packager: Bundler: use transform module hash instead of mtime

Summary: mtime is a problem for the global cache, because everyone has different times at which they rebased/cloned the repo. Use the actual content instead. Of course, the file could depend on other files we don't take into account... but the mtime solution was already problematic in that regard, so I assume this is a safe change.

Reviewed By: davidaurelio

Differential Revision: D4237826

fbshipit-source-id: 9dc18eb422cddd4d5ed097d1ebeef4b8c361ff39

Author: Jean Lauliac

packager/react-packager/src/Bundler/index.js

@@ -12,6 +12,7 @@
 'use strict';
 
 const assert = require('assert');
+const crypto = require('crypto');
 const fs = require('fs');
 const Cache = require('../node-haste').Cache;
 const Transformer = require('../JSTransformer');
@@ -146,20 +147,21 @@ class Bundler {
 
     opts.projectRoots.forEach(verifyRootExists);
 
-    let mtime;
+    let transformModuleHash;
     try {
-      ({mtime} = fs.statSync(opts.transformModulePath));
-      mtime = String(mtime.getTime());
+      const transformModuleStr = fs.readFileSync(opts.transformModulePath);
+      transformModuleHash =
+        crypto.createHash('sha1').update(transformModuleStr).digest('hex');
     } catch (error) {
-      mtime = '';
+      transformModuleHash = '';
     }
 
     const cacheKeyParts =  [
       'react-packager-cache',
       version,
       opts.cacheVersion,
       opts.projectRoots.join(',').split(pathSeparator).join('-'),
-      mtime,
+      transformModuleHash,
     ];
 
     this._getModuleId = createModuleIdFactory();