fix(tree-differ): treat symlinks to deleted paths as removals

Previously, tree-differ would not correctly handle symlinks to deleted files, resulting in
an ENOENT errno being tossed by libuv.

This change fixes this to ensure that symlinks are safely handled, performantly.

Closes #1961

tools/broccoli/tree-differ.spec.ts

@@ -104,6 +104,79 @@ describe('TreeDiffer', () => {
     });
 
 
+    it('should handle changes via symbolic links', () => {
+      let testDir = {
+        'orig_path': {
+          'file-1.txt': mockfs.file({content: 'file-1.txt content', mtime: new Date(1000)}),
+          'file-2.txt': mockfs.file({content: 'file-2.txt content', mtime: new Date(1000)}),
+          'subdir-1': {
+            'file-1.1.txt':
+                mockfs.file({content: 'file-1.1.txt content', mtime: new Date(1000)})
+          }
+        },
+        'symlinks': {
+          'file-1.txt': mockfs.symlink({path: '../orig_path/file-1.txt'}),
+          'file-2.txt': mockfs.symlink({path: '../orig_path/file-2.txt'}),
+          'subdir-1':
+              {'file-1.1.txt': mockfs.symlink({path: '../../orig_path/subdir-1/file-1.1.txt'})}
+        }
+      };
+      mockfs(testDir);
+
+      let differ = new TreeDiffer('symlinks');
+
+      let diffResult = differ.diffTree();
+
+      expect(diffResult.changedPaths)
+          .toEqual(['file-1.txt', 'file-2.txt', 'subdir-1/file-1.1.txt']);
+
+      // change two files
+      testDir['orig_path']['file-1.txt'] =
+          mockfs.file({content: 'new content', mtime: new Date(1000)});
+      testDir['orig_path']['subdir-1']['file-1.1.txt'] =
+          mockfs.file({content: 'file-1.1.txt content', mtime: new Date(9999)});
+      mockfs(testDir);
+
+      diffResult = differ.diffTree();
+
+      expect(diffResult.changedPaths).toEqual(['file-1.txt', 'subdir-1/file-1.1.txt']);
+
+      expect(diffResult.removedPaths).toEqual([]);
+
+      // change one file
+      testDir['orig_path']['file-1.txt'] =
+          mockfs.file({content: 'super new', mtime: new Date(1000)});
+      mockfs(testDir);
+
+      diffResult = differ.diffTree();
+      expect(diffResult.changedPaths).toEqual(['file-1.txt']);
+
+      // remove a link
+      delete testDir['orig_path']['file-1.txt'];
+      mockfs(testDir);
+
+      diffResult = differ.diffTree();
+      expect(diffResult.changedPaths).toEqual([]);
+      expect(diffResult.removedPaths).toEqual(['file-1.txt']);
+
+      // don't report it as a removal twice
+      mockfs(testDir);
+
+      diffResult = differ.diffTree();
+      expect(diffResult.changedPaths).toEqual([]);
+      expect(diffResult.removedPaths).toEqual([]);
+
+      // re-add it.
+      testDir['orig_path']['file-1.txt'] =
+          mockfs.file({content: 'super new', mtime: new Date(1000)});
+      mockfs(testDir);
+
+      diffResult = differ.diffTree();
+      expect(diffResult.changedPaths).toEqual(['file-1.txt']);
+      expect(diffResult.removedPaths).toEqual([]);
+    });
+
+
     it('should ignore files with extensions not listed in includeExtensions', () => {
       let testDir = {
         'dir1': {

tools/broccoli/tree-differ.ts

@@ -4,6 +4,16 @@ import fs = require('fs');
 import path = require('path');
 
 
+function tryStatSync(path) {
+  try {
+    return fs.statSync(path);
+  } catch (e) {
+    if (e.code === "ENOENT") return null;
+    throw e;
+  }
+}
+
+
 export class TreeDiffer {
   private fingerprints: {[key: string]: string} = Object.create(null);
   private nextFingerprints: {[key: string]: string} = Object.create(null);
@@ -41,7 +51,11 @@ export class TreeDiffer {
   private dirtyCheckPath(rootDir: string, result: DirtyCheckingDiffResult) {
     fs.readdirSync(rootDir).forEach((segment) => {
       let absolutePath = path.join(rootDir, segment);
-      let pathStat = fs.statSync(absolutePath);
+      let pathStat = fs.lstatSync(absolutePath);
+      if (pathStat.isSymbolicLink()) {
+        pathStat = tryStatSync(absolutePath);
+        if (pathStat === null) return;
+      }
 
       if (pathStat.isDirectory()) {
         result.directoriesChecked++;