Bug 1005818 - Part 2: Only limited browser API are available to a widget. r=kanru

1. Add |ownerIsWidget| in nsIFrameLoader.idl
2. Add |GetReallyIsWidget| in nsIMozBrowserFrame.idl
3. Hide the methods of browser API of a widget
4. Hide security-sensitive mozbrowser events of a widget

content/base/public/nsIFrameLoader.idl

@@ -112,7 +112,7 @@ interface nsIContentViewManager : nsISupports
   readonly attribute nsIContentView rootContentView;
 };
 
-[scriptable, builtinclass, uuid(d52ca6a8-8237-4ae0-91d7-7be4f1db24ef)]
+[scriptable, builtinclass, uuid(55a772b8-855a-4c5f-b4a1-284b6b3bec28)]
 interface nsIFrameLoader : nsISupports
 {
   /**
@@ -291,6 +291,13 @@ interface nsIFrameLoader : nsISupports
    * Especially, a widget frame is regarded as an app frame.
    */
   readonly attribute boolean ownerIsBrowserOrAppFrame;
+
+  /**
+   * Find out whether the owner content really is a widget. If this attribute
+   * returns true, |ownerIsBrowserOrAppFrame| must return true.
+   */
+  readonly attribute boolean ownerIsWidget;
+
 };
 
 %{C++

content/base/src/nsFrameLoader.cpp

@@ -1474,6 +1474,22 @@ nsFrameLoader::GetOwnerIsBrowserOrAppFrame(bool* aResult)
   return NS_OK;
 }
 
+bool
+nsFrameLoader::OwnerIsWidget()
+{
+  nsCOMPtr<nsIMozBrowserFrame> browserFrame = do_QueryInterface(mOwnerContent);
+  return browserFrame ? browserFrame->GetReallyIsWidget() : false;
+}
+
+
+// The xpcom getter version
+NS_IMETHODIMP
+nsFrameLoader::GetOwnerIsWidget(bool* aResult)
+{
+  *aResult = OwnerIsWidget();
+  return NS_OK;
+}
+
 bool
 nsFrameLoader::OwnerIsAppFrame()
 {

content/base/src/nsFrameLoader.h

@@ -333,6 +333,12 @@ private:
    */
   bool OwnerIsBrowserOrAppFrame();
 
+  /**
+   * Is this a frameloader for a bona fide <iframe mozwidget>?  (I.e., does the
+   * frame return true for nsIMozBrowserFrame::GetReallyIsWidget()?)
+   */
+  bool OwnerIsWidget();
+
   /**
    * Is this a frameloader for a bona fide <iframe mozapp>?  (I.e., does the
    * frame return true for nsIMozBrowserFrame::GetReallyIsApp()?)

content/html/content/src/nsGenericHTMLFrameElement.cpp

@@ -319,6 +319,44 @@ nsGenericHTMLFrameElement::GetReallyIsApp(bool *aOut)
   return NS_OK;
 }
 
+namespace {
+
+bool WidgetsEnabled()
+{
+  static bool sMozWidgetsEnabled = false;
+  static bool sBoolVarCacheInitialized = false;
+
+  if (!sBoolVarCacheInitialized) {
+    sBoolVarCacheInitialized = true;
+    Preferences::AddBoolVarCache(&sMozWidgetsEnabled,
+                                 "dom.enable_widgets");
+  }
+
+  return sMozWidgetsEnabled;
+}
+
+} // anonymous namespace
+
+/* [infallible] */ NS_IMETHODIMP
+nsGenericHTMLFrameElement::GetReallyIsWidget(bool *aOut)
+{
+  *aOut = false;
+  if (!WidgetsEnabled()) {
+    return NS_OK;
+  }
+
+  nsAutoString appManifestURL;
+  GetManifestURLByType(nsGkAtoms::mozapp, appManifestURL);
+  bool isApp = !appManifestURL.IsEmpty();
+
+  nsAutoString widgetManifestURL;
+  GetManifestURLByType(nsGkAtoms::mozwidget, widgetManifestURL);
+  bool isWidget = !widgetManifestURL.IsEmpty();
+
+  *aOut = isWidget && !isApp;
+  return NS_OK;
+}
+
 /* [infallible] */ NS_IMETHODIMP
 nsGenericHTMLFrameElement::GetIsExpectingSystemMessage(bool *aOut)
 {
@@ -404,7 +442,7 @@ nsGenericHTMLFrameElement::GetAppManifestURL(nsAString& aOut)
 
