Preload scan external css for @import

third_party/WebKit/Source/core/html/parser/CSSPreloadScanner.cpp

Index: third_party/WebKit/Source/core/html/parser/CSSPreloadScanner.cpp
diff --git a/third_party/WebKit/Source/core/html/parser/CSSPreloadScanner.cpp b/third_party/WebKit/Source/core/html/parser/CSSPreloadScanner.cpp
index 15a5aaf1d36714052c7ccaf8db2548737a75454c..532e5a6182ccfec551f3607032fa143d45bf2422 100644
--- a/third_party/WebKit/Source/core/html/parser/CSSPreloadScanner.cpp
+++ b/third_party/WebKit/Source/core/html/parser/CSSPreloadScanner.cpp
@@ -27,8 +27,11 @@
 
 #include "core/html/parser/CSSPreloadScanner.h"
 
+#include "core/fetch/CSSStyleSheetResource.h"
 #include "core/fetch/FetchInitiatorTypeNames.h"
 #include "core/html/parser/HTMLParserIdioms.h"
+#include "core/html/parser/HTMLResourcePreloader.h"
+#include "platform/Histogram.h"
 #include "platform/text/SegmentedString.h"
 
 namespace blink {
@@ -233,4 +236,33 @@ void CSSPreloadScanner::emitRule(const SegmentedString& source)
     m_ruleValue.clear();
 }
 
+CSSPreloaderResourceClient::CSSPreloaderResourceClient(Resource* resource, HTMLResourcePreloader* preloader)
+    : m_resource(resource)
+    , m_preloader(preloader)
+{
+}
+
+void CSSPreloaderResourceClient::notifyFinished(Resource* resource)
+{
+    resource->removeClient(this);
+}
+
+// Only attach for one appendData call, as that's where most imports will likely
+// be (according to spec).
+void CSSPreloaderResourceClient::didAppendFirstData(const CSSStyleSheetResource* resource)
+{
+    const String& chunk = resource->decodedText();
+    if (!chunk.isNull() && m_preloader) {
+        CSSPreloadScanner cssPreloadScanner;
+        PreloadRequestStream preloads;
+        // TODO(csharrison): Should SegmentedString be populated?

[kouhei (in TOK), 2016/04/25 22:10:05]

The only caveat of using empty SegmentedString here is that we will get
PreloadRequests without source file/line info. I'm not sure how important that
is.

[Charlie Harrison, 2016/04/25 22:29:12]

Yeah I know :/ I think leaving it out is okay, because @import should be the
first thing in the file besides charset. We could include it, but it would
entail a copy.

I'll leave it out for now and we can revisit if we decide to land this
experiment.

+        cssPreloadScanner.scan(chunk, SegmentedString(), preloads, resource->response().url());
+        int currentPreloadCount = m_preloader->countPreloads();
+        m_preloader->takeAndPreload(preloads);
+        DEFINE_STATIC_LOCAL(CustomCountHistogram, cssImportHistogram, ("CSSPreloaderResourceClient.PreloadCount", 1, 100, 50));
+        cssImportHistogram.count(m_preloader->countPreloads() - currentPreloadCount);
+    }
+    m_resource->removeClient(this);
+}
+
 } // namespace blink