Whitelist common, side-effect-free IDL callbacks

Marks common callbacks that do not produce JS-observable side-effects
on: Document, DOMTokenList, Element, Node, Window, HTMLElement.

Note that several Window properties are lazy and need extra V8 support
to be whitelisted.

Bug: 829571
Change-Id: I9ad50c1ca9b8c260ec898bc94423286c142d6bec
Reviewed-on: https://chromium-review.googlesource.com/1024863Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Erik Luo <luoe@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553672}

third_party/WebKit/LayoutTests/http/tests/inspector-protocol/side-effects/evaluate-embedder-side-effect-free-attributes-expected.txt

@@ -17,6 +17,10 @@ Expression `document.body`
 has side effect: false, expected: false
 Expression `document.head`
 has side effect: false, expected: false
+Expression `document.location`
+has side effect: false, expected: false
+Expression `document.defaultView`
+has side effect: false, expected: false
 Expression `document.activeElement`
 has side effect: false, expected: false
 Expression `div.tagName`
@@ -25,6 +29,22 @@ Expression `div.id`
 has side effect: false, expected: false
 Expression `div.className`
 has side effect: false, expected: false
+Expression `div.classList`
+has side effect: false, expected: false
+Expression `div.attributes`
+has side effect: false, expected: false
+Expression `shadowContainer.shadowRoot`
+has side effect: false, expected: false
+Expression `div.innerHTML`
+has side effect: false, expected: false
+Expression `div.outerHTML`
+has side effect: false, expected: false
+Expression `div.hidden`
+has side effect: false, expected: false
+Expression `div.tabIndex`
+has side effect: false, expected: false
+Expression `div.style`
+has side effect: false, expected: false
 Expression `div.nodeType`
 has side effect: false, expected: false
 Expression `div.nodeName`
@@ -49,6 +69,8 @@ Expression `div.previousSibling`
 has side effect: false, expected: false
 Expression `div.nextSibling`
 has side effect: false, expected: false
+Expression `div.ownerDocument`
+has side effect: false, expected: false
 Expression `document.nodeType`
 has side effect: false, expected: false
 Expression `document.nodeName`
@@ -73,6 +95,8 @@ Expression `document.previousSibling`
 has side effect: false, expected: false
 Expression `document.nextSibling`
 has side effect: false, expected: false
+Expression `document.ownerDocument`
+has side effect: false, expected: false
 Expression `textNode.nodeType`
 has side effect: false, expected: false
 Expression `textNode.nodeName`
@@ -97,6 +121,8 @@ Expression `textNode.previousSibling`
 has side effect: false, expected: false
 Expression `textNode.nextSibling`
 has side effect: false, expected: false
+Expression `textNode.ownerDocument`
+has side effect: false, expected: false
 Expression `div.childElementCount`
 has side effect: false, expected: false
 Expression `div.children`
@@ -127,4 +153,16 @@ Expression `screenX`
 has side effect: false, expected: false
 Expression `screenY`
 has side effect: false, expected: false
+Expression `document`
+has side effect: false, expected: false
+Expression `history`
+has side effect: false, expected: false
+Expression `navigator`
+has side effect: false, expected: false
+Expression `performance`
+has side effect: false, expected: false
+Expression `window`
+has side effect: true, expected: true
+Expression `window.location`
+has side effect: true, expected: true
 

third_party/WebKit/LayoutTests/http/tests/inspector-protocol/side-effects/evaluate-embedder-side-effect-free-attributes.js

@@ -6,10 +6,19 @@
     var div = document.createElement('div');
     div.id = 'foo';
     div.className = 'bar baz';
+    div.setAttribute('attr1', 'attr1-value');
+    div.tabIndex = -1;
+    div.style.color = 'red';
+
     var textNode = document.createTextNode('footext');
     div.appendChild(textNode);
     var textNode2 = document.createTextNode('bartext');
     div.appendChild(textNode2);
+
+    var shadowContainer = document.createElement('div');
+    var shadowRoot = shadowContainer.attachShadow({mode: 'open'});
+    var divInShadow = document.createElement('div');
+    shadowRoot.appendChild(divInShadow);
   `);
 