   GetManifestURLByType(nsGkAtoms::mozapp, appManifestURL);
 
-  if (Preferences::GetBool("dom.enable_widgets")) {
+  if (WidgetsEnabled()) {
     GetManifestURLByType(nsGkAtoms::mozwidget, widgetManifestURL);
   }
 

dom/apps/tests/file_app.sjs

@@ -46,7 +46,7 @@ function handleRequest(request, response) {
 
   // Get the app type.
   var apptype = query.apptype;
-  if (apptype != 'hosted' && apptype != 'cached' && apptype != 'widget')
+  if (apptype != 'hosted' && apptype != 'cached' && apptype != 'widget'  && apptype != 'invalidWidget')
     throw "Invalid app type: " + apptype;
 
   // Get the version from server state and handle the etag.
@@ -83,7 +83,7 @@ function handleRequest(request, response) {
     response.write(makeResource(gAppcacheTemplatePath, version, apptype));
     return;
   }
-  else if (apptype == 'widget')
+  else if (apptype == 'widget' || apptype == 'invalidWidget')
   {
     response.setHeader("Content-Type", "text/html", false);
     response.write(makeResource(gWidgetTemplatePath, version, apptype));

dom/apps/tests/file_invalidWidget_app.template.webapp

@@ -0,0 +1,11 @@
+{
+  "name": "Really Rapid Release (invalid widget)",
+  "description": "Updated even faster than <a href='http://mozilla.org'>Firefox</a>, just to annoy slashdotters.",
+  "launch_path": "/tests/dom/apps/tests/file_app.sjs?apptype=invalidWidget",
+  "icons": {
+    "128": "ICONTOKEN"
+  },
+  "widgetPages": [
+    "/tests/dom/apps/tests/file_app.sjs?apptype=widget"
+  ]
+}

dom/apps/tests/file_widget_app.template.html

@@ -1,68 +1,8 @@
 <html>
 <head>
-<script>
-
-function sendMessage(msg) {
-  alert(msg);
-}
-
-function ok(p, msg) {
-  if (p)
-    sendMessage("OK: " + msg);
-  else
-    sendMessage("KO: " + msg);
-}
-
-function is(a, b, msg) {
-  if (a == b)
-    sendMessage("OK: " + a + " == " + b + " - " + msg);
-  else
-    sendMessage("KO: " + a + " != " + b + " - " + msg);
-}
-
-function installed(p) {
-  if (p)
-    sendMessage("IS_INSTALLED");
-  else
-    sendMessage("NOT_INSTALLED");
-}
-
-function finish() {
-  sendMessage("VERSION: MyWebApp vVERSIONTOKEN");
-  sendMessage("DONE");
-}
-
-function cbError() {
-  ok(false, "Error callback invoked");
-  finish();
-}
-
-function go() {
-  ok(true, "Launched app");
-  var request = window.navigator.mozApps.getSelf();
-  request.onsuccess = function() {
-    var widget = request.result;
-    ok(widget,"Should be a widget");
-    checkWidget(widget);
-  }
-  request.onerror = cbError;
-}
-
-function checkWidget(widget) {
-  // If the widget is installed, |widget| will be non-null. If it is, verify its state.
-  installed(!!widget);
-  if (widget) {
-    var widgetName = "Really Rapid Release (APPTYPETOKEN)";
-    var manifest = SpecialPowers.wrap(widget.manifest);
-    is(manifest.name, widgetName, "Manifest name should be correct");
-    is(widget.origin, "http://test", "Widget origin should be correct");
-    is(widget.installOrigin, "http://mochi.test:8888", "Install origin should be correct");
-  }
-  finish();
-}
-
-</script>
 </head>
-<body onload="go();">
+<body>
 App Body. Version: VERSIONTOKEN
-</body></html>
+<br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br />
+</body>
+</html>

dom/apps/tests/mochitest.ini

@@ -7,6 +7,7 @@ support-files =
   file_cached_app.template.appcache
   file_cached_app.template.webapp
   file_hosted_app.template.webapp
+  file_invalidWidget_app.template.webapp
   file_packaged_app.sjs
   file_packaged_app.template.html
   file_packaged_app.template.webapp

dom/apps/tests/test_widget.html

@@ -11,16 +11,17 @@
   <script type="application/javascript;version=1.7">
 
   var gWidgetManifestURL = 'http://test/tests/dom/apps/tests/file_app.sjs?apptype=widget&getmanifest=true';
+  var gInvalidWidgetManifestURL = 'http://test/tests/dom/apps/tests/file_app.sjs?apptype=invalidWidget&getmanifest=true';
   var gApp;
 
