Do not include assets source code in the sourcemaps

Summary:
As reported in https://github.com/facebook/metro/issues/109, the sourcemaps generated by metro are including the binary data of all the assets, which adds a lot of overhead and is not useful at all.

This commit removes the assets contents from the sourcemaps to fix https://github.com/facebook/metro/issues/109

Reviewed By: jeanlauliac

Differential Revision: D6649741

fbshipit-source-id: 8b09d429a1aa8d487557c22440bfa73ae55d03bd

packages/metro/src/node-haste/AssetModule.js

@@ -14,6 +14,9 @@
 
 const Module = require('./Module');
 
+import type {TransformedCode} from '../JSTransformer/worker';
+import type {ReadResult} from './Module';
+
 class AssetModule extends Module {
   getPackage() {
     return null;
@@ -26,6 +29,14 @@ class AssetModule extends Module {
   isAsset() {
     return true;
   }
+
+  _finalizeReadResult(source: string, result: TransformedCode): ReadResult {
+    // We do not want to return the "source code" of assets, since it's going to
+    // be binary data and can potentially be very large. This source property
+    // is only used to generate the sourcemaps (since we include all the
+    // modules original sources in the sourcemaps).
+    return {...result, source: ''};
+  }
 }
 
 module.exports = AssetModule;

packages/metro/src/node-haste/__tests__/AssetModule-test.js

@@ -12,12 +12,44 @@
 
 'use strict';
 
+jest.mock('fs').mock('../../lib/TransformCaching');
+
 const AssetModule = require('../AssetModule');
+const DependencyGraphHelpers = require('../DependencyGraph/DependencyGraphHelpers');
+const ModuleCache = require('../ModuleCache');
+const TransformCaching = require('../../lib/TransformCaching');
+const fs = require('fs');
 
 describe('AssetModule:', () => {
   const defaults = {file: '/arbitrary.png'};
 
+  beforeEach(() => {
+    fs.__setMockFilesystem({root: {'image.png': 'png data'}});
+  });
+
   it('is an asset', () => {
     expect(new AssetModule(defaults).isAsset()).toBe(true);
   });
+
+  it('returns an empty source code for an asset', async () => {
+    const module = new AssetModule({
+      depGraphHelpers: new DependencyGraphHelpers({
+        providesModuleNodeModules: [],
+        assetExts: ['png'],
+      }),
+      file: '/root/image.png',
+      getTransformCacheKey: () => 'foo',
+      localPath: 'image.png',
+      moduleCache: new ModuleCache({}),
+      options: {transformCache: TransformCaching.mocked()},
+      transformCode: () => {
+        return Promise.resolve({code: 'module.exports = "asset";'});
+      },
+    });
+
+    const data = await module.read();
+
+    expect(data.code).toBe('module.exports = "asset";');
+    expect(data.source).toBe('');
+  });
 });