Show a warning when using sync xhr.

Source/core/xml/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 572 matching lines @@
         if (m_responseTypeCode != ResponseTypeDefault) {
             exceptionState.throwDOMException(InvalidAccessError, "Synchronous requests from a document must not set a response type.");
             return;
         }
 
         // Similarly, timeouts are disabled for synchronous requests as well.
         if (m_timeoutMilliseconds > 0) {
             exceptionState.throwDOMException(InvalidAccessError, "Synchronous requests must not set a timeout.");
             return;
         }
+
+        // Here we just warn that firing sync XHR's may affect responsiveness.
+        // Eventually sync xhr will be deprecated and an "InvalidAccessError" exception thrown.
+        // Refer : https://xhr.spec.whatwg.org/#sync-warning
+        // Use count for XHR synchronous requests on main thread only.
+        if (!document()->processingBeforeUnload())
+            UseCounter::countDeprecation(executionContext(), UseCounter::XMLHttpRequestSynchronous);

[kouhei (in TOK), 2014/10/14 05:08:49]

I'm still worried about this change. This will break the current use counter, so
I would add a new one to avoid troubles.

The old one was measured at send(), and this one is measured at open().

     }
 
     m_method = uppercaseKnownHTTPMethod(method);
 
     m_url = url;
 
     m_async = async;
 
     ASSERT(!m_loader);
 
@@ skipping 269 matching lines @@
     if (m_async) {
         if (m_upload)
             request.setReportUploadProgress(true);
 
         // ThreadableLoader::create can return null here, for example if we're no longer attached to a page.
         // This is true while running onunload handlers.
         // FIXME: Maybe we need to be able to send XMLHttpRequests from onunload, <http://bugs.webkit.org/show_bug.cgi?id=10904>.
         // FIXME: Maybe create() can return null for other reasons too?
         ASSERT(!m_loader);
         m_loader = ThreadableLoader::create(executionContext, this, request, options, resourceLoaderOptions);
-    } else {
-        // Use count for XHR synchronous requests.
-        UseCounter::count(&executionContext, UseCounter::XMLHttpRequestSynchronous);
+    } else
         ThreadableLoader::loadResourceSynchronously(executionContext, request, *this, options, resourceLoaderOptions);
-    }
 
     if (!m_exceptionCode && m_error)
         m_exceptionCode = NetworkError;
     if (m_exceptionCode)
         exceptionState.throwDOMException(m_exceptionCode, "Failed to load '" + m_url.elidedString() + "'.");
 }
 
 void XMLHttpRequest::abort()
 {
     WTF_LOG(Network, "XMLHttpRequest %p abort()", this);
@@ skipping 673 matching lines @@
     visitor->trace(m_responseStream);
     visitor->trace(m_streamSource);
     visitor->trace(m_responseDocument);
     visitor->trace(m_responseDocumentParser);
     visitor->trace(m_progressEventThrottle);
     visitor->trace(m_upload);
     XMLHttpRequestEventTarget::trace(visitor);
 }
 
 } // namespace blink