fix: skip double compression for child compilation

src/Webpack5Cache.js

@@ -14,7 +14,7 @@ export default class Cache {
   async get(task) {
     // eslint-disable-next-line no-param-reassign
     task.cacheIdent =
-      task.cacheIdent || `${task.file}|${serialize(task.cacheKeys)}`;
+      task.cacheIdent || `${task.name}|${serialize(task.cacheKeys)}`;
     // eslint-disable-next-line no-param-reassign
     task.cacheETag =
       task.cacheETag || this.cache.getLazyHashedEtag(task.assetSource);

src/index.js

@@ -171,8 +171,38 @@ class CssMinimizerPlugin {
       : Math.min(Number(parallel) || 0, cpus.length - 1);
   }
 
-  *taskGenerator(compiler, compilation, file) {
-    const assetSource = compilation.assets[file];
+  // eslint-disable-next-line consistent-return
+  static getAsset(compilation, name) {
+    // New API
+    if (compilation.getAsset) {
+      return compilation.getAsset(name);
+    }
+
+    if (compilation.assets[name]) {
+      return { name, source: compilation.assets[name], info: {} };
+    }
+  }
+
+  static updateAsset(compilation, name, newSource, assetInfo) {
+    // New API
+    if (compilation.updateAsset) {
+      compilation.updateAsset(name, newSource, assetInfo);
+    }
+
+    // eslint-disable-next-line no-param-reassign
+    compilation.assets[name] = newSource;
+  }
+
+  *taskGenerator(compiler, compilation, name) {
+    const { info, source: assetSource } = CssMinimizerPlugin.getAsset(
+      compilation,
+      name
+    );
+
+    // Skip double minimize assets from child compilation
+    if (info.minimized) {
+      yield false;
+    }
 
     let input;
     let inputSourceMap;
@@ -190,7 +220,7 @@ class CssMinimizerPlugin {
           inputSourceMap = map;
 
           compilation.warnings.push(
-            new Error(`${file} contains invalid source map`)
+            new Error(`${name} contains invalid source map`)
           );
         }
       }
@@ -222,7 +252,7 @@ class CssMinimizerPlugin {
         compilation.errors.push(
           CssMinimizerPlugin.buildError(
             error,
-            file,
+            name,
             sourceMap,
             new RequestShortener(compiler.context)
           )
@@ -236,7 +266,7 @@ class CssMinimizerPlugin {
       if (map) {
         outputSource = new SourceMapSource(
           code,
-          file,
+          name,
           map,
           input,
           inputSourceMap,
@@ -246,16 +276,17 @@ class CssMinimizerPlugin {
         outputSource = new RawSource(code);
       }
 
-      // Updating assets
-      // eslint-disable-next-line no-param-reassign
-      compilation.assets[file] = outputSource;
+      CssMinimizerPlugin.updateAsset(compilation, name, outputSource, {
+        ...info,
+        minimized: true,
+      });
 
       // Handling warnings
       if (warnings && warnings.length > 0) {
         warnings.forEach((warning) => {
           const builtWarning = CssMinimizerPlugin.buildWarning(
             warning,
-            file,
+            name,
             sourceMap,
             new RequestShortener(compiler.context),
             this.options.warningsFilter
@@ -269,7 +300,7 @@ class CssMinimizerPlugin {
     };
 
     const task = {
-      file,
+      name,
       input,
       inputSourceMap,
       map: this.options.sourceMap,
@@ -299,11 +330,11 @@ class CssMinimizerPlugin {
           'css-minimizer-webpack-plugin': require('../package.json').version,
           'css-minimizer-webpack-plugin-options': this.options,
           nodeVersion: process.version,
-          filename: file,
+          filename: name,
           contentHash: digest.substr(0, hashDigestLength),
         };
 
-        task.cacheKeys = this.options.cacheKeys(defaultCacheKeys, file);
+        task.cacheKeys = this.options.cacheKeys(defaultCacheKeys, name);
       }
     } else {
       // For webpack@5 cache

src/minify.js

@@ -10,15 +10,15 @@ function warningsToString(warnings) {
 
 const minify = async (options) => {
   const {
-    file,
+    name,
     input,
     minimizerOptions,
     map,
     inputSourceMap,
     minify: minifyFn,
   } = options;
 
