timers: assure setTimeout callback only runs once

This fixes an edge case where running this.unref() during the callback caused the
callback to get executed multiple times.

lib/timers.js

@@ -85,6 +85,7 @@ function listOnTimeout() {
         if (domain)
           domain.enter();
         threw = true;
+        first._called = true;
         first._onTimeout();
         if (domain)
           domain.exit();
@@ -305,6 +306,9 @@ Timeout.prototype.unref = function() {
   if (this._handle) {
     this._handle.unref();
   } else if (typeof(this._onTimeout) === 'function') {
+    // Prevent running callback multiple times
+    // when unref() is called during the callback
+    if (this._called) return;
     var now = Timer.now();
     if (!this._idleStart) this._idleStart = now;
     var delay = this._idleStart + this._idleTimeout - now;
@@ -492,6 +496,7 @@ function unrefTimeout() {
       if (domain) domain.enter();
       threw = true;
       debug('unreftimer firing timeout');
+      first._called = true;
       first._onTimeout();
       threw = false;
       if (domain)

test/parallel/test-timers-unref.js

@@ -5,6 +5,7 @@ var interval_fired = false,
     timeout_fired = false,
     unref_interval = false,
     unref_timer = false,
+    unref_callbacks = 0,
     interval, check_unref, checks = 0;
 
 var LONG_TIME = 10 * 1000;
@@ -34,6 +35,11 @@ check_unref = setInterval(function() {
   checks += 1;
 }, 100);
 
+setTimeout(function() {
+  unref_callbacks++;
+  this.unref();
+}, SHORT_TIME);
+
 // Should not assert on args.Holder()->InternalFieldCount() > 0. See #4261.
 (function() {
   var t = setInterval(function() {}, 1);
@@ -46,4 +52,5 @@ process.on('exit', function() {
   assert.strictEqual(timeout_fired, false, 'Timeout should not fire');
   assert.strictEqual(unref_timer, true, 'An unrefd timeout should still fire');
   assert.strictEqual(unref_interval, true, 'An unrefd interval should still fire');
+  assert.strictEqual(unref_callbacks, 1, 'Callback should only run once');
 });