XMLHttpRequest: check if 'loadstart' handler cancelled send().

third_party/WebKit/Source/core/xmlhttprequest/XMLHttpRequest.cpp

 /*
  *  Copyright (C) 2004, 2006, 2008 Apple Inc. All rights reserved.
  *  Copyright (C) 2005-2007 Alexey Proskuryakov <ap@webkit.org>
  *  Copyright (C) 2007, 2008 Julien Chaffraix <jchaffraix@webkit.org>
  *  Copyright (C) 2008, 2011 Google Inc. All rights reserved.
  *  Copyright (C) 2012 Intel Corporation
  *
  *  This library is free software; you can redistribute it and/or
  *  modify it under the terms of the GNU Lesser General Public
  *  License as published by the Free Software Foundation; either
@@ skipping 923 matching lines @@
   m_sendFlag = true;
   // The presence of upload event listeners forces us to use preflighting
   // because POSTing to an URL that does not permit cross origin requests should
   // look exactly like POSTing to an URL that does not respond at all.
   // Also, only async requests support upload progress events.
   bool uploadEvents = false;
   if (m_async) {
     InspectorInstrumentation::asyncTaskScheduled(
         &executionContext, "XMLHttpRequest.send", this, true);
     dispatchProgressEvent(EventTypeNames::loadstart, 0, 0);
+    // Event handler could have invalidated this send operation,
+    // (re)setting the send flag and/or initiating another send
+    // operation; leave quietly if so.
+    if (!m_sendFlag || m_loader)
+      return;
     if (httpBody && m_upload) {
       uploadEvents = m_upload->hasEventListeners();
       m_upload->dispatchEvent(
           ProgressEvent::create(EventTypeNames::loadstart, false, 0, 0));
+      // See above.
+      if (!m_sendFlag || m_loader)
+        return;
     }
   }
 
   m_sameOriginRequest = getSecurityOrigin()->canRequestNoSuborigin(m_url);
 
   // Per https://w3c.github.io/webappsec-suborigins/#security-model-opt-outs,
   // credentials are forced when credentials mode is "same-origin", the
   // 'unsafe-credentials' option is set, and the request's physical origin is
   // the same as the URL's.
   bool includeCredentials =
@@ skipping 881 matching lines @@
   visitor->traceWrappers(m_responseDocument);
   visitor->traceWrappers(m_responseArrayBuffer);
   XMLHttpRequestEventTarget::traceWrappers(visitor);
 }
 
 std::ostream& operator<<(std::ostream& ostream, const XMLHttpRequest* xhr) {
   return ostream << "XMLHttpRequest " << static_cast<const void*>(xhr);
 }
 
 }  // namespace blink