-  const postcssOptions = { to: file, from: file };
+  const postcssOptions = { to: name, from: name };
 
   if (minifyFn) {
     const result = await minifyFn(

test/CssMinimizerPlugin.test.js

@@ -340,4 +340,35 @@ describe('CssMinimizerPlugin', () => {
       }
     });
   });
+
+  it('should work with child compilation', async () => {
+    const compiler = getCompiler({
+      entry: {
+        entry: `${__dirname}/fixtures/entry.js`,
+      },
+      module: {
+        rules: [
+          {
+            test: /entry.js$/i,
+            use: [
+              {
+                loader: path.resolve(__dirname, './helpers/preLoader'),
+              },
+            ],
+          },
+          {
+            test: /.s?css$/i,
+            use: [MiniCssExtractPlugin.loader, 'css-loader'],
+          },
+        ],
+      },
+    });
+    new CssMinimizerPlugin().apply(compiler);
+
+    const stats = await compile(compiler);
+
+    expect(readAssets(compiler, stats, '.css')).toMatchSnapshot('assets');
+    expect(getErrors(stats)).toMatchSnapshot('errors');
+    expect(getWarnings(stats)).toMatchSnapshot('warnings');
+  });
 });

test/__snapshots__/CssMinimizerPlugin.test.js.snap.webpack4

@@ -66,6 +66,16 @@ exports[`CssMinimizerPlugin should work with assets using querystring: entry.css
 /*# sourceMappingURL=entry.css.map?v=test*/"
 `;
 
+exports[`CssMinimizerPlugin should work with child compilation: assets 1`] = `
+Object {
+  "entry.css": ".entry{text-align:center}",
+}
+`;
+
+exports[`CssMinimizerPlugin should work with child compilation: errors 1`] = `Array []`;
+
+exports[`CssMinimizerPlugin should work with child compilation: warnings 1`] = `Array []`;
+
 exports[`CssMinimizerPlugin should write stdout and stderr of workers to stdout and stderr of main process in parallel mode: assets 1`] = `
 Object {
   "one.css": ".minify {};",

test/__snapshots__/CssMinimizerPlugin.test.js.snap.webpack5

@@ -66,6 +66,16 @@ exports[`CssMinimizerPlugin should work with assets using querystring: entry.css
 /*# sourceMappingURL=entry.css.map?v=test*/"
 `;
 
+exports[`CssMinimizerPlugin should work with child compilation: assets 1`] = `
+Object {
+  "entry.css": ".entry{text-align:center}",
+}
+`;
+
+exports[`CssMinimizerPlugin should work with child compilation: errors 1`] = `Array []`;
+
+exports[`CssMinimizerPlugin should work with child compilation: warnings 1`] = `Array []`;
+
 exports[`CssMinimizerPlugin should write stdout and stderr of workers to stdout and stderr of main process in parallel mode: assets 1`] = `
 Object {
   "one.css": ".minify {};",

test/helpers/preLoader.js

@@ -0,0 +1,42 @@
+import { RawSource } from 'webpack-sources';
+
+class PreCopyPlugin {
+  constructor(options = {}) {
+    this.options = options.options || {};
+  }
+
+  // eslint-disable-next-line class-methods-use-this
+  apply(compiler) {
+    const plugin = { name: 'PreCopyPlugin' };
+
+    compiler.hooks.compilation.tap(plugin, (compilation) => {
+      compilation.hooks.additionalAssets.tapAsync(plugin, (callback) => {
+        compilation.emitAsset(
+          'entry.css',
+          new RawSource('.entry {\n  text-align: center;\n}\n\n')
+        );
+
+        callback();
+      });
+    });
+  }
+}
+
+export default function loader() {
+  const callback = this.async();
+
+  const childCompiler = this._compilation.createChildCompiler(
+    `preloader`,
+    this.options
+  );
+
+  new PreCopyPlugin().apply(childCompiler);
+
+  childCompiler.runAsChild((error) => {
+    if (error) {
+      return callback(error);
+    }
+
+    return callback(null, 'export default 1');
+  });
+}

test/worker.test.js

@@ -7,7 +7,7 @@ import { normalizeErrors } from './helpers';
 describe('worker', () => {
   it('should minify css', async () => {
     const options = {
-      file: 'entry.css',
+      name: 'entry.css',
       input: '.foo{color:red;}\n.bar{color:coral;}',
       inputSourceMap: {
         version: 3,
@@ -27,7 +27,7 @@ describe('worker', () => {
 
   it('should work inputSourceMap as prev', async () => {
     const options = {
-      file: 'entry.css',
+      name: 'entry.css',
       input: '.foo{color:red;}\n.bar{color:coral;}',
       minimizerOptions: { discardComments: false },
       inputSourceMap: {
@@ -47,7 +47,7 @@ describe('worker', () => {
 
   it('should work options.minify function', async () => {
     const options = {
-      file: 'entry.css',
+      name: 'entry.css',
       input: '.foo{color:red;}\n.bar{color:coral;}',
       minimizerOptions: { discardComments: false },
       minify: () => {
@@ -62,7 +62,7 @@ describe('worker', () => {
 
   it('should emit error', async () => {
     const options = {
-      file: 'entry.css',
+      name: 'entry.css',
       input: false,
     };
 
@@ -79,7 +79,7 @@ describe('worker', () => {
 
   it('should emit minimizer error', async () => {
     const options = {
-      file: 'entry.css',
+      name: 'entry.css',
       input: false,
       minify: () => {
         return { error: new Error('css minimizer error') };