   // Sanity check: test that setters are not allowed on whitelisted accessors.
@@ -24,6 +33,8 @@
   await checkHasNoSideEffect(`document.scrollingElement`);
   await checkHasNoSideEffect(`document.body`);
   await checkHasNoSideEffect(`document.head`);
+  await checkHasNoSideEffect(`document.location`);
+  await checkHasNoSideEffect(`document.defaultView`);
 
   // DocumentOrShadowRoot
   await checkHasNoSideEffect(`document.activeElement`);
@@ -32,6 +43,16 @@
   await checkHasNoSideEffect(`div.tagName`);
   await checkHasNoSideEffect(`div.id`);
   await checkHasNoSideEffect(`div.className`);
+  await checkHasNoSideEffect(`div.classList`);
+  await checkHasNoSideEffect(`div.attributes`);
+  await checkHasNoSideEffect(`shadowContainer.shadowRoot`);
+  await checkHasNoSideEffect(`div.innerHTML`);
+  await checkHasNoSideEffect(`div.outerHTML`);
+
+  // HTMLElement
+  await checkHasNoSideEffect(`div.hidden`);
+  await checkHasNoSideEffect(`div.tabIndex`);
+  await checkHasNoSideEffect(`div.style`);
 
   // Node
   var testNodes = ['div', 'document', 'textNode'];
@@ -48,6 +69,7 @@
     await checkHasNoSideEffect(`${node}.lastChild`);
     await checkHasNoSideEffect(`${node}.previousSibling`);
     await checkHasNoSideEffect(`${node}.nextSibling`);
+    await checkHasNoSideEffect(`${node}.ownerDocument`);
   }
 
   // ParentNode
@@ -62,6 +84,14 @@
   await checkHasNoSideEffect(`devicePixelRatio`);
   await checkHasNoSideEffect(`screenX`);
   await checkHasNoSideEffect(`screenY`);
+  await checkHasNoSideEffect(`document`);
+  await checkHasNoSideEffect(`history`);
+  await checkHasNoSideEffect(`navigator`);
+  await checkHasNoSideEffect(`performance`);
+
+  // TODO(luoe): add support for LazyData properties.
+  await checkHasSideEffect(`window`);
+  await checkHasSideEffect(`window.location`);
 
   testRunner.completeTest();
 

third_party/WebKit/LayoutTests/http/tests/inspector-protocol/side-effects/evaluate-embedder-side-effect-free-methods-expected.txt

@@ -27,12 +27,22 @@ Expression `unmonitorEvents.toString()`
 has side effect: false, expected: false
 Expression `document.getElementsByTagName('div')`
 has side effect: false, expected: false
-Expression `document.getElementsByTagNameNS('http://www.w3.org/1999/xhtml', 'div')`
+Expression `document.getElementsByTagNameNS(namespace, 'div')`
 has side effect: false, expected: false
 Expression `document.getElementsByClassName('foo')`
 has side effect: false, expected: false
 Expression `document.getElementsByName('div-name')`
 has side effect: false, expected: false
+Expression `document.hasFocus()`
+has side effect: false, expected: false
+Expression `document.getSelection()`
+has side effect: false, expected: false
+Expression `domTokenList.contains('foo')`
+has side effect: false, expected: false
+Expression `domTokenList.contains({})`
+has side effect: false, expected: false
+Expression `domTokenList.contains()`
+has side effect: false, expected: false
 Expression `div.getAttributeNames()`
 has side effect: false, expected: false
 Expression `divNoAttrs.getAttributeNames()`
@@ -53,6 +63,26 @@ Expression `div.hasAttribute({})`
 has side effect: false, expected: false
 Expression `divNoAttrs.hasAttribute('attr1')`
 has side effect: false, expected: false
+Expression `div.getAttributeNS(namespace, 'attr1')`
+has side effect: false, expected: false
+Expression `div.getAttributeNS(namespace)`
+has side effect: false, expected: false
+Expression `div.getAttributeNS()`
+has side effect: false, expected: false
+Expression `divNoAttrs.getAttributeNS(namespace, 'attr1')`
+has side effect: false, expected: false
+Expression `div.hasAttributeNS(namespace, 'attr1')`
+has side effect: false, expected: false
+Expression `div.hasAttributeNS(namespace)`
+has side effect: false, expected: false
+Expression `div.hasAttributeNS()`
+has side effect: false, expected: false
+Expression `divNoAttrs.hasAttributeNS(namespace, 'attr1')`
+has side effect: false, expected: false
+Expression `divNoAttrs.hasAttributeNS(namespace)`
+has side effect: false, expected: false
+Expression `div.hasAttributes()`
+has side effect: false, expected: false
 Expression `div.contains(textNode)`
 has side effect: false, expected: false
 Expression `div.contains()`
@@ -63,6 +93,8 @@ Expression `div.querySelector('div')`
 has side effect: false, expected: false
 Expression `div.querySelectorAll('div')`
 has side effect: false, expected: false
+Expression `div.hasChildNodes()`
+has side effect: false, expected: false
 Expression `document.contains(textNode)`
 has side effect: false, expected: false
 Expression `document.contains()`
@@ -73,6 +105,8 @@ Expression `document.querySelector('div')`
 has side effect: false, expected: false
 Expression `document.querySelectorAll('div')`
 has side effect: false, expected: false
+Expression `document.hasChildNodes()`
+has side effect: false, expected: false
 Expression `textNode.contains(textNode)`
 has side effect: false, expected: false
 Expression `textNode.contains()`
@@ -83,7 +117,11 @@ Expression `textNode.querySelector('div')`
 has side effect: false, expected: false
 Expression `textNode.querySelectorAll('div')`
 has side effect: false, expected: false
-Expression `global_performance.now()`
+Expression `textNode.hasChildNodes()`
+has side effect: false, expected: false
+Expression `performance.now()`
+has side effect: false, expected: false
+Expression `global_getSelection()`
 has side effect: false, expected: false
 Expression `htmlCollection[0]`
 has side effect: false, expected: false

