Add integrity to link preload tags

Closes #111

README.md

@@ -155,11 +155,13 @@ Webpack >= 4).
 
 ### Preloading
 
-`<link rel="preload">` doesn't work as expected in current Chrome versions, even
-if the integrity attribute is added to the `link tag (which the current version
-of webpack-subresource-integrity does _not_ do.) The resource will be loaded
-twice, defeating the purpose of preloading.  This issue doesn't appear to exist
-in Firefox or Safari.  See issue #111 for more information.
+This plugin adds the integrity attribute to `<link rel="preload">`
+tags, but preloading with SRI doesn't work as expected in current
+Chrome versions.  The resource will be loaded twice, defeating the
+purpose of preloading.  This problem doesn't appear to exist in
+Firefox or Safari.  See [issue
+#111](https://github.com/waysact/webpack-subresource-integrity/issues/111)
+for more information.
 
 ### Proxies
 

examples/webpack-preload/README.md

@@ -0,0 +1,3 @@
+# With `/* webpackPreload: "true" */`
+
+https://github.com/waysact/webpack-subresource-integrity/issues/111

examples/webpack-preload/index.js

@@ -0,0 +1 @@
+import('./lazy-chunk-1.js');

examples/webpack-preload/lazy-chunk-1.js

@@ -0,0 +1,5 @@
+setTimeout(() => {
+  import(/* webpackPreload: true */ './lazy-chunk-2.js').then(mod =>
+    mod.test()
+  );
+}, 750);

examples/webpack-preload/lazy-chunk-2.js

@@ -0,0 +1,16 @@
+module.exports.test = () => {
+  const linkTag = Array.from(document.getElementsByTagName('link')).find(
+    el => el.rel === 'preload'
+  );
+  const scriptTag = Array.from(document.getElementsByTagName('script')).find(
+    el => linkTag && el.src === linkTag.href
+  );
+  console.log(
+    scriptTag &&
+      linkTag &&
+      scriptTag.integrity &&
+      scriptTag.integrity === linkTag.integrity
+      ? 'ok'
+      : 'error'
+  );
+};

examples/webpack-preload/test.js

@@ -0,0 +1,8 @@
+var webpackVersion = Number(
+  require('webpack/package.json').version.split('.')[0]
+);
+
+module.exports.skip = function skip() {
+  // webpack-preload tag requires Webpack 4
+  return webpackVersion < 4;
+};

examples/webpack-preload/webpack.config.js

@@ -0,0 +1,18 @@
+var SriPlugin = require('webpack-subresource-integrity');
+var HtmlWebpackPlugin = require('html-webpack-plugin');
+
+module.exports = {
+  entry: {
+    index: './index.js'
+  },
+  output: {
+    crossOriginLoading: 'anonymous'
+  },
+  plugins: [
+    new SriPlugin({
+      hashFuncNames: ['sha256', 'sha384'],
+      enabled: true
+    }),
+    new HtmlWebpackPlugin()
+  ]
+};

jmtp.js

@@ -46,8 +46,8 @@ WebIntegrityJsonpMainTemplatePlugin.prototype.addSriHashes =
 /*
  *  Patch jsonp-script code to add the integrity attribute.
  */
-WebIntegrityJsonpMainTemplatePlugin.prototype.jsonpScriptPlugin =
-  function jsonpScriptPlugin(mainTemplate, source) {
+WebIntegrityJsonpMainTemplatePlugin.prototype.addAttribute =
+  function addAttribute(mainTemplate, elName, source) {
     if (!mainTemplate.outputOptions.crossOriginLoading) {
       this.sriPlugin.errorOnce(
         this.compilation,
@@ -56,22 +56,24 @@ WebIntegrityJsonpMainTemplatePlugin.prototype.jsonpScriptPlugin =
     }
     return (Template.asString || mainTemplate.asString)([
       source,
-      'script.integrity = sriHashes[chunkId];',
-      'script.crossOrigin = ' + JSON.stringify(mainTemplate.outputOptions.crossOriginLoading) + ';',
+      elName + '.integrity = sriHashes[chunkId];',
+      elName + '.crossOrigin = ' + JSON.stringify(mainTemplate.outputOptions.crossOriginLoading) + ';',
     ]);
   };
 
 WebIntegrityJsonpMainTemplatePlugin.prototype.apply = function apply(
   mainTemplate
 ) {
-  var jsonpScriptPlugin = this.jsonpScriptPlugin.bind(this, mainTemplate);
+  var jsonpScriptPlugin = this.addAttribute.bind(this, mainTemplate, "script");
+  var linkPreloadPlugin = this.addAttribute.bind(this, mainTemplate, "link");
   var addSriHashes = this.addSriHashes.bind(this, mainTemplate);
 
   if (!mainTemplate.hooks) {
     mainTemplate.plugin('jsonp-script', jsonpScriptPlugin);
     mainTemplate.plugin('local-vars', addSriHashes);
   } else if (mainTemplate.hooks.jsonpScript && mainTemplate.hooks.localVars) {
     mainTemplate.hooks.jsonpScript.tap('SriPlugin', jsonpScriptPlugin);
+    mainTemplate.hooks.linkPreload.tap('SriPlugin', linkPreloadPlugin);
     mainTemplate.hooks.localVars.tap('SriPlugin', addSriHashes);
   } else {
     this.sriPlugin.warnOnce(