fs: fix watchFile() missing deletion events

Make sure the deletion event gets reported in the following scenario:

  1. Watch a file.
  2. The initial stat() goes okay.
  3. Something deletes the watched file.
  4. The second stat() fails with ENOENT.

The second stat() translates into the first 'change' event but a logic error
stopped it from getting emitted.

Fixes #4027.

lib/fs.js

@@ -881,7 +881,10 @@ function StatWatcher() {
   var oldStatus = -1;
 
   this._handle.onchange = function(current, previous, newStatus) {
-    if (oldStatus == -1 && newStatus == -1) return;
+    if (oldStatus === -1 &&
+        newStatus === -1 &&
+        current.nlink === previous.nlink) return;
+
     oldStatus = newStatus;
     self.emit('change', current, previous);
   };

test/simple/test-regress-GH-4027.js

@@ -0,0 +1,41 @@
+// Copyright Joyent, Inc. and other Node contributors.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a
+// copy of this software and associated documentation files (the
+// "Software"), to deal in the Software without restriction, including
+// without limitation the rights to use, copy, modify, merge, publish,
+// distribute, sublicense, and/or sell copies of the Software, and to permit
+// persons to whom the Software is furnished to do so, subject to the
+// following conditions:
+//
+// The above copyright notice and this permission notice shall be included
+// in all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN
+// NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
+// DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
+// USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+var common = require('../common');
+var assert = require('assert');
+var path = require('path');
+var fs = require('fs');
+
+var filename = path.join(common.tmpDir, 'watched');
+fs.writeFileSync(filename, 'quis custodiet ipsos custodes');
+setTimeout(fs.unlinkSync, 100, filename);
+
+var seenEvent = 0;
+process.on('exit', function() {
+  assert.equal(seenEvent, 1);
+});
+
+fs.watchFile(filename, { interval: 50 }, function(curr, prev) {
+  assert.equal(prev.nlink, 1);
+  assert.equal(curr.nlink, 0);
+  fs.unwatchFile(filename);
+  seenEvent++;
+});