VM eventhandler: Read "old" event mask before it can be modified.

runtime/bin/eventhandler_fuchsia.cc

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #if !defined(DART_IO_DISABLED)
 
 #include "platform/globals.h"
 #if defined(TARGET_OS_FUCHSIA)
 
 #include "bin/eventhandler.h"
@@ skipping 370 matching lines @@
 
 void EventHandlerImplementation::HandleEvents(struct epoll_event* events,
                                               int size) {
   bool interrupt_seen = false;
   for (int i = 0; i < size; i++) {
     if (events[i].data.ptr == NULL) {
       interrupt_seen = true;
     } else {
       DescriptorInfo* di =
           reinterpret_cast<DescriptorInfo*>(events[i].data.ptr);
+      const intptr_t old_mask = di->Mask();
       intptr_t event_mask = GetPollEvents(events[i].events, di);
-
       if ((event_mask & (1 << kErrorEvent)) != 0) {
         di->NotifyAllDartPorts(event_mask);
       }
       event_mask &= ~(1 << kErrorEvent);
 
       LOG_INFO("HandleEvents: fd=%ld events=%ld\n", di->fd(), event_mask);
       if (event_mask != 0) {
-        intptr_t old_mask = di->Mask();
         Dart_Port port = di->NextNotifyDartPort(event_mask);
         ASSERT(port != 0);
         UpdateEpollInstance(old_mask, di);
         LOG_INFO("HandleEvents: Posting %ld to %ld for fd=%ld\n", event_mask,
                  port, di->fd());
         bool success = DartUtils::PostInt32(port, event_mask);
         if (!success) {
           // This can happen if e.g. the isolate that owns the port has died
           // for some reason.
           LOG_ERR("Failed to post event for fd %ld to port %ld", di->fd(),
@@ skipping 94 matching lines @@
   // The hashmap does not support keys with value 0.
   return dart::Utils::WordHash(fd + 1);
 }
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_FUCHSIA)
 
 #endif  // !defined(DART_IO_DISABLED)