timers: do not fire timer if unenrolled

The previous commits introduced a behavior change. If two timers fired
at the same time, and the first one to be processed in the list would
unenroll the next one, the next one would still fire.

This change puts back the old behavior of not firing the next timer to
be processed in the list if it has been unenrolled.

lib/timers.js

@@ -406,8 +406,15 @@ var unrefList, unrefTimer;
 
 function _makeTimerTimeout(timer) {
   var domain = timer.domain;
+  var msecs = timer._idleTimeout;
 
-  if (!timer._onTimeout) return;
+  // Timer has been unenrolled by another timer that fired at the same time,
+  // so don't make it timeout.
+  if (!msecs || msecs < 0)
+    return;
+
+  if (!timer._onTimeout)
+    return;
 
   if (domain && domain._disposed)
     return;

test/simple/test-timers-unref-remove-other-unref-timers-only-one-fires.js

@@ -0,0 +1,56 @@
+// 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.
+
+/*
+ * The goal of this test is to make sure that, after the regression introduced
+ * by 934bfe23a16556d05bfb1844ef4d53e8c9887c3d, the fix preserves the following
+ * behavior of unref timers: if two timers are scheduled to fire at the same
+ * time, if one unenrolls the other one in its _onTimeout callback, the other
+ * one will *not* fire.
+ */
+var timers = require('timers');
+var assert = require('assert');
+
+var nbTimersFired = 0;
+
+var foo = new function() {
+  this._onTimeout = function() {
+    ++nbTimersFired;
+    timers.unenroll(bar);
+  };
+}();
+
+var bar = new function() {
+  this._onTimeout = function() {
+    ++nbTimersFired;
+    timers.unenroll(foo);
+  };
+}();
+
+timers.enroll(bar, 1);
+timers._unrefActive(bar);
+
+timers.enroll(foo, 1);
+timers._unrefActive(foo);
+
+setTimeout(function() {
+  assert.notEqual(nbTimersFired, 2);
+}, 20);