Prefetch @import files from CSS files.

Source/core/html/HTMLLinkElement.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2003, 2006, 2007, 2008, 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2009 Rob Buis (rwlbuis@gmail.com)
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
@@ skipping 418 matching lines @@
     HTMLElement::trace(visitor);
 }
 
 PassOwnPtrWillBeRawPtr<LinkStyle> LinkStyle::create(HTMLLinkElement* owner)
 {
     return adoptPtrWillBeNoop(new LinkStyle(owner));
 }
 
 LinkStyle::LinkStyle(HTMLLinkElement* owner)
     : LinkResource(owner)
+    , m_preloadScanner(adoptPtr(new CSSPreloadScanner))
+    , m_preloadInput(adoptPtr(new SegmentedString))
     , m_disabledState(Unset)
     , m_pendingSheetType(None)
     , m_loading(false)
     , m_firedLoad(false)
     , m_loadedSheet(false)
 {
 }
 
 LinkStyle::~LinkStyle()
 {
@@ skipping 46 matching lines @@
     styleSheet->parseAuthorStyleSheet(cachedStyleSheet, m_owner->document().securityOrigin());
 
     m_loading = false;
     styleSheet->notifyLoadedSheet(cachedStyleSheet);
     styleSheet->checkLoaded();
 
     if (styleSheet->isCacheable())
         const_cast<CSSStyleSheetResource*>(cachedStyleSheet)->saveParsedStyleSheet(styleSheet);
 }
 
+void LinkStyle::dataReceived(const CSSStyleSheetResource* cachedStyleSheet, const char* data, int length)
+{
+    if (!m_preloadScanner)
+        return;
+
+    PreloadRequestStream pendingPreloads;
+    m_preloadInput->append(SegmentedString(data));

[abarth-chromium, 2014/06/26 16:37:59]

Why don't we need to use |length| on this line?

+    m_preloadScanner->scan(data, *m_preloadInput, pendingPreloads);
+    HTMLResourcePreloader m_preloader(&document());
+    m_preloader.takeAndPreload(pendingPreloads);
+
+    if (m_preloadScanner->isDoneParsingImports()) {

[eseidel, 2014/05/28 23:52:52]

Could you explain the magic here a bit more?

// Note:  Although dataRecieved will be called for each packet of data returned
from the network
// @import statements always are at the top of the CSS file.  As soon as we
reach
// a part of the CSS file which is not an @import we can delete our preloader
and ignore
// the rest of file.  If we ever make CSS preloading more complicated
// (to look for URLs other than @imports) we will need to re-think this logic.

+        m_preloadScanner.clear();
+        m_preloadInput.clear();
+    }
+}
+
 bool LinkStyle::sheetLoaded()
 {
     if (!styleSheetIsLoading()) {
         removePendingSheet();
         return true;
     }
     return false;
 }
 
 void LinkStyle::notifyLoadedSheetAndAllCriticalSubresources(bool errorOccurred)
@@ skipping 189 matching lines @@
         removePendingSheet(RemovePendingSheetNotifyLater);
 }
 
 void LinkStyle::trace(Visitor* visitor)
 {
     visitor->trace(m_sheet);
     LinkResource::trace(visitor);
 }
 
 } // namespace WebCore