Bug 1873772 - Only report XULStore load errors in the parent process; r=mossop

Differential Revision: https://phabricator.services.mozilla.com/D198086

dom/base/Document.cpp

@@ -12249,7 +12249,7 @@ void Document::SetReadyStateInternal(ReadyState aReadyState,
 
   if (READYSTATE_INTERACTIVE == aReadyState &&
       NodePrincipal()->IsSystemPrincipal()) {
-    if (!mXULPersist) {
+    if (!mXULPersist && XRE_IsParentProcess()) {
       mXULPersist = new XULPersist(this);
       mXULPersist->Init();
     }

dom/xul/XULPersist.h

@@ -19,6 +19,13 @@ class nsCOMArray;
 
 namespace mozilla::dom {
 
+/**
+ * This class synchronizes element attributes (such as window sizing) with the
+ * live elements in a Document and with the XULStore. The XULStore persists
+ * these attributes to the file system. This class is created and owned by the
+ * Document and must only be created in the parent process. It only presists
+ * chrome document element attributes.
+ */
 class XULPersist final : public nsStubDocumentObserver {
  public:
   NS_DECL_ISUPPORTS

toolkit/components/xulstore/XULStore.sys.mjs

@@ -12,6 +12,30 @@ const WRITE_DELAY_MS = (debugMode ? 3 : 30) * 1000;
 const XULSTORE_CID = Components.ID("{6f46b6f4-c8b1-4bd4-a4fa-9ebbed0753ea}");
 const STOREDB_FILENAME = "xulstore.json";
 
+/**
+ * The XULStore retains XULElement attributes such as the sizing and styling. These are
+ * stored in the user's profile directory inside of "xulstore.json". The JSON is
+ * stored based on the chrome URL of the resource.
+ *
+ * For instance the "chrome://browser/content/browser.xhtml" main window sizing could be
+ * stored like so:
+ *
+ * {
+ *   "chrome://browser/content/browser.xhtml": {
+ *     "main-window": {
+ *       "screenX": "1104",
+ *       "screenY": "25",
+ *       "width": "1904",
+ *       "height": "1612",
+ *       "sizemode": "normal"
+ *     },
+ *     ...
+ *   }
+ * }
+ *
+ * The XULStore can only be loaded in the parent process. See the XULStore
+ * and XULPersist C++ classes for how this is integrated from the C++ side.
+ */
 export function XULStore() {
   if (!Services.appinfo.inSafeMode) {
     this.load();

toolkit/components/xulstore/components.conf

@@ -12,5 +12,6 @@ Classes = [
         'interfaces': ['nsIXULStore'],
         'esModule': 'resource://gre/modules/XULStore.sys.mjs',
         'constructor': 'XULStore',
+        'processes': ProcessSelector.MAIN_PROCESS_ONLY,
     },
 ]

xpfe/appshell/AppWindow.cpp

@@ -1704,6 +1704,11 @@ static void ConvertWindowSize(nsIAppWindow* aWin, const nsAtom* aAttr,
 }
 
 nsresult AppWindow::GetPersistentValue(const nsAtom* aAttr, nsAString& aValue) {
+  if (!XRE_IsParentProcess()) {
+    // The XULStore is only available in the parent process.
+    return NS_ERROR_UNEXPECTED;
+  }
+
   nsCOMPtr<dom::Element> docShellElement = GetWindowDOMElement();
   if (!docShellElement) {
     return NS_ERROR_FAILURE;
@@ -1941,6 +1946,11 @@ nsresult AppWindow::MaybeSaveEarlyWindowPersistentValues(
 
 nsresult AppWindow::SetPersistentValue(const nsAtom* aAttr,
                                        const nsAString& aValue) {
+  if (!XRE_IsParentProcess()) {
+    // The XULStore is only available in the parent process.
+    return NS_ERROR_UNEXPECTED;
+  }
+
   nsAutoString uri;
   nsAutoString windowElementId;
   nsresult rv = GetDocXulStoreKeys(uri, windowElementId);