Lazy Parse CSS

third_party/WebKit/Source/core/css/StyleSheetContents.cpp

Index: third_party/WebKit/Source/core/css/StyleSheetContents.cpp
diff --git a/third_party/WebKit/Source/core/css/StyleSheetContents.cpp b/third_party/WebKit/Source/core/css/StyleSheetContents.cpp
index 9ad890082c3cc4b20fc52366b5317ba5fd16ad06..640048b5a2fd716da2783dee1199e76e6c8ed2cc 100644
--- a/third_party/WebKit/Source/core/css/StyleSheetContents.cpp
+++ b/third_party/WebKit/Source/core/css/StyleSheetContents.cpp
@@ -333,6 +333,8 @@ void StyleSheetContents::parseAuthorStyleSheet(const CSSStyleSheetResource* cach
 
     CSSStyleSheetResource::MIMETypeCheck mimeTypeCheck = isQuirksModeBehavior(m_parserContext.mode()) && isSameOriginRequest ? CSSStyleSheetResource::MIMETypeCheck::Lax : CSSStyleSheetResource::MIMETypeCheck::Strict;
     String sheetText = cachedStyleSheet->sheetText(mimeTypeCheck);
+    if (RuntimeEnabledFeatures::lazyParseCSSEnabled())
+        m_sheetText = sheetText;
 
     const ResourceResponse& response = cachedStyleSheet->response();
     m_sourceMapURL = response.httpHeaderField(HTTPNames::SourceMap);
@@ -341,8 +343,9 @@ void StyleSheetContents::parseAuthorStyleSheet(const CSSStyleSheetResource* cach
         m_sourceMapURL = response.httpHeaderField(HTTPNames::X_SourceMap);
     }
 
-    CSSParserContext context(parserContext(), UseCounter::getFrom(this));

[Timothy Loh, 2016/09/26 05:18:00]

Presumably there was some reason for this to be written this way -- it looks
like we're overriding the UseCounter on parserContext(). Could you try and work
out what's happening here? I worry that we'll end up failing to count a whole
lot of things.

[Charlie Harrison, 2016/09/26 22:14:45]

After looking through the code I think you're right.

I've attempted to solve this by persisting the StyleSheetContents in the
closure, and calling UseCounter::getFrom(stylesheet) when lazy parsing is
triggered. However, this is not performant for reasons I'm not sure about
(probably every Persistent<> triggers a lot of bookkeeping in Oilpan to add
another root to the graph).

The solution I ended up with is passing (unretained)
- The stylesheet's context
- The stylesheet's usecounter
This allows us to use this pattern when the closure is triggered.

-    CSSParser::parseSheet(context, this, sheetText);
+    // Note: pass the member context here, as lazy parsing requires retaining a
+    // reference to it.
+    CSSParser::parseSheet(parserContext(), this, sheetText, RuntimeEnabledFeatures::lazyParseCSSEnabled());
 }
 
 void StyleSheetContents::parseString(const String& sheetText)
@@ -474,7 +477,7 @@ static bool childRulesHaveFailedOrCanceledSubresources(const HeapVector<Member<S
         const StyleRuleBase* rule = rules[i].get();
         switch (rule->type()) {
         case StyleRuleBase::Style:
-            if (toStyleRule(rule)->properties().hasFailedOrCanceledSubresources())
+            if (toStyleRule(rule)->propertiesHaveFailedOrCanceledSubresources())
                 return true;
             break;
         case StyleRuleBase::FontFace: