Fix the exception thrown when writeToDisk is enabled in multi-compiler mode

lib/fs.js

@@ -12,42 +12,45 @@ const { mkdirp } = new NodeOutputFileSystem();
 
 module.exports = {
   toDisk(context) {
-    context.compiler.hooks.afterEmit.tap('WebpackDevMiddleware', (compilation) => {
-      const { assets, compiler } = compilation;
-      const { log } = context;
-      const { writeToDisk: filter } = context.options;
-      let { outputPath } = compiler;
+    const compilers = context.compiler.compilers || [context.compiler];
+    for (const compiler of compilers) {
+      compiler.hooks.afterEmit.tap('WebpackDevMiddleware', (compilation) => {
+        const { assets } = compilation;
+        const { log } = context;
+        const { writeToDisk: filter } = context.options;
+        let { outputPath } = compiler;
 
-      if (outputPath === '/') {
-        outputPath = compiler.context;
-      }
+        if (outputPath === '/') {
+          outputPath = compiler.context;
+        }
 
-      for (const assetPath of Object.keys(assets)) {
-        const asset = assets[assetPath];
-        const source = asset.source();
-        const isAbsolute = pathabs(assetPath);
-        const writePath = isAbsolute ? assetPath : path.join(outputPath, assetPath);
-        const relativePath = path.relative(process.cwd(), writePath);
-        const allowWrite = filter && typeof filter === 'function' ? filter(writePath) : true;
+        for (const assetPath of Object.keys(assets)) {
+          const asset = assets[assetPath];
+          const source = asset.source();
+          const isAbsolute = pathabs(assetPath);
+          const writePath = isAbsolute ? assetPath : path.join(outputPath, assetPath);
+          const relativePath = path.relative(process.cwd(), writePath);
+          const allowWrite = filter && typeof filter === 'function' ? filter(writePath) : true;
 
-        if (allowWrite) {
-          let output = source;
+          if (allowWrite) {
+            let output = source;
 
-          mkdirp.sync(path.dirname(writePath));
+            mkdirp.sync(path.dirname(writePath));
 
-          if (Array.isArray(source)) {
-            output = source.join('\n');
-          }
+            if (Array.isArray(source)) {
+              output = source.join('\n');
+            }
 
-          try {
-            fs.writeFileSync(writePath, output, 'utf-8');
-            log.debug(chalk`{cyan Asset written to disk}: ${relativePath}`);
-          } catch (e) {
-            log.error(`Unable to write asset to disk:\n${e}`);
+            try {
+              fs.writeFileSync(writePath, output, 'utf-8');
+              log.debug(chalk`{cyan Asset written to disk}: ${relativePath}`);
+            } catch (e) {
+              log.error(`Unable to write asset to disk:\n${e}`);
+            }
           }
         }
-      }
-    });
+      });
+    }
   },
 
   setFs(context, compiler) {

test/fixtures/server-test/webpack.array.config.js

@@ -1,12 +1,14 @@
 'use strict';
 
+const path = require('path');
+
 module.exports = [{
   mode: 'development',
   context: __dirname,
   entry: './foo.js',
   output: {
     filename: 'foo.js',
-    path: '/js1',
+    path: path.resolve(__dirname, 'js1'),
     publicPath: '/js1/'
   },
   module: {
@@ -24,7 +26,7 @@ module.exports = [{
   entry: './bar.js',
   output: {
     filename: 'bar.js',
-    path: '/js2',
+    path: path.resolve(__dirname, 'js2'),
     publicPath: '/js2/'
   }
 }];

test/tests/server.js

@@ -377,4 +377,49 @@ describe('Server', () => {
         });
     });
   });
+
+  function writeToDiskWithMultiCompiler(value, done) {
+    app = express();
+    const compiler = webpack(webpackMultiConfig);
+    instance = middleware(compiler, {
+      stats: 'errors-only',
+      logLevel,
+      writeToDisk: value
+    });
+    app.use(instance);
+    app.use((req, res) => {
+      res.sendStatus(200);
+    });
+    listen = listenShorthand(done);
+  }
+
+  describe('write to disk with MultiCompiler', () => {
+    before((done) => {
+      writeToDiskWithMultiCompiler(true, done);
+    });
+    after(close);
+
+    it('should find the bundle files on disk', (done) => {
+      request(app).get('/foo/bar')
+        .expect(200, () => {
+          const bundleFiles = [
+            '../fixtures/server-test/js1/foo.js',
+            '../fixtures/server-test/js1/index.html',
+            '../fixtures/server-test/js1/svg.svg',
+            '../fixtures/server-test/js2/bar.js'
+          ];
+
+          for (const bundleFile of bundleFiles) {
+            const bundlePath = path.join(__dirname, bundleFile);
+            assert(fs.existsSync(bundlePath));
+            fs.unlinkSync(bundlePath);
+          }
+
+          fs.rmdirSync(path.join(__dirname, '../fixtures/server-test/js1/'));
+          fs.rmdirSync(path.join(__dirname, '../fixtures/server-test/js2/'));
+
+          done();
+        });
+    });
+  });
 });