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

runtime/bin/eventhandler_macos.cc

 // Copyright (c) 2012, 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_MACOS)
 
 #include "bin/eventhandler.h"
@@ skipping 370 matching lines @@
     if ((events[i].flags & EV_ERROR) != 0) {
       const int kBufferSize = 1024;
       char error_message[kBufferSize];
       Utils::StrError(events[i].data, error_message, kBufferSize);
       FATAL1("kevent failed %s\n", error_message);
     }
     if (events[i].udata == NULL) {
       interrupt_seen = true;
     } else {
       DescriptorInfo* di = reinterpret_cast<DescriptorInfo*>(events[i].udata);
+      const intptr_t old_mask = di->Mask();
       intptr_t event_mask = GetEvents(events + i, di);
       if ((event_mask & (1 << kErrorEvent)) != 0) {
         di->NotifyAllDartPorts(event_mask);
       }
       event_mask &= ~(1 << kErrorEvent);
 
       if (event_mask != 0) {
-        intptr_t old_mask = di->Mask();
         Dart_Port port = di->NextNotifyDartPort(event_mask);
         ASSERT(port != 0);
         UpdateKQueueInstance(old_mask, di);
         DartUtils::PostInt32(port, event_mask);
       }
     }
   }
   if (interrupt_seen) {
     // Handle after socket events, so we avoid closing a socket before we handle
     // the current events.
@@ skipping 97 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_MACOS)
 
 #endif  // !defined(DART_IO_DISABLED)