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 458 matching lines @@
     HTMLElement::attributeWillChange(name, oldValue, newValue);
 }
 
 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_preloadLength(0)
     , 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)
+{
+    if (!m_preloadScanner)
+        return;
+
+    PreloadRequestStream pendingPreloads;
+    String sheetText = cachedStyleSheet->sheetText();
+    if (sheetText.length() == m_preloadLength)
+        return;
+
+    m_preloadInput->append(SegmentedString(sheetText.substring(m_preloadLength)));
+    m_preloadScanner->scan(sheetText.substring(m_preloadLength), *m_preloadInput, pendingPreloads);
+    m_preloadLength = sheetText.length();

[abarth-chromium, 2014/07/31 14:54:23]

Why do you call sheetText.substring(m_preloadLength) twice?  That's an expensive
operation that mallocs a new string buffer and copies the data into the buffer.

I don't understand why we need to pass the sheetText substring in twice, once in
m_preloadInput and once as the first argument to m_preloadScanner->scan.  In
fact, this is wrong:

void CSSPreloadScanner::scan(const String& tagName,  const SegmentedString&
source, PreloadRequestStream& requests)

The first argument is the tagName, not the CSS source.

[jonnyr, 2015/01/05 10:34:52]

First of all sorry for the delay in following up on this, I am back from five
months on parental leave.

I rewrote this section and started using the html tokenizer so that tagName is
passed to CSSPreloadScanner::scan(), just like it is done in the html background
parser. The main reason was to reuse the tokenizer so that the second parameter,
source, can keep track of request origin. This is used in Developer Tools to see
initiator on the network tab.

+
+    OwnPtr<HTMLResourcePreloader> preloader = HTMLResourcePreloader::create(document());
+    preloader->takeAndPreload(pendingPreloads);
+    if (m_preloadScanner->isDoneParsingImports()) {
+        // Note: Although dataRecieved will be called for each packet of data
+        // returned from the network, @import statements are always 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 186 matching lines @@
         removePendingSheet();
 }
 
 void LinkStyle::trace(Visitor* visitor)
 {
     visitor->trace(m_sheet);
     LinkResource::trace(visitor);
 }
 
 } // namespace blink