fs: do not emit 'stop' watch event synchronously

Emits 'stop' event for fs.watchFile on process.nextTick
to fix 'maximum call stack size exceeded' error when
`stop` is called synchronously after listener is attached.

PR-URL: https://github.com/nodejs/node/pull/8524
Fixes: https://github.com/nodejs/node/issues/8421
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Yorkie Liu <yorkiefixer@gmail.com>

lib/fs.js

@@ -1443,6 +1443,10 @@ fs.watch = function(filename, options, listener) {
 
 // Stat Change Watchers
 
+function emitStop(self) {
+  self.emit('stop');
+}
+
 function StatWatcher() {
   EventEmitter.call(this);
 
@@ -1463,7 +1467,7 @@ function StatWatcher() {
   };
 
   this._handle.onstop = function() {
-    self.emit('stop');
+    process.nextTick(emitStop, self);
   };
 }
 util.inherits(StatWatcher, EventEmitter);

test/parallel/test-fs-watch-stop-async.js

@@ -0,0 +1,19 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const fs = require('fs');
+
+const watch = fs.watchFile(__filename, () => {});
+let triggered;
+const listener = common.mustCall(() => {
+  triggered = true;
+});
+
+triggered = false;
+watch.once('stop', listener);  // Should trigger.
+watch.stop();
+assert.equal(triggered, false);
+setImmediate(() => {
+  assert.equal(triggered, true);
+  watch.removeListener('stop', listener);
+});

test/parallel/test-fs-watch-stop-sync.js

@@ -0,0 +1,9 @@
+'use strict';
+require('../common');
+const assert = require('assert');
+const fs = require('fs');
+
+const watch = fs.watchFile(__filename, () => {});
+watch.once('stop', assert.fail);  // Should not trigger.
+watch.stop();
+watch.removeListener('stop', assert.fail);