timers: don't mutate unref list while iterating it

Commit 934bfe23a1 had introduced a
regression where node would crash trying to access a null unref timer if
a given unref timer's callback would remove other unref timers set to
fire in the future.

More generally, it makes the unrefTimeout function more solid by not
mutating the unrefList while traversing it.

Fixes: https://github.com/joyent/node/issues/8897

Conflicts:
	lib/timers.js

Fixes: https://github.com/nodejs/node-convergence-archive/issues/23
Ref: https://github.com/nodejs/node/issues/268
PR-URL: https://github.com/nodejs/node/pull/2540
Reviewed-By: bnoordhuis - Ben Noordhuis <info@bnoordhuis.nl>

lib/timers.js

@@ -477,6 +477,39 @@ exports.clearImmediate = function(immediate) {
 
 var unrefList, unrefTimer;
 
+function _makeTimerTimeout(timer) {
+  var domain = timer.domain;
+  var msecs = timer._idleTimeout;
+
+  // 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;
+
+  try {
+    var threw = true;
+
+    if (domain) domain.enter();
+
+    debug('unreftimer firing timeout');
+    L.remove(timer);
+    timer._called = true;
+    timer._onTimeout();
+
+    threw = false;
+
+    if (domain)
+      domain.exit();
+  } finally {
+    if (threw) process.nextTick(unrefTimeout);
+  }
+}
 
 function unrefTimeout() {
   var now = Timer.now();
@@ -487,7 +520,7 @@ function unrefTimeout() {
   var nextTimeoutTime;
   var nextTimeoutDuration;
   var minNextTimeoutTime;
-  var itemToDelete;
+  var timersToTimeout = [];
 
   // The actual timer fired and has not yet been rearmed,
   // let's consider its next firing time is invalid for now.
@@ -518,45 +551,21 @@ function unrefTimeout() {
         // we scanned through the whole list.
         minNextTimeoutTime = nextTimeoutTime;
       }
-
-      // This timer hasn't expired yet, skipping
-      cur = cur._idlePrev;
-      continue;
+    } else {
+      // We found a timer that expired. Do not call its _onTimeout callback
+      // right now, as it could mutate any item of the list (including itself).
+      // Instead, add it to another list that will be processed once the list
+      // of current timers has been fully traversed.
+      timersToTimeout.push(cur);
     }
 
-    // We found a timer that expired
-    var domain = cur.domain;
-
-    if (!cur._onTimeout) continue;
-
-    if (domain && domain._disposed)
-      continue;
-
-    try {
-      var threw = true;
-
-      if (domain) domain.enter();
-
-      itemToDelete = cur;
-      // Move to the previous item before calling the _onTimeout callback,
-      // as it can mutate the list.
     cur = cur._idlePrev;
+  }
 
-      // Remove the timeout from the list because it expired.
-      L.remove(itemToDelete);
-
-      debug('unreftimer firing timeout');
-      itemToDelete._called = true;
-      itemToDelete._onTimeout();
-
-      threw = false;
+  var nbTimersToTimeout = timersToTimeout.length;
+  for (var timerIdx = 0; timerIdx < nbTimersToTimeout; ++timerIdx)
+    _makeTimerTimeout(timersToTimeout[timerIdx]);
 
-      if (domain)
-        domain.exit();
-    } finally {
-      if (threw) process.nextTick(unrefTimeout);
-    }
-  }
 
   // Rearm the actual timer with the timeout delay
   // of the earliest timeout found.

test/parallel/test-timers-socket-timeout-removes-other-socket-unref-timer.js

@@ -0,0 +1,50 @@
+'use strict';
+
+/*
+ * This test is a regression test for joyent/node#8897.
+ */
+
+const common = require('../common');
+const assert = require('assert');
+const net = require('net');
+
+const clients = [];
+
+const server = net.createServer(function onClient(client) {
+  clients.push(client);
+
+  if (clients.length === 2) {
+    /*
+     * Enroll two timers, and make the one supposed to fire first
+     * unenroll the other one supposed to fire later. This mutates
+     * the list of unref timers when traversing it, and exposes the
+     * original issue in joyent/node#8897.
+     */
+    clients[0].setTimeout(1, function onTimeout() {
+      clients[1].setTimeout(0);
+      clients[0].end();
+      clients[1].end();
+    });
+
+    // Use a delay that is higher than the lowest timer resolution accross all
+    // supported platforms, so that the two timers don't fire at the same time.
+    clients[1].setTimeout(50);
+  }
+});
+
+server.listen(common.PORT, common.localhostIPv4, function() {
+  var nbClientsEnded = 0;
+
+  function addEndedClient(client) {
+    ++nbClientsEnded;
+    if (nbClientsEnded === 2) {
+      server.close();
+    }
+  };
+
+  const client1 = net.connect({ port: common.PORT });
+  client1.on('end', addEndedClient);
+
+  const client2 = net.connect({ port: common.PORT });
+  client2.on('end', addEndedClient);
+});

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

@@ -0,0 +1,41 @@
+'use strict';
+
+/*
+ * 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.
+ *
+ * This behavior is a private implementation detail and should not be
+ * considered public interface.
+ */
+const common = require('../common');
+const timers = require('timers');
+const assert = require('assert');
+
+var nbTimersFired = 0;
+
+const foo = {
+  _onTimeout: function() {
+    ++nbTimersFired;
+    timers.unenroll(bar);
+  }
+};
+
+const bar = {
+  _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);

test/parallel/test-timers-unref-remove-other-unref-timers.js

@@ -0,0 +1,33 @@
+'use strict';
+
+/*
+ * This test is a regression test for joyent/node#8897.
+ *
+ * It tests some private implementation details that should not be
+ * considered public interface.
+ */
+const common = require('../common');
+const assert = require('assert');
+const timers = require('timers');
+
+const foo = {
+  _onTimeout: assert.fail
+};
+
+const bar = {
+  _onTimeout: common.mustCall(function() {
+    timers.unenroll(foo);
+  })
+};
+
+// We use timers with expiration times that are sufficiently apart to make
+// sure that they're not fired at the same time on platforms where the timer
+// resolution is a bit coarse (e.g Windows with a default resolution of ~15ms).
+timers.enroll(bar, 1);
+timers._unrefActive(bar);
+
+timers.enroll(foo, 50);
+timers._unrefActive(foo);
+
+// Keep the process open.
+setTimeout(function() {}, 100);