-  function cbError() {
+  function onError() {
     ok(false, "Error callback invoked");
     finish();
   }
 
-  function installApp() {
-    var request = navigator.mozApps.install(gWidgetManifestURL);
-    request.onerror = cbError;
+  function installApp(path) {
+    var request = navigator.mozApps.install(path);
+    request.onerror = onError;
     request.onsuccess = function() {
       gApp = request.result;
 
@@ -31,40 +32,134 @@
   function uninstallApp() {
     // Uninstall the app.
     var request = navigator.mozApps.mgmt.uninstall(gApp);
-    request.onerror = cbError;
+    request.onerror = onError;
     request.onsuccess = function() {
       // All done.
       info("All done");
+
       runTest();
     }
   }
 
-  function testApp() {
+  function testApp(isValidWidget) {
+    info("Test widget feature. IsValidWidget: " + isValidWidget);
+
     var ifr = document.createElement('iframe');
     ifr.setAttribute('mozbrowser', 'true');
     ifr.setAttribute('mozwidget', gApp.manifestURL);
     ifr.setAttribute('src', gApp.origin+gApp.manifest.launch_path);
 
     var domParent = document.getElementById('container');
+    domParent.appendChild(ifr);
 
-    // Set us up to listen for messages from the app.
-    var listener = function(e) {
-      var message = e.detail.message;
-      if (/^OK/.exec(message)) {
-        ok(true, "Message from widget: " + message);
-      } else if (/KO/.exec(message)) {
-        ok(false, "Message from widget: " + message);
-      } else if (/DONE/.exec(message)) {
-        ok(true, "Message from widget complete");
-        ifr.removeEventListener('mozbrowsershowmodalprompt', listener);
-        domParent.removeChild(ifr);
-        runTest();
+    var mm = SpecialPowers.getBrowserFrameMessageManager(ifr);
+    mm.addMessageListener('OK', function(msg) {
+      ok(isValidWidget, "Message from widget: " + SpecialPowers.wrap(msg).json);
+    });
+    mm.addMessageListener('KO', function(msg) {
+      ok(!isValidWidget, "Message from widget: " + SpecialPowers.wrap(msg).json);
+    });
+    mm.addMessageListener('DONE', function(msg) {
+      ok(true, "Message from widget complete: "+SpecialPowers.wrap(msg).json);
+      domParent.removeChild(ifr);
+      runTest();
+    });
+
+    ifr.addEventListener('mozbrowserloadend', function() {
+      ok(true, "receive mozbrowserloadend");
+
+      // Test limited browser API feature only for valid widget case
+      if (isValidWidget) {
+        testLimitedBrowserAPI(ifr);
       }
+      SimpleTest.executeSoon(()=>loadFrameScript(mm));
+    }, false);
+
+    // Test limited browser API feature only for valid widget case
+    if (!isValidWidget) {
+      return;
     }
 