third_party/WebKit/LayoutTests/http/tests/inspector-protocol/side-effects/evaluate-embedder-side-effect-free-methods.js

@@ -3,9 +3,10 @@
       `Tests that evaluating V8-embedder callbacks allows side-effect-free methods. Should not crash.`);
 
   await session.evaluate(`
-    var global_performance = window.performance;
+    var global_getSelection = window.getSelection;
 
-    document.documentElement.setAttribute('xmlns', 'http://www.w3.org/1999/xhtml');
+    var namespace = 'http://www.w3.org/1999/xhtml';
+    document.documentElement.setAttribute('xmlns', namespace);
 
     var div = document.createElement('div');
     div.setAttribute('attr1', 'attr1-value');
@@ -52,9 +53,18 @@
 
   // Document
   await checkHasNoSideEffect(`document.getElementsByTagName('div')`);
-  await checkHasNoSideEffect(`document.getElementsByTagNameNS('http://www.w3.org/1999/xhtml', 'div')`);
+  await checkHasNoSideEffect(`document.getElementsByTagNameNS(namespace, 'div')`);
   await checkHasNoSideEffect(`document.getElementsByClassName('foo')`);
   await checkHasNoSideEffect(`document.getElementsByName('div-name')`);
+  await checkHasNoSideEffect(`document.hasFocus()`);
+
+  // DocumentOrShadowRoot
+  await checkHasNoSideEffect(`document.getSelection()`);
+
+  // DOMTokenList
+  await checkHasNoSideEffect(`domTokenList.contains('foo')`);
+  await checkHasNoSideEffect(`domTokenList.contains({})`);
+  await checkHasNoSideEffect(`domTokenList.contains()`);
 
   // Element
   await checkHasNoSideEffect(`div.getAttributeNames()`);
@@ -68,6 +78,18 @@
   await checkHasNoSideEffect(`div.hasAttribute({})`);
   await checkHasNoSideEffect(`divNoAttrs.hasAttribute('attr1')`);
 
