style: Add a way to update an empty stylesheet for @import.

We'll update the empty stylesheet we've created when instantiating the
ImportRule when the stylesheet finishes loading.

components/style/media_queries.rs

@@ -297,4 +297,8 @@ impl MediaList {
             }
         })
     }
+
+    pub fn is_empty(&self) -> bool {
+        self.media_queries.is_empty()
+    }
 }

components/style/stylesheets.rs

@@ -56,6 +56,12 @@ pub struct Namespaces {
 #[derive(Debug)]
 pub struct CssRules(pub Vec<CssRule>);
 
+impl CssRules {
+    pub fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+}
+
 pub enum RulesMutateError {
     Syntax,
     IndexSize,
@@ -106,7 +112,8 @@ impl CssRules {
 
         // Step 3, 4
         // XXXManishearth should we also store the namespace map?
-        let (new_rule, new_state) = try!(CssRule::parse(&rule, parent_stylesheet,
+        let (new_rule, new_state) =
+            try!(CssRule::parse(&rule, parent_stylesheet,
                                 ParserContextExtraData::default(), state));
 
         // Step 5
@@ -464,34 +471,67 @@ impl Stylesheet {
                       environment_encoding: Option<EncodingRef>,
                       origin: Origin,
                       media: MediaList,
+                      stylesheet_loader: Option<&StylesheetLoader>,
                       error_reporter: Box<ParseErrorReporter + Send>,
                       extra_data: ParserContextExtraData)
                       -> Stylesheet {
         let (string, _) = decode_stylesheet_bytes(
             bytes, protocol_encoding_label, environment_encoding);
-        Stylesheet::from_str(&string, base_url, origin, media, error_reporter, extra_data)
+        Stylesheet::from_str(&string,
+                             base_url,
+                             origin,
+                             media,
+                             stylesheet_loader,
+                             error_reporter,
+                             extra_data)
     }
 
-    pub fn from_str(css: &str, base_url: ServoUrl, origin: Origin, media: MediaList,
+    pub fn update_from_bytes(existing: &Stylesheet,
+                             bytes: &[u8],
+                             protocol_encoding_label: Option<&str>,
+                             environment_encoding: Option<EncodingRef>,
+                             stylesheet_loader: Option<&StylesheetLoader>,
                              error_reporter: Box<ParseErrorReporter + Send>,
-                    extra_data: ParserContextExtraData) -> Stylesheet {
+                             extra_data: ParserContextExtraData) {
-        let mut namespaces = Namespaces::default();
+        assert!(existing.rules.read().is_empty());
-        let mut rules = vec![];
+        let (string, _) = decode_stylesheet_bytes(
-        let dirty_on_viewport_size_change;
+            bytes, protocol_encoding_label, environment_encoding);
-        {
+        Self::update_from_str(existing,
+                              &string,
+                              stylesheet_loader,
+                              error_reporter,
+                              extra_data)
+    }
+
+    pub fn update_from_str(existing: &Stylesheet,
+                           css: &str,
+                           stylesheet_loader: Option<&StylesheetLoader>,
+                           error_reporter: Box<ParseErrorReporter + Send>,
+                           extra_data: ParserContextExtraData) {
+        let mut rules = existing.rules.write();
+        let mut namespaces = existing.namespaces.write();
+
+        assert!(rules.is_empty());
+
+        let mut input = Parser::new(css);
         let rule_parser = TopLevelRuleParser {
-                stylesheet_origin: origin,
+            stylesheet_origin: existing.origin,
             namespaces: &mut namespaces,
-                context: ParserContext::new_with_extra_data(origin, &base_url, error_reporter, extra_data),
+            loader: stylesheet_loader,
+            context: ParserContext::new_with_extra_data(existing.origin,
+                                                        &existing.base_url,
+                                                        error_reporter,
+                                                        extra_data),
             state: Cell::new(State::Start),
         };
-            let mut input = Parser::new(css);
+
         input.look_for_viewport_percentages();
+
         {
             let mut iter = RuleListParser::new_for_stylesheet(&mut input, rule_parser);
             while let Some(result) = iter.next() {
                 match result {
-                        Ok(rule) => rules.push(rule),
+                    Ok(rule) => rules.0.push(rule),
                     Err(range) => {
                         let pos = range.start;
                         let message = format!("Invalid rule: '{}'", iter.input.slice(range));
@@ -500,18 +540,33 @@ impl Stylesheet {
                 }
             }
         }
-            dirty_on_viewport_size_change = input.seen_viewport_percentages();
+
+        existing.dirty_on_viewport_size_change
+            .store(input.seen_viewport_percentages(), Ordering::Release);
     }
 
-        Stylesheet {
+    pub fn from_str(css: &str, base_url: ServoUrl, origin: Origin,
+                    media: MediaList,
+                    stylesheet_loader: Option<&StylesheetLoader>,
+                    error_reporter: Box<ParseErrorReporter + Send>,
+                    extra_data: ParserContextExtraData) -> Stylesheet {
+        let s = Stylesheet {
             origin: origin,
             base_url: base_url,
-            namespaces: RwLock::new(namespaces),
+            namespaces: RwLock::new(Namespaces::default()),
-            rules: CssRules::new(rules),
+            rules: CssRules::new(vec![]),
             media: Arc::new(RwLock::new(media)),
-            dirty_on_viewport_size_change: AtomicBool::new(dirty_on_viewport_size_change),
+            dirty_on_viewport_size_change: AtomicBool::new(false),
             disabled: AtomicBool::new(false),
-        }
+        };
+
+        Self::update_from_str(&s,
+                              css,
+                              stylesheet_loader,
+                              error_reporter,
+                              extra_data);
+
+        s
     }
 
     pub fn dirty_on_viewport_size_change(&self) -> bool {