-    // This event is triggered when the app calls "alert".
-    ifr.addEventListener('mozbrowsershowmodalprompt', listener, false);
-    domParent.appendChild(ifr);
+    [
+      'mozbrowsertitlechange',
+      'mozbrowseropenwindow',
+      'mozbrowserscroll',
+      'mozbrowserasyncscroll'
+    ].forEach( function(topic) {
+      ifr.addEventListener(topic, function() {
+        ok(false, topic + " should be hidden");
+      }, false);
+    });
+  }
+
+  function testLimitedBrowserAPI(ifr) {
+    var securitySensitiveCalls = [
+      'sendMouseEvent',
+      'sendTouchEvent',
+      'goBack',
+      'goForward',
+      'reload',
+      'stop',
+      'download',
+      'purgeHistory',
+      'getScreenshot',
+      'zoom',
+      'getCanGoBack',
+      'getCanGoForward'
+    ];
+    securitySensitiveCalls.forEach( function(call) {
+      is(typeof ifr[call], "undefined", call + " should be hidden for widget");
+    });
+  }
+
+  function loadFrameScript(mm) {
+    var script = 'data:,\
+      function ok(p, msg) { \
+        if (p) { \
+          sendAsyncMessage("OK", msg); \
+        } else { \
+          sendAsyncMessage("KO", msg); \
+        } \
+      } \
+      \
+      function is(a, b, msg) { \
+        if (a == b) { \
+          sendAsyncMessage("OK", a + " == " + b + " - " + msg); \
+        } else { \
+          sendAsyncMessage("KO", a + " != " + b + " - " + msg); \
+        } \
+      } \
+      \
+      function finish() { \
+          sendAsyncMessage("DONE",""); \
+      } \
+      \
+      function onError() { \
+        ok(false, "Error callback invoked"); \
+        finish(); \
+      } \
+      \
+      function checkWidget(widget) { \
+        /*For invalid widget case, ignore the following check*/\
+        if (widget) { \
+          var widgetName = "Really Rapid Release (APPTYPETOKEN)"; \
+          is(widget.origin, "http://test", "Widget origin should be correct"); \
+          is(widget.installOrigin, "http://mochi.test:8888", "Install origin should be correct"); \
+        } \
+        finish(); \
+      } \
+      \
+      var request = content.window.navigator.mozApps.getSelf(); \
+      request.onsuccess = function() { \
+        var widget = request.result; \
+        ok(widget,"Should be a widget"); \
+        checkWidget(widget); \
+      }; \
+      request.onerror = onError; \
+      content.window.open("about:blank"); /*test mozbrowseropenwindow*/ \
+      content.window.scrollTo(4000, 4000); /*test mozbrowser(async)scroll*/ \
+      ';
+    mm.loadFrameScript(script, /* allowDelayedLoad = */ false);
   }
 
   var tests = [
@@ -93,10 +188,19 @@
     },
 
     // Installing the app
-    installApp,
+    ()=>installApp(gWidgetManifestURL),
 
     // Run tests in app
-    testApp,
+    ()=>testApp(true),
+
+    // Uninstall the app
+    uninstallApp,
+
+    // Installing the app for invalid widget case
+    ()=>installApp(gInvalidWidgetManifestURL),
+
+    // Run tests in app for invalid widget case
+    ()=>testApp(false),
 
     // Uninstall the app
     uninstallApp

dom/browser-element/BrowserElementParent.cpp

@@ -164,6 +164,13 @@ BrowserElementParent::DispatchOpenWindowEvent(Element* aOpenerFrameElement,
     return BrowserElementParent::OPEN_WINDOW_IGNORED;
   }
 
+  // Do not dispatch a mozbrowseropenwindow event of a widget to its embedder
+  nsCOMPtr<nsIMozBrowserFrame> browserFrame =
+    do_QueryInterface(aOpenerFrameElement);
+  if (browserFrame && browserFrame->GetReallyIsWidget()) {
+    return BrowserElementParent::OPEN_WINDOW_CANCELLED;
+  }
+
   nsEventStatus status;
   bool dispatchSucceeded =
     DispatchCustomDOMEvent(aOpenerFrameElement,
@@ -349,6 +356,14 @@ BrowserElementParent::DispatchAsyncScrollEvent(TabParent* aTabParent,
                                                const CSSRect& aContentRect,
                                                const CSSSize& aContentSize)
 {
+  // Do not dispatch a mozbrowserasyncscroll event of a widget to its embedder
+  nsCOMPtr<Element> frameElement = aTabParent->GetOwnerElement();
+  NS_ENSURE_TRUE(frameElement, false);
+  nsCOMPtr<nsIMozBrowserFrame> browserFrame = do_QueryInterface(frameElement);
+  if (browserFrame && browserFrame->GetReallyIsWidget()) {
+    return true;
+  }
+
   nsRefPtr<DispatchAsyncScrollEventRunnable> runnable =
     new DispatchAsyncScrollEventRunnable(aTabParent, aContentRect,
                                          aContentSize);

dom/browser-element/BrowserElementParent.jsm

@@ -116,24 +116,30 @@ function BrowserElementParent(frameLoader, hasRemoteFrame, isPendingFrame) {
   // Define methods on the frame element.
   defineNoReturnMethod('setVisible', this._setVisible);
   defineDOMRequestMethod('getVisible', 'get-visible');
-  defineNoReturnMethod('sendMouseEvent', this._sendMouseEvent);
 
