Don't trigger a link resource fetch on media change.

third_party/WebKit/Source/core/html/HTMLLinkElement.cpp

Index: third_party/WebKit/Source/core/html/HTMLLinkElement.cpp
diff --git a/third_party/WebKit/Source/core/html/HTMLLinkElement.cpp b/third_party/WebKit/Source/core/html/HTMLLinkElement.cpp
index f6d32a7a5cff7fcb0e4ea9357b1ce220e256ad9a..e3babb089a2c16b38db1374be050feb7458c6932 100644
--- a/third_party/WebKit/Source/core/html/HTMLLinkElement.cpp
+++ b/third_party/WebKit/Source/core/html/HTMLLinkElement.cpp
@@ -192,7 +192,8 @@ void HTMLLinkElement::parseAttribute(const QualifiedName& name, const AtomicStri
         m_sizes->setValue(value);
     } else if (name == mediaAttr) {
         m_media = value.lower();
-        process();
+        if (m_link)
+            m_link->mediaChanged();

[Yoav Weiss, 2016/03/14 15:23:51]

Seems to me that we would want "process()" not to trigger a second download if
it's not necessary rather than (or on top of) handling this `media` specific
case.

Maybe "process()" can check current URL and see if it is changed since it was
fetched? (and perhaps special-case media changes as to trigger mediaChanged
rather than a download? Not sure it's necessary)

Long term, I think we may want to move the loading code from here and into
loader (maybe split LinkLoader into LinkStyleLoader, LinkPreloadLoader, etc?)

[rune, 2016/04/07 07:56:07]

Yes, mediaChanged() is necessary to trigger a style update. Having process()
checking attributes without given the knowledge of what changed sounds
cumbersome. Instead of using the process method, we could have an
attributeChanged() for each of the LinkResource types. If there is no
LinkResources, call process(). If the rel type changed in such a way that the
LinkResource type should change, dispose the old one and create a new one with
process().

This code seems broken in so many ways and needs an overhaul. A couple of
issues:

- If we create a LinkResource for one rel-type, it cannot be changed afterwards
(works in Gecko).
- If we set the any of the known attributes to the same value ten times, it will
trigger ten resource loads.

If my media specific fix is not acceptable, I'll defer this to loading-dev
people for now as I need to focus elsewhere.

     } else if (name == disabledAttr) {
         UseCounter::count(document(), UseCounter::HTMLLinkElementDisabled);
         if (LinkStyle* link = linkStyle())
@@ -687,6 +688,14 @@ void LinkStyle::setDisabledState(bool disabled)
     }
 }
 
+void LinkStyle::mediaChanged()
+{
+    if (m_sheet) {
+        m_sheet->setMediaQueries(MediaQuerySet::create(m_owner->media()));
+        document().modifiedStyleSheet(m_sheet.get());
+    }
+}
+
 void LinkStyle::setCrossOriginStylesheetStatus(CSSStyleSheet* sheet)
 {
     if (m_fetchFollowingCORS && resource() && !resource()->errorOccurred()) {