+  await checkHasNoSideEffect(`div.getAttributeNS(namespace, 'attr1')`);
+  await checkHasNoSideEffect(`div.getAttributeNS(namespace)`);
+  await checkHasNoSideEffect(`div.getAttributeNS()`);
+  await checkHasNoSideEffect(`divNoAttrs.getAttributeNS(namespace, 'attr1')`);
+  await checkHasNoSideEffect(`div.hasAttributeNS(namespace, 'attr1')`);
+  await checkHasNoSideEffect(`div.hasAttributeNS(namespace)`);
+  await checkHasNoSideEffect(`div.hasAttributeNS()`);
+  await checkHasNoSideEffect(`divNoAttrs.hasAttributeNS(namespace, 'attr1')`);
+  await checkHasNoSideEffect(`divNoAttrs.hasAttributeNS(namespace)`);
+
+  await checkHasNoSideEffect(`div.hasAttributes()`);
+
   // Node
   var testNodes = ['div', 'document', 'textNode'];
   for (var node of testNodes) {
@@ -76,10 +98,14 @@
     await checkHasNoSideEffect(`${node}.contains({})`);
     await checkHasNoSideEffect(`${node}.querySelector('div')`);
     await checkHasNoSideEffect(`${node}.querySelectorAll('div')`);
+    await checkHasNoSideEffect(`${node}.hasChildNodes()`);
   }
 
+  // Performance
+  await checkHasNoSideEffect(`performance.now()`);
+
   // Window
-  await checkHasNoSideEffect(`global_performance.now()`);
+  await checkHasNoSideEffect(`global_getSelection()`);
 
   // Collection getters (e.g. HTMLCollection, NodeList)
   var indexedCollections = [

third_party/blink/renderer/core/dom/document.idl

@@ -92,7 +92,7 @@ typedef (HTMLScriptElement or SVGScriptElement) HTMLOrSVGScriptElement;
 
     // resource metadata management
 
-    [PutForwards=href, Unforgeable] readonly attribute Location? location;
+    [Affects=Nothing, PutForwards=href, Unforgeable] readonly attribute Location? location;
     [Affects=Nothing, RaisesException=Setter] attribute USVString domain;
     [Affects=Nothing] readonly attribute USVString referrer;
     [Affects=Nothing, RaisesException, RuntimeCallStatsCounter=DocumentCookie] attribute DOMString cookie;
@@ -129,8 +129,8 @@ typedef (HTMLScriptElement or SVGScriptElement) HTMLOrSVGScriptElement;
     [RuntimeEnabled=TrustedDOMTypes, CallWith=EnteredWindow, CEReactions, CustomElementCallbacks, RaisesException] void writeln(TrustedHTML text);
 
     // user interaction
-    readonly attribute Window? defaultView;
-    boolean hasFocus();
+    [Affects=Nothing] readonly attribute Window? defaultView;
+    [Affects=Nothing] boolean hasFocus();
     [CEReactions, CustomElementCallbacks, MeasureAs=DocumentDesignMode] attribute DOMString designMode;
     [CEReactions, CustomElementCallbacks, RaisesException] boolean execCommand(DOMString commandId, optional boolean showUI = false, optional DOMString value = "");
     [RaisesException] boolean queryCommandEnabled(DOMString commandId);

third_party/blink/renderer/core/dom/document_or_shadow_root.idl

@@ -10,7 +10,7 @@
 ] interface DocumentOrShadowRoot {
     // Selection API
     // https://w3c.github.io/selection-api/#extensions-to-document-interface
-    Selection? getSelection();
+    [Affects=Nothing] Selection? getSelection();
     // CSSOM View Module
     // https://drafts.csswg.org/cssom-view/#extensions-to-the-document-interface
     Element? elementFromPoint(double x, double y);

third_party/blink/renderer/core/dom/dom_token_list.idl

@@ -30,7 +30,7 @@
 ] interface DOMTokenList {
     [Affects=Nothing] readonly attribute unsigned long length;
     [Affects=Nothing] getter DOMString? item(unsigned long index);
-    boolean contains(DOMString token);
+    [Affects=Nothing] boolean contains(DOMString token);
     [RaisesException, CEReactions, CustomElementCallbacks] void add(DOMString... tokens);
     [RaisesException, CEReactions, CustomElementCallbacks] void remove(DOMString... tokens);
     [RaisesException, CEReactions, CustomElementCallbacks] boolean toggle(DOMString token, optional boolean force);

third_party/blink/renderer/core/dom/element.idl

@@ -34,7 +34,7 @@ interface Element : Node {
 
     [Affects=Nothing, CEReactions, Reflect] attribute DOMString id;
     [Affects=Nothing, CEReactions, Reflect=class] attribute DOMString className;
-    [SameObject, PerWorldBindings, PutForwards=value] readonly attribute DOMTokenList classList;
+    [Affects=Nothing, SameObject, PerWorldBindings, PutForwards=value] readonly attribute DOMTokenList classList;
     [Unscopable, CEReactions, Reflect] attribute DOMString slot;
 