-  // 0 = disabled, 1 = enabled, 2 - auto detect
-  if (getIntPref(TOUCH_EVENTS_ENABLED_PREF, 0) != 0) {
-    defineNoReturnMethod('sendTouchEvent', this._sendTouchEvent);
+  // Not expose security sensitive browser API for widgets
+  if (!this._frameLoader.QueryInterface(Ci.nsIFrameLoader).ownerIsWidget) {
+    defineNoReturnMethod('sendMouseEvent', this._sendMouseEvent);
+
+    // 0 = disabled, 1 = enabled, 2 - auto detect
+    if (getIntPref(TOUCH_EVENTS_ENABLED_PREF, 0) != 0) {
+      defineNoReturnMethod('sendTouchEvent', this._sendTouchEvent);
+    }
+    defineNoReturnMethod('goBack', this._goBack);
+    defineNoReturnMethod('goForward', this._goForward);
+    defineNoReturnMethod('reload', this._reload);
+    defineNoReturnMethod('stop', this._stop);
+    defineMethod('download', this._download);
+    defineDOMRequestMethod('purgeHistory', 'purge-history');
+    defineMethod('getScreenshot', this._getScreenshot);
+    defineNoReturnMethod('zoom', this._zoom);
+
+    defineDOMRequestMethod('getCanGoBack', 'get-can-go-back');
+    defineDOMRequestMethod('getCanGoForward', 'get-can-go-forward');
   }
-  defineNoReturnMethod('goBack', this._goBack);
-  defineNoReturnMethod('goForward', this._goForward);
-  defineNoReturnMethod('reload', this._reload);
-  defineNoReturnMethod('stop', this._stop);
-  defineNoReturnMethod('zoom', this._zoom);
-  defineMethod('download', this._download);
-  defineDOMRequestMethod('purgeHistory', 'purge-history');
-  defineMethod('getScreenshot', this._getScreenshot);
+
   defineMethod('addNextPaintListener', this._addNextPaintListener);
   defineMethod('removeNextPaintListener', this._removeNextPaintListener);
-  defineDOMRequestMethod('getCanGoBack', 'get-can-go-back');
-  defineDOMRequestMethod('getCanGoForward', 'get-can-go-forward');
 
   let principal = this._frameElement.ownerDocument.nodePrincipal;
   let perm = Services.perms
@@ -217,6 +223,9 @@ BrowserElementParent.prototype = {
   _setupMessageListener: function() {
     this._mm = this._frameLoader.messageManager;
     let self = this;
+    let isWidget = this._frameLoader
+                       .QueryInterface(Ci.nsIFrameLoader)
+                       .ownerIsWidget;
 
     // Messages we receive are handed to functions which take a (data) argument,
     // where |data| is the message manager's data object.
@@ -224,26 +233,14 @@ BrowserElementParent.prototype = {
     // on data.msg_name
     let mmCalls = {
       "hello": this._recvHello,
-      "contextmenu": this._fireCtxMenuEvent,
-      "locationchange": this._fireEventFromMsg,
       "loadstart": this._fireProfiledEventFromMsg,
       "loadend": this._fireProfiledEventFromMsg,
-      "titlechange": this._fireProfiledEventFromMsg,
-      "iconchange": this._fireEventFromMsg,
-      "manifestchange": this._fireEventFromMsg,
-      "metachange": this._fireEventFromMsg,
       "close": this._fireEventFromMsg,
-      "resize": this._fireEventFromMsg,
-      "activitydone": this._fireEventFromMsg,
-      "opensearch": this._fireEventFromMsg,
-      "securitychange": this._fireEventFromMsg,
       "error": this._fireEventFromMsg,
-      "scroll": this._fireEventFromMsg,
       "firstpaint": this._fireProfiledEventFromMsg,
       "documentfirstpaint": this._fireProfiledEventFromMsg,
       "nextpaint": this._recvNextPaint,
       "keyevent": this._fireKeyEvent,
-      "showmodalprompt": this._handleShowModalPrompt,
       "got-purge-history": this._gotDOMRequestResult,
       "got-screenshot": this._gotDOMRequestResult,
       "got-can-go-back": this._gotDOMRequestResult,
@@ -257,9 +254,31 @@ BrowserElementParent.prototype = {
       "selectionchange": this._handleSelectionChange
     };
 
