disable fetches for parser blocking scripts inserted via doc.write

third_party/WebKit/Source/core/loader/FrameFetchContext.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 153 matching lines @@
         return ReloadIgnoringCacheData;
     if (policy == CachePolicyReload)
         return ReloadBypassingCache;
     if (policy == CachePolicyHistoryBuffer)
         return ReturnCacheDataElseLoad;
     return UseProtocolCachePolicy;
 }
 
 ResourceRequestCachePolicy FrameFetchContext::resourceRequestCachePolicy(const ResourceRequest& request, Resource::Type type) const
 {
+    ASSERT(frame());
     if (type == Resource::MainResource) {
         FrameLoadType frameLoadType = frame()->loader().loadType();
         if (request.httpMethod() == "POST" && frameLoadType == FrameLoadTypeBackForward)
             return ReturnCacheDataDontLoad;
         if (!frame()->host()->overrideEncoding().isEmpty())
             return ReturnCacheDataElseLoad;
         if (frameLoadType == FrameLoadTypeSame || request.isConditional() || request.httpMethod() == "POST")
             return ReloadIgnoringCacheData;
 
         for (Frame* f = frame(); f; f = f->tree().parent()) {
             if (!f->isLocalFrame())
                 continue;
             frameLoadType = toLocalFrame(f)->loader().loadType();
             if (frameLoadType == FrameLoadTypeBackForward)
                 return ReturnCacheDataElseLoad;
             if (frameLoadType == FrameLoadTypeReloadFromOrigin)
                 return ReloadBypassingCache;
             if (frameLoadType == FrameLoadTypeReload)
                 return ReloadIgnoringCacheData;
         }
         return UseProtocolCachePolicy;
     }
 
+    // For users on slow connections, we want to avoid blocking the parser in
+    // the main frame on script loads inserted via document.write, since it can
+    // add significant delays before page content is displayed on the screen.
+    // For now, as a prototype, we block fetches for main frame scripts
+    // inserted via document.write as long as the
+    // disallowFetchForDocWrittenScriptsInMainFrame setting is enabled. In the
+    // future, we'll extend this logic to only block if estimated network RTT
+    // is above some threshold.
+    if (type == Resource::Script && isMainFrame()) {
+        const bool isInDocumentWrite = m_document && m_document->isInDocumentWrite();
+        const bool disallowFetchForDocWriteScripts = frame()->settings() && frame()->settings()->disallowFetchForDocWrittenScriptsInMainFrame();
+        if (isInDocumentWrite && disallowFetchForDocWriteScripts)
+            return ReturnCacheDataDontLoad;
+    }
+
     if (request.isConditional())
         return ReloadIgnoringCacheData;
 
     if (m_documentLoader && m_document && !m_document->loadEventFinished()) {
         // For POST requests, we mutate the main resource's cache policy to avoid form resubmission.
         // This policy should not be inherited by subresources.
         ResourceRequestCachePolicy mainResourceCachePolicy = m_documentLoader->request().cachePolicy();
         if (m_documentLoader->request().httpMethod() == "POST") {
             if (mainResourceCachePolicy == ReturnCacheDataDontLoad)
                 return ReturnCacheDataElseLoad;
@@ skipping 558 matching lines @@
 }
 
 DEFINE_TRACE(FrameFetchContext)
 {
     visitor->trace(m_document);
     visitor->trace(m_documentLoader);
     FetchContext::trace(visitor);
 }
 
 } // namespace blink