Require transform file eagerly in transform worker

Summary:
One consequence we didn't predict after introducing the Internal Transform Pipeline, was that when the workers would get started, we won't require the external transformer the user specified up until the first time each worker received a job. There're 2 visible consequences of this: (1) the transform progress bar seems to get stuck for about 5 seconds the first time the packager receives a request and (2) the first N (# of cores) HMR requests take way longer (about 4 seconds with FB's transformer instead of << 1 second) as we need to require lots of modules.

This diff creates a temporary file for the js transformer workers that requires the user-specified transform file eagerly.
That makes sure workers have imported babel and the transforms before receiving the first request.

There are better ways to do this, like adding an `init()` method to the workers and call that eagerly. I will follow with another diff doing that.

public

Reviewed By: javache

Differential Revision: D2812153

fb-gh-sync-id: 15be316b792d1acd878ed9303bea398aa0b52e1d

react-packager/src/JSTransformer/__tests__/Transformer-test.js

@@ -13,6 +13,7 @@ jest
   .dontMock('../');
 
 jest.mock('fs');
+jest.setMock('temp', {path: () => '/arbitrary/path'});
 
 var Cache = require('../../DependencyResolver/Cache');
 var Transformer = require('../');

react-packager/src/JSTransformer/index.js

@@ -12,6 +12,7 @@ const ModuleTransport = require('../lib/ModuleTransport');
 const Promise = require('promise');
 const declareOpts = require('../lib/declareOpts');
 const fs = require('fs');
+const temp = require('temp');
 const util = require('util');
 const workerFarm = require('worker-farm');
 const debug = require('debug')('ReactNativePackager:JStransformer');
@@ -63,13 +64,22 @@ class Transformer {
     this._transformModulePath = opts.transformModulePath;
 
     if (opts.transformModulePath != null) {
+      this._workerWrapperPath = temp.path();
+      fs.writeFileSync(
+        this._workerWrapperPath,
+        `
+        module.exports = require(${JSON.stringify(require.resolve('./worker'))});
+        require(${JSON.stringify(String(opts.transformModulePath))});
+        `
+      );
+
       this._workers = workerFarm({
         autoStart: true,
         maxConcurrentCallsPerWorker: 1,
         maxCallsPerWorker: MAX_CALLS_PER_WORKER,
         maxCallTime: opts.transformTimeoutInterval,
         maxRetries: MAX_RETRIES,
-      }, require.resolve('./worker'));
+      }, this._workerWrapperPath);
 
       this._transform = Promise.denodeify(this._workers);
     }
@@ -77,6 +87,9 @@ class Transformer {
 
   kill() {
     this._workers && workerFarm.end(this._workers);
+    if (typeof this._workerWrapperPath === 'string') {
+      fs.unlink(this._workerWrapperPath, () => {}); // we don't care about potential errors here
+    }
   }
 
   invalidateFile(filePath) {