     // Pointer Events
@@ -43,17 +43,17 @@ interface Element : Node {
     [RaisesException] void releasePointerCapture (long pointerId);
     boolean hasPointerCapture (long pointerId);
 
-    boolean hasAttributes();
-    [SameObject, PerWorldBindings, ImplementedAs=attributesForBindings] readonly attribute NamedNodeMap attributes;
+    [Affects=Nothing] boolean hasAttributes();
+    [Affects=Nothing, SameObject, PerWorldBindings, ImplementedAs=attributesForBindings] readonly attribute NamedNodeMap attributes;
     [Affects=Nothing] sequence<DOMString> getAttributeNames();
     [Affects=Nothing] DOMString? getAttribute(DOMString name);
-    DOMString? getAttributeNS(DOMString? namespaceURI, DOMString localName);
+    [Affects=Nothing] DOMString? getAttributeNS(DOMString? namespaceURI, DOMString localName);
     [RaisesException, CEReactions, CustomElementCallbacks] void setAttribute(DOMString name, DOMString value);
     [RaisesException, CEReactions, CustomElementCallbacks] void setAttributeNS(DOMString? namespaceURI, DOMString name, DOMString value);
     [CEReactions, CustomElementCallbacks] void removeAttribute(DOMString name);
     [CEReactions, CustomElementCallbacks] void removeAttributeNS(DOMString? namespaceURI, DOMString localName);
     [Affects=Nothing] boolean hasAttribute(DOMString name);
-    boolean hasAttributeNS(DOMString? namespaceURI, DOMString localName);
+    [Affects=Nothing] boolean hasAttributeNS(DOMString? namespaceURI, DOMString localName);
 
     Attr? getAttributeNode(DOMString name);
     Attr? getAttributeNodeNS(DOMString? namespaceURI, DOMString localName);
@@ -66,7 +66,7 @@ interface Element : Node {
     [RaisesException, ImplementedAs=matches, MeasureAs=ElementPrefixedMatchesSelector] boolean webkitMatchesSelector(DOMString selectors); // historical alias of .matches
 
     [RaisesException, CallWith=ScriptState, MeasureAs=ElementAttachShadow] ShadowRoot attachShadow(ShadowRootInit shadowRootInitDict);
-    [PerWorldBindings, ImplementedAs=OpenShadowRoot] readonly attribute ShadowRoot? shadowRoot;
+    [Affects=Nothing, PerWorldBindings, ImplementedAs=OpenShadowRoot] readonly attribute ShadowRoot? shadowRoot;
 
     [Affects=Nothing] HTMLCollection getElementsByTagName(DOMString localName);
     [Affects=Nothing] HTMLCollection getElementsByTagNameNS(DOMString? namespaceURI, DOMString localName);
@@ -83,8 +83,8 @@ interface Element : Node {
     // https://w3c.github.io/DOM-Parsing/#extensions-to-the-element-interface
     //
     // TODO(mkwst): Write a spec for the `TrustedHTML` variants.
-    [CEReactions, CustomElementCallbacks, Custom=Setter, RuntimeCallStatsCounter=ElementInnerHTML] attribute HTMLString innerHTML;
-    [CEReactions, CustomElementCallbacks, Custom=Setter] attribute HTMLString outerHTML;
+    [Affects=Nothing, CEReactions, CustomElementCallbacks, Custom=Setter, RuntimeCallStatsCounter=ElementInnerHTML] attribute HTMLString innerHTML;
+    [Affects=Nothing, CEReactions, CustomElementCallbacks, Custom=Setter] attribute HTMLString outerHTML;
     [CEReactions, CustomElementCallbacks, RaisesException] void insertAdjacentHTML(DOMString position, HTMLString text);
 
     // Pointer Lock

third_party/blink/renderer/core/dom/node.idl

@@ -39,10 +39,10 @@ interface Node : EventTarget {
     readonly attribute USVString baseURI;
 
