timers: fix cleanup of nested same-timeout timers

For nested timers with the same timeout, we can get into a situation
where we have recreated a timer list immediately before we need to
clean up an old timer list with the same key. Fix: make sure the list
to be deleted is the same instance as the list whose reference was used
to determine that a cleanup is necessary. If it's not the same instance,
a new list with the same key has been created, and it should not be
deleted.

Fixes: #7722

lib/timers.js

@@ -211,9 +211,13 @@ function listOnTimeout() {
   debug('%d list empty', msecs);
   assert(L.isEmpty(list));
   this.close();
-  if (list._unrefed === true) {
+
+  // Either refedLists[msecs] or unrefedLists[msecs] may have been removed and
+  // recreated since the reference to `list` was created. Make sure they're
+  // the same instance of the list before destroying.
+  if (list._unrefed === true && list === unrefedLists[msecs]) {
     delete unrefedLists[msecs];
-  } else {
+  } else if (list === refedLists[msecs]) {
     delete refedLists[msecs];
   }
 }

test/parallel/test-timers-same-timeout-wrong-list-deleted.js

@@ -0,0 +1,80 @@
+'use strict';
+
+/*
+ * This is a regression test for https://github.com/nodejs/node/issues/7722.
+ *
+ * When nested timers have the same timeout, calling clearTimeout on the
+ * older timer after it has fired causes the list the newer timer is in
+ * to be deleted. Since the newer timer was not cleared, it still blocks
+ * the event loop completing for the duration of its timeout, however, since
+ * no reference exists to it in its list, it cannot be canceled and its
+ * callback is not called when the timeout elapses.
+ */
+
+const common = require('../common');
+const assert = require('assert');
+const Timer = process.binding('timer_wrap').Timer;
+
+const TIMEOUT = common.platformTimeout(100);
+const start = Timer.now();
+
+// This bug also prevents the erroneously dereferenced timer's callback
+// from being called, so we can't use it's execution or lack thereof
+// to assert that the bug is fixed.
+process.on('exit', function() {
+  const end = Timer.now();
+  assert.equal(end - start < TIMEOUT * 2, true,
+               'Elapsed time does not include second timer\'s timeout.');
+});
+
+const handle1 = setTimeout(common.mustCall(function() {
+  // Cause the old TIMEOUT list to be deleted
+  clearTimeout(handle1);
+
+  // Cause a new list with the same key (TIMEOUT) to be created for this timer
+  const handle2 = setTimeout(function() {
+    common.fail('Inner callback is not called');
+  }, TIMEOUT);
+
+  setTimeout(common.mustCall(function() {
+    // Attempt to cancel the second timer. Fix for this bug will keep the
+    // newer timer from being dereferenced by keeping its list from being
+    // erroneously deleted. If we are able to cancel the timer successfully,
+    // the bug is fixed.
+    clearTimeout(handle2);
+    setImmediate(common.mustCall(function() {
+      setImmediate(common.mustCall(function() {
+        const activeHandles = process._getActiveHandles();
+        const activeTimers = activeHandles.filter(function(handle) {
+          return handle instanceof Timer;
+        });
+
+        // Make sure our clearTimeout succeeded. One timer finished and
+        // the other was canceled, so none should be active.
+        assert.equal(activeTimers.length, 0, 'No Timers remain.');
+      }));
+    }));
+  }), 10);
+
+  // Make sure our timers got added to the list.
+  const activeHandles = process._getActiveHandles();
+  const activeTimers = activeHandles.filter(function(handle) {
+    return handle instanceof Timer;
+  });
+  const shortTimer = activeTimers.find(function(handle) {
+    return handle._list.msecs === 10;
+  });
+  const longTimers = activeTimers.filter(function(handle) {
+    return handle._list.msecs === TIMEOUT;
+  });
+
+  // Make sure our clearTimeout succeeded. One timer finished and
+  // the other was canceled, so none should be active.
+  assert.equal(activeTimers.length, 3, 'There are 3 timers in the list.');
+  assert(shortTimer instanceof Timer, 'The shorter timer is in the list.');
+  assert.equal(longTimers.length, 2, 'Both longer timers are in the list.');
+
+  // When this callback completes, `listOnTimeout` should now look at the
+  // correct list and refrain from removing the new TIMEOUT list which
+  // contains the reference to the newer timer.
+}), TIMEOUT);