gzip.bind(zlib));
+
 class TestServer {
   /**
    * @param {string} dirPath
@@ -236,7 +241,7 @@ class TestServer {
    * @param {!http.ServerResponse} response
    * @param {string|undefined} filePath
    */
-  serveFile(request, response, filePath) {
+  async serveFile(request, response, filePath) {
     let pathName = url.parse(request.url).path;
     if (!filePath) {
       if (pathName === '/')
@@ -258,7 +263,10 @@ class TestServer {
     if (this._csp.has(pathName))
       response.setHeader('Content-Security-Policy', this._csp.get(pathName));
 
-    fs.readFile(filePath, (err, data) => {
+    const {err, data} = await readFileAsync(filePath).then(data => ({data})).catch(err => ({err}));
+    // The HTTP transaction might be already terminated after async hop here - do nothing in this case.
+    if (response.writableEnded)
+      return;
     if (err) {
       response.statusCode = 404;
       response.end(`File not found: ${filePath}`);
@@ -271,14 +279,13 @@ class TestServer {
     response.setHeader('Content-Type', contentType);
     if (this._gzipRoutes.has(pathName)) {
       response.setHeader('Content-Encoding', 'gzip');
-        const zlib = require('zlib');
-        zlib.gzip(data, (_, result) => {
+      const result = await gzipAsync(data);
+      // The HTTP transaction might be already terminated after async hop here.
+      if (!response.writableEnded)
         response.end(result);
-        });
     } else {
       response.end(data);
     }
-    });
   }
 
   _onWebSocketConnection(ws) {