     [Affects=Nothing, Measure] readonly attribute boolean isConnected;
-    [PerWorldBindings] readonly attribute Document? ownerDocument;
+    [Affects=Nothing, PerWorldBindings] readonly attribute Document? ownerDocument;
     [Affects=Nothing, PerWorldBindings] readonly attribute Node? parentNode;
     [Affects=Nothing, PerWorldBindings] readonly attribute Element? parentElement;
-    [ImplementedAs=hasChildren] boolean hasChildNodes();
+    [Affects=Nothing, ImplementedAs=hasChildren] boolean hasChildNodes();
     [Affects=Nothing, SameObject, PerWorldBindings] readonly attribute NodeList childNodes;
     [Affects=Nothing, PerWorldBindings] readonly attribute Node? firstChild;
     [Affects=Nothing, PerWorldBindings] readonly attribute Node? lastChild;

third_party/blink/renderer/core/frame/window.idl

@@ -36,11 +36,11 @@
     // FIXME: The spec uses the WindowProxy type for this and many other attributes.
     [Unforgeable, CrossOrigin] readonly attribute Window window;
     [Replaceable, CrossOrigin] readonly attribute Window self;
-    [Unforgeable, CachedAccessor] readonly attribute Document document;
+    [Affects=Nothing, Unforgeable, CachedAccessor] readonly attribute Document document;
     [Replaceable] readonly attribute DOMString origin;
     attribute DOMString name;
     [PutForwards=href, Unforgeable, CrossOrigin=(Getter,Setter), Custom=Getter] readonly attribute Location location;
-    readonly attribute History history;
+    [Affects=Nothing] readonly attribute History history;
     [Replaceable, MeasureAs=BarPropLocationbar] readonly attribute BarProp locationbar;
     [Replaceable, MeasureAs=BarPropMenubar] readonly attribute BarProp menubar;
     [Replaceable, MeasureAs=BarPropPersonalbar] readonly attribute BarProp personalbar;
@@ -84,7 +84,7 @@
     [Custom, NotEnumerable, CrossOrigin] getter object (DOMString name);
 
     // the user agent
-    [LogActivity=GetterOnly] readonly attribute Navigator navigator;
+    [Affects=Nothing, LogActivity=GetterOnly] readonly attribute Navigator navigator;
     [LogActivity=GetterOnly, SecureContext=RestrictAppCacheToSecureContexts] readonly attribute ApplicationCache applicationCache;
 
     // user prompts
@@ -157,7 +157,7 @@
 
     // Selection API
     // https://w3c.github.io/selection-api/#extensions-to-window-interface
-    Selection? getSelection();
+    [Affects=Nothing] Selection? getSelection();
 
     // Console API
     // https://console.spec.whatwg.org/#console-interface

third_party/blink/renderer/core/html/html_element.idl

@@ -29,9 +29,9 @@ interface HTMLElement : Element {
     [SameObject, PerWorldBindings] readonly attribute DOMStringMap dataset;
 
     // user interaction
-    [CEReactions, Reflect] attribute boolean hidden;
+    [Affects=Nothing, CEReactions, Reflect] attribute boolean hidden;
     [RuntimeCallStatsCounter=HTMLElementClick] void click();
-    [CEReactions, CustomElementCallbacks] attribute long tabIndex;
+    [Affects=Nothing, CEReactions, CustomElementCallbacks] attribute long tabIndex;
     [CEReactions, RuntimeEnabled=InertAttribute, Reflect] attribute boolean inert;
     void focus(optional FocusOptions options);
     void blur();
@@ -56,7 +56,7 @@ interface HTMLElement : Element {
 
     // CSS Object Model (CSSOM)
     // https://drafts.csswg.org/cssom/#the-elementcssinlinestyle-interface
-    [SameObject, PerWorldBindings, PutForwards=cssText] readonly attribute CSSStyleDeclaration style;
+    [Affects=Nothing, SameObject, PerWorldBindings, PutForwards=cssText] readonly attribute CSSStyleDeclaration style;
 
     // Non-standard APIs
     [CEReactions, CustomElementCallbacks, RaisesException=Setter, MeasureAs=HTMLElementInnerText] attribute [TreatNullAs=NullString] DOMString innerText;

third_party/blink/renderer/core/timing/window_performance.idl

@@ -9,5 +9,5 @@
 [
     ImplementedAs=DOMWindowPerformance
 ] partial interface Window {
-    [Replaceable] readonly attribute Performance performance;
+    [Affects=Nothing, Replaceable] readonly attribute Performance performance;
 };