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 5b1e253e9ae9ad5a0940b25817d3fca1bca46862..b05a3d082cf0da3abdbd1c5de2647f3faa9fc43a 100644
--- a/Source/core/html/HTMLLinkElement.cpp
+++ b/Source/core/html/HTMLLinkElement.cpp
@@ -476,6 +476,9 @@ PassOwnPtrWillBeRawPtr<LinkStyle> LinkStyle::create(HTMLLinkElement* 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)
@@ -542,6 +545,34 @@ void LinkStyle::setCSSStyleSheet(const String& href, const KURL& baseURL, const
         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()) {