[LazyParseCSS] Ensure UseCounting has parity with strict parsing

third_party/WebKit/Source/core/css/parser/CSSLazyParsingState.cpp

Index: third_party/WebKit/Source/core/css/parser/CSSLazyParsingState.cpp
diff --git a/third_party/WebKit/Source/core/css/parser/CSSLazyParsingState.cpp b/third_party/WebKit/Source/core/css/parser/CSSLazyParsingState.cpp
index 8370f0a0dbd447696b93fc7dd2c55b6cb6698e6b..601ac6003830e83ac39a9bdcac43d01da8027924 100644
--- a/third_party/WebKit/Source/core/css/parser/CSSLazyParsingState.cpp
+++ b/third_party/WebKit/Source/core/css/parser/CSSLazyParsingState.cpp
@@ -4,6 +4,7 @@
 
 #include "core/css/parser/CSSLazyParsingState.h"
 #include "core/css/parser/CSSParserTokenRange.h"
+#include "core/dom/Document.h"
 #include "core/frame/UseCounter.h"
 
 namespace blink {
@@ -19,9 +20,14 @@ CSSLazyParsingState::CSSLazyParsingState(const CSSParserContext& context,
 
 const CSSParserContext& CSSLazyParsingState::context() {
   DCHECK(m_owningContents);
-  UseCounter* sheetCounter = UseCounter::getFrom(m_owningContents);
-  if (sheetCounter != m_context.useCounter())
-    m_context = CSSParserContext(m_context, sheetCounter);
+  Document* document = m_owningContents->anyOwnerDocument();
+  if (document != m_document) {

[rune, 2016/12/02 22:18:09]

Couldn't you check if m_document is nullptr before calling anyOwnerDocument()?

Like:

if (!m_document) {
  m_document = m_owningContents->anyOwnerDocument();
  m_context = ...;
}

Or is it a problem if you use the use-counter from a document which is no longer
a client, but before it is garbage collected and the weak reference is not
cleared?

If it's theoretically possible that you enter lazy parsing after all clients
have been cleared, but documents are not garbage collected, the if (document !=
m_document) variant could set m_document from a non-null to a nullptr.

[Charlie Harrison, 2016/12/02 22:38:46]

Yes I think we can do this. My reason for doing it like this is that it is much
easier to unit test this code. In addition, using a if (!m_document) check is
much faster than pulling the owning document.

+    // Since all StyleSheetContents are parsed with a single owner document, try
+    // as best as possible to grab a valid UseCounter if the underlying document
+    // has gone away.
+    m_document = document;
+    m_context = CSSParserContext(m_context, UseCounter::getFrom(m_document));
+  }
   return m_context;
 }
 
@@ -53,4 +59,9 @@ bool CSSLazyParsingState::shouldLazilyParseProperties(
   return true;
 }
 
+DEFINE_TRACE(CSSLazyParsingState) {
+  visitor->trace(m_owningContents);
+  visitor->trace(m_document);
+}
+
 }  // namespace blink