Prefetch @import files from CSS files.

Source/core/html/HTMLLinkElement.cpp

Index: Source/core/html/HTMLLinkElement.cpp
diff --git a/Source/core/html/HTMLLinkElement.cpp b/Source/core/html/HTMLLinkElement.cpp
index 6b6d8b195ff4c55adb09468cb5ede4836628fbea..e039960f2479721a6dc1e9b43396b6e3b88d70a8 100644
--- a/Source/core/html/HTMLLinkElement.cpp
+++ b/Source/core/html/HTMLLinkElement.cpp
@@ -46,6 +46,7 @@
 #include "core/frame/csp/ContentSecurityPolicy.h"
 #include "core/html/LinkManifest.h"
 #include "core/html/imports/LinkImport.h"
+#include "core/html/parser/CompactHTMLToken.h"
 #include "core/loader/FrameLoader.h"
 #include "core/loader/FrameLoaderClient.h"
 #include "core/rendering/style/StyleInheritedData.h"
@@ -480,12 +481,19 @@ PassOwnPtrWillBeRawPtr<LinkStyle> LinkStyle::create(HTMLLinkElement* owner)
 
 LinkStyle::LinkStyle(HTMLLinkElement* owner)
     : LinkResource(owner)
+    , m_token(adoptPtr(new HTMLToken))
+    , m_tokenizer(HTMLTokenizer::create(m_options))
+    , m_preloadScanner(adoptPtr(new CSSPreloadScanner))
+    , m_preloadLength(0)
     , m_disabledState(Unset)
     , m_pendingSheetType(None)
     , m_loading(false)
     , m_firedLoad(false)
     , m_loadedSheet(false)
 {
+    // The tokenizer uses RAWTEXTState to identify the origin of a request.
+    // This state is also used while handling CSS prefetch in the html parser.
+    m_tokenizer->setState(HTMLTokenizer::RAWTEXTState);
 }
 
 LinkStyle::~LinkStyle()
@@ -554,6 +562,43 @@ void LinkStyle::setCSSStyleSheet(const String& href, const KURL& baseURL, const
         const_cast<CSSStyleSheetResource*>(cachedStyleSheet)->saveParsedStyleSheet(styleSheet);
 }
 
+void LinkStyle::dataReceived(const CSSStyleSheetResource* cachedStyleSheet)
+{
+    // An absence of preload scanner means we are done with @import statements.
+    if (!m_preloadScanner)
+        return;
+
+    PreloadRequestStream pendingPreloads;
+    String sheetText = cachedStyleSheet->sheetText();
+    if (sheetText.length() == m_preloadLength)
+        return;
+
+    m_input.append(sheetText.substring(m_preloadLength));
+
+    while (m_tokenizer->nextToken(m_input.current(), *m_token)) {
+        CompactHTMLToken token(m_token.get(), TextPosition(m_input.current().currentLine(), m_input.current().currentColumn()));
+
+        m_preloadScanner->scan(token.data(), m_input.current(), pendingPreloads);
+        m_token->clear();
+    }
+
+    m_preloadLength = sheetText.length();
+
+    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_tokenizer.clear();
+        m_token.clear();
+    }
+}
+
 bool LinkStyle::sheetLoaded()
 {
     if (!styleSheetIsLoading()) {