Bug 1599846 - Enable Background WebExtension scripts in FxR on PC r=zombie

This change allows for background scripts from WebExtensions to run in the FxR on PC chrome window. In this scenario, the promise ExtensionParent.browserStartupPromise is never fulfilled because it depends on the notification sessionstore-windows-restored, which doesn't occur because the FxR window does not participate in SessionStore.
To address this issue, browserStartupPromise is now a race between the original notification and a new one, extensions-late-startup, which is fired from fxrui.js.

Differential Revision: https://phabricator.services.mozilla.com/D55058

--HG--
extra : moz-landing-system : lando

Author: thomasmo

browser/fxr/content/fxrui.js

@@ -136,6 +136,11 @@ function setupBrowser() {
     );
 
     FullScreen.init();
+
+    // Send this notification to start and allow background scripts for
+    // WebExtensions, since this FxR UI doesn't participate in typical
+    // startup activities
+    Services.obs.notifyObservers(window, "extensions-late-startup");
   }
 }
 

gfx/vr/VRShMem.cpp

@@ -317,7 +317,7 @@ bool VRShMem::JoinShMem() {
       msg.AppendPrintf("VRService OpenMutex error \"%lu\".", GetLastError());
       NS_WARNING(msg.get());
 #  endif
-      MOZ_ASSERT(false);
+      return false;
     }
     MOZ_ASSERT(GetLastError() == 0);
   }

gfx/vr/nsFxrCommandLineHandler.cpp

@@ -21,6 +21,7 @@
 #include "nsString.h"
 #include "nsArray.h"
 #include "nsCOMPtr.h"
+#include "mozilla/StaticPrefs_extensions.h"
 
 #include "windows.h"
 #include "WinUtils.h"
@@ -69,6 +70,10 @@ nsFxrCommandLineHandler::Handle(nsICommandLine* aCmdLine) {
       MOZ_CRASH("--fxr not supported without e10s");
     }
 
+    MOZ_ASSERT(mozilla::StaticPrefs::extensions_webextensions_remote(),
+               "Remote extensions are the only supported configuration on "
+               "desktop platforms");
+
     aCmdLine->SetPreventDefault(true);
 
     nsCOMPtr<nsIWindowWatcher> wwatch =

toolkit/components/extensions/ExtensionParent.jsm

@@ -2079,15 +2079,19 @@ var ExtensionParent = {
 
 // browserPaintedPromise and browserStartupPromise are promises that
 // resolve after the first browser window is painted and after browser
-// windows have been restored, respectively.
+// windows have been restored, respectively. Alternatively,
+// browserStartupPromise also resolves from the extensions-late-startup
+// notification sent by Firefox Reality on desktop platforms, because it
+// doesn't support SessionStore.
 // _resetStartupPromises should only be called from outside this file in tests.
 ExtensionParent._resetStartupPromises = () => {
   ExtensionParent.browserPaintedPromise = promiseObserved(
     "browser-delayed-startup-finished"
   ).then(() => {});
-  ExtensionParent.browserStartupPromise = promiseObserved(
-    "sessionstore-windows-restored"
-  ).then(() => {});
+  ExtensionParent.browserStartupPromise = Promise.race([
+    promiseObserved("sessionstore-windows-restored"),
+    promiseObserved("extensions-late-startup"),
+  ]).then(() => {});
 };
 ExtensionParent._resetStartupPromises();