+    let mmSecuritySensitiveCalls = {
+      "showmodalprompt": this._handleShowModalPrompt,
+      "contextmenu": this._fireCtxMenuEvent,
+      "securitychange": this._fireEventFromMsg,
+      "locationchange": this._fireEventFromMsg,
+      "iconchange": this._fireEventFromMsg,
+      "titlechange": this._fireProfiledEventFromMsg,
+      "opensearch": this._fireEventFromMsg,
+      "manifestchange": this._fireEventFromMsg,
+      "metachange": this._fireEventFromMsg,
+      "resize": this._fireEventFromMsg,
+      "activitydone": this._fireEventFromMsg,
+      "scroll": this._fireEventFromMsg
+    };
+
     this._mm.addMessageListener('browser-element-api:call', function(aMsg) {
-      if (self._isAlive() && (aMsg.data.msg_name in mmCalls)) {
+      if (!self._isAlive()) {
+        return;
+      }
+
+      if (aMsg.data.msg_name in mmCalls) {
         return mmCalls[aMsg.data.msg_name].apply(self, arguments);
+      } else if (!isWidget && aMsg.data.msg_name in mmSecuritySensitiveCalls) {
+        return mmSecuritySensitiveCalls[aMsg.data.msg_name]
+                 .apply(self, arguments);
       }
     });
   },
@@ -294,26 +313,29 @@ BrowserElementParent.prototype = {
       }
     };
 
-    if (authDetail.isOnlyPassword) {
-      // We don't handle password-only prompts, so just cancel it.
+    // 1. We don't handle password-only prompts.
+    // 2. We don't handle for widget case because of security concern.
+    if (authDetail.isOnlyPassword ||
+        this._frameLoader.QueryInterface(Ci.nsIFrameLoader).ownerIsWidget) {
       cancelCallback();
       return;
-    } else { /* username and password */
-      let detail = {
-        host:     authDetail.host,
-        realm:    authDetail.realm
-      };
-
-      evt = this._createEvent('usernameandpasswordrequired', detail,
-                              /* cancelable */ true);
-      Cu.exportFunction(function(username, password) {
-        if (callbackCalled)
-          return;
-        callbackCalled = true;
-        callback(true, username, password);
-      }, evt.detail, { defineAs: 'authenticate' });
     }
 
+    /* username and password */
+    let detail = {
+      host:     authDetail.host,
+      realm:    authDetail.realm
+    };
+
+    evt = this._createEvent('usernameandpasswordrequired', detail,
+                            /* cancelable */ true);
+    Cu.exportFunction(function(username, password) {
+      if (callbackCalled)
+        return;
+      callbackCalled = true;
+      callback(true, username, password);
+    }, evt.detail, { defineAs: 'authenticate' });
+
     Cu.exportFunction(cancelCallback, evt.detail, { defineAs: 'cancel' });
 
     this._frameElement.dispatchEvent(evt);

dom/interfaces/html/nsIMozBrowserFrame.idl

@@ -9,7 +9,7 @@
 
 interface nsITabParent;
 
-[scriptable, builtinclass, uuid(929AED00-3E15-49B7-8CA2-75003715B7E7)]
+[scriptable, builtinclass, uuid(0c0a862c-1a47-43c0-ae9e-d51835e3e1a6)]
 interface nsIMozBrowserFrame : nsIDOMMozBrowserFrame
 {
   /**
@@ -33,6 +33,16 @@ interface nsIMozBrowserFrame : nsIDOMMozBrowserFrame
    */
   [infallible] readonly attribute boolean reallyIsApp;
 
+  /**
+   * Gets whether this frame really is a widget frame.
+   *
+   * In order to really be a frame, this frame must really be a browser
+   * frame (this requirement will go away eventually), the frame's mozwidget
+   * attribute must point to the manifest of a valid app, and the src should
+   * be in the |widgetPages| specified by the manifest.
+   */
+  [infallible] readonly attribute boolean reallyIsWidget;
+
   /**
    * This corresponds to the expecting-system-message attribute, which tells us
    * whether we should expect that this frame will receive a system message once