MHTML saving optimization: don't generate CSS code that will be discarded.

third_party/WebKit/Source/core/frame/FrameSerializer.cpp

 /*
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 310 matching lines @@
     } else if (isHTMLStyleElement(element)) {
       HTMLStyleElement& styleElement = toHTMLStyleElement(element);
       if (CSSStyleSheet* sheet = styleElement.sheet())
         serializeCSSStyleSheet(*sheet, KURL());
     }
   }
 }
 
 void FrameSerializer::serializeCSSStyleSheet(CSSStyleSheet& styleSheet,
                                              const KURL& url) {
+  // If the URL is invalid or if it is a data URL this means that this CSS is
+  // defined inline, respectively in a <style> tag or in the data URL itself.
+  bool isInlineCss = !url.isValid() || url.protocolIsData();
+  // If this CSS is not inline then it is identifiable by its URL. So just skip
+  // it if it has already been analyzed before.
+  if (!isInlineCss && (m_resourceURLs.contains(url) ||
+                       m_delegate.shouldSkipResourceWithURL(url))) {
+    return;
+  }
+
   TRACE_EVENT2("page-serialization", "FrameSerializer::serializeCSSStyleSheet",
                "type", "CSS", "url", url.elidedString().utf8().data());
   // Only report UMA metric if this is not a reentrant CSS serialization call.
   double cssStartTime = 0;
   if (!m_isSerializingCss) {
     m_isSerializingCss = true;
     cssStartTime = monotonicallyIncreasingTime();
   }
 
-  StringBuilder cssText;
-  cssText.append("@charset \"");
-  cssText.append(styleSheet.contents()->charset().lower());
-  cssText.append("\";\n\n");
+  // If this CSS is inlined its definition was already serialized with the frame
+  // HTML code that was previously generated. No need to regenerate it here.
+  if (!isInlineCss) {
+    StringBuilder cssText;
+    cssText.append("@charset \"");
+    cssText.append(styleSheet.contents()->charset().lower());
+    cssText.append("\";\n\n");
 
-  for (unsigned i = 0; i < styleSheet.length(); ++i) {
-    CSSRule* rule = styleSheet.item(i);
-    String itemText = rule->cssText();
-    if (!itemText.isEmpty()) {
-      cssText.append(itemText);
-      if (i < styleSheet.length() - 1)
-        cssText.append("\n\n");
+    for (unsigned i = 0; i < styleSheet.length(); ++i) {
+      CSSRule* rule = styleSheet.item(i);
+      String itemText = rule->cssText();
+      if (!itemText.isEmpty()) {
+        cssText.append(itemText);
+        if (i < styleSheet.length() - 1)
+          cssText.append("\n\n");
+      }
+
+      // Some rules have resources associated with them that we need to

[dcheng, 2016/10/07 22:34:28]

Nit: maybe just do this in one for loop at the end at line 381? We want this for
both branches anyway.

[carlosk, 2016/10/07 23:16:07]

Yes, done. I talked to lukasza@ and understood his previous suggestion of doing
that.

+      // retrieve.
+      serializeCSSRule(rule);
     }
 
-    // Some rules have resources associated with them that we need to retrieve.
-    serializeCSSRule(rule);
-  }
-
-  if (shouldAddURL(url)) {
     WTF::TextEncoding textEncoding(styleSheet.contents()->charset());
     ASSERT(textEncoding.isValid());
     String textString = cssText.toString();
     CString text =
         textEncoding.encode(textString, WTF::CSSEncodedEntitiesForUnencodables);
     m_resources->append(
         SerializedResource(url, String("text/css"),
                            SharedBuffer::create(text.data(), text.length())));
     m_resourceURLs.add(url);
+  } else {
+    // Sub resources need to be serialized even if the CSS definition doesn't
+    // need to be.
+    for (unsigned i = 0; i < styleSheet.length(); ++i)
+      serializeCSSRule(styleSheet.item(i));
   }
 
   if (cssStartTime != 0) {
     m_isSerializingCss = false;
     DEFINE_STATIC_LOCAL(CustomCountHistogram, cssHistogram,
                         ("PageSerialization.SerializationTime.CSSElement", 0,
                          maxSerializationTimeUmaMicroseconds, 50));
     cssHistogram.count(
         static_cast<int64_t>((monotonicallyIncreasingTime() - cssStartTime) *
                              secondsToMicroseconds));
@@ skipping 163 matching lines @@
     emitsMinus = ch == '-';
     builder.append(ch);
   }
   CString escapedUrl = builder.toString().ascii();
   return String::format("saved from url=(%04d)%s",
                         static_cast<int>(escapedUrl.length()),
                         escapedUrl.data());
 }
 
 